Systems and methods for providing tactile information and tactile actuators

Abstract

A tactile information device, actuator, system, and method. A substrate surface of the device has a first instance with a touch-perceptible attribute that differs from the corresponding touch-perceptible attribute a second instance of the surface, which difference defines a predetermined information signal. The difference, which may be a degree of slipperiness or stickiness, is defined by presence of a first coating in a first portion absent in a second portion, different chemical coatings in the first and second portions, a singular coating material having different states in the first and second portions or during different first and second time instances in the same portion. In actuators and coatings in which the coating has at least two stimulus-dependent states, a controller controls a degree, absence or presence of the stimulus.

Description

[0001] SYSTEMS AND METHODS FOR PROVIDING

[0002] TACTILE INFORMATION AND TACTILE ACTUATORS

[0003] STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0004] This invention was made with government support under Grant No. R01EY032584 NIH (NEI) awarded by the National Institutes of Health (NIH). The government has certain rights in the invention.

[0005] CROSS REFERENCE TO RELATED APPLICATIONS

[0006] This application claims priority from U.S. Provisional Application Ser. No. 63 / 617,093, titled “CHEMICAL COATINGS FOR TACTILE INFORMATION,” filed January 3, 2024, U.S. Provisional Application Ser. No. 63 / 715,964, titled “TRANSPARENT AND ELECTRICALLY SWITCHABLE THIN FILM TACTILE ACTUATORS BY MOLECULAR ORIENTATION,” filed November 4, 2024, and U.S. Provisional Application Ser. No. 63 / 741,322, titled “CONNECTING MATERIALS CHEMISTRY TO HUMAN FINE TOUCH THROUGH STRUCTURE-FRICTION- PROPERTY RELATIONSHIPS,” filed January 2, 2025, all of which are incorporated herein by reference in their entireties.

[0007] TECHNICAL FIELD

[0008] The present invention relates generally to haptic technologies, and more particularly, to tactile actuators that use liquid crystal films to acuate fine-touch sensations through liquid crystal films’ inherent molecular alignment and ordering.

[0009] BACKGROUND OF THE INVENTION

[0010] Haptic technologies are still limited in the types of fine-touch sensations they can create, such as a feeling of everyday surfaces and objects (e.g., wood, plastic) and in offering descriptive, high-fidelity tactile sensations for fine touch. Current tactile actuators create tactile sensations through vibrations or the mechanical and electrochemical formation of bumps to signal sensation. As a result, they only create a narrow range of feelings. While there has been extensive development in mechanical actuators for more realistic, broader range tactile sensations, haptics devices are still incapable of producing every day fine- touch sensations (i.e., feel of wood versus plastic surface). Current tactile actuators are also slow, which creates significant lag between displaying an image and changing its texture. SUMMARY OF THE INVENTION

[0011] The examples described herein provide a transparent, electrically switchable tactile actuator that can be placed on top of screens or smartphones to create switchable tactile sensations. The deposition and binding of chemical molecules or macromolecules to tactile aids or other objects can be used to convey tactile information. Depending on their chemistry, these molecules can provide a different perceptible feel on the user’s finger. For example, two or more different coatings can be used on a surface to provide discriminable (e.g., distinguishable by touch) tactile information. These textures or tactile sensations can be tuned to be perceived as sticky, slippery, or other perceptible attributes, all of which are derived due to the chemical properties of the surface, not the physical shape. Details of the use of chemical coatings to convey tactile information are provided, for example, in U.S. Provisional Application Ser. No. 63 / 617,093, filed January 3, 2024, and titled CHEMICAL COATINGS FOR TACTILE INFORMATION, U.S. Provisional Application Ser. No. 63 / 715,964, titled “TRANSPARENT AND ELECTRICALLY SWITCHABLE THIN FILM TACTILE ACTUATORS BY MOLECULAR ORIENTATION,” filed November 4, 2024, and U.S. Provisional Application Ser. No. 63 / 741,322, titled “CONNECTING MATERIALS CHEMISTRY TO HUMAN FINE TOUCH THROUGH STRUCTUREFRICTION-PROPERTY RELATIONSHIPS,” filed January 2, 2025, the disclosures of which are incorporated herein by reference in their entireties.

[0012] One exemplary tactile actuator described herein is made from a thin polymer film and provides wide sensations ranging from slippery to tacky, instead of forming bumps. The actuator also works very quickly (at least 60 Hz), so it can be refreshed quickly. Because the actuator is clear, it can be arranged on top of a screen of a mobile device or any other surface. Aspects of the present invention can be applied to virtual reality, online shopping, or making switchable furniture for hotels. The disclosed examples demonstrate that molecular rearrangement can be leveraged to create new classes of tactile actuators based on the phases of liquid crystals embedded in a solid and transparent polymer film.

[0013] One aspect of the invention relates to a method for conveying information by touch. The method includes providing at least one substrate having a surface; disposing one or more coatings on the surface or portions of the surface of the substrate; and conveying with the one or more coatings tactile information in the form of different touch-perceptible attributes due to chemical properties of the one or more coatings. In embodiments, the different touch-perceptible attributes include differences in friction perceptible as degrees of slipperiness or stickiness.

[0014] In embodiments, the one or more coatings comprises at least two coatings, each having a different chemical structure.

[0015] In embodiments, the one or more coatings comprises a single coating having among its chemical properties at least two stimulus-dependent states, wherein conveying the tactile information comprises dynamically modifying the coating from at least one of the two states to another of the at least two states by controlling a level or absence or presence of the stimulus.

[0016] In embodiments, the single coating comprises a thermotropic liquid crystal that displays a reversable change in orientation and surface morphology upon application of heat and / or electrical stimulus.

[0017] In embodiments, each of the at least two coatings comprises a macromolecular structure that is different from the at least two coatings, and the method further comprises depositing and binding the at least two coatings to respective surfaces or surface portions of the at least one substrate.

[0018] In embodiments, the method for conveying information by touch further includes coating braille characters in discriminable chemical coatings to differentiate characters that otherwise feel the same.

[0019] In embodiments, the at least two coatings are applied to the surfaces of tactile aids or objects by plasma treating of a target surface to receive the coating and vacuum depositing of a liquid silane on the plasma treated target surface.

[0020] In embodiments, a first coating of the at least two coatings comprises pentyltrichlorislane (C5) and a second coating of the at least two coatings comprises n- butylaminopropyltriethoxysilane (C4-APTMS).

[0021] In embodiments, the method for conveying information by touch further includes identifying objects based on the conveyed tactile information.

[0022] In embodiments, the single coating comprises a liquid crystal (LC), more specifically a Liquid Crystal Polymer (LCP), most specifically a Polymer Network- Stabilized Liquid Crystal (PSLC) film applied to the at least one substrate, the PSLC film configured to display reversable changes in orientation and surface morphology based upon application of heat and / or electrical stimulus.

[0023] In embodiments, the at least one substrate comprises a surface treated low-roughness quartz wafer substrate.

[0024] In embodiments, the at least one substrate comprises a glass substrate overcoated with a patterned electrode.

[0025] In embodiments, the patterned electrode comprises an interdigitated, comb gold electrode pattern.

[0026] In embodiments, the PSLC film comprises a mixture of a nematic liquid crystal 4- Cyano-4’ -pentylbiphenyl (5CB) and a liquid crystalline photoreactive monomer 1,4-Bis[4- (3 -acryloyloxypropoxy) benzoyloxy]-2-methylbenzene (RM257).

[0027] Another aspect of the invention relates to a tactile information system comprising a tactile actuator. The tactile actuator comprises a substrate having a surface; a first portion of the surface having at least a first touch-perceptible attribute defining a difference from a second touch-perceptible attribute of a second portion of the surface, the difference defining a predetermined information signal; the difference between the first portion and the second portion defined by at least one of a presence a first coating in the first portion and an absence of the first coating in the second portion; a first coating having a first chemical composition in the first portion and a second coating having a second chemical composition different from the first chemical composition in the second portion; and a first coating having a first state in the first portion and a second state in the second portion.

[0028] In embodiments, the difference between the first touch-perceptible attribute and the second touch-perceptible attribute comprises a degree of slipperiness or stickiness.

[0029] In embodiments, the first coating has at least two stimulus-dependent states responsive to a stimulus, the system further comprising a controller for applying and controlling the stimulus to dynamically modify the coating from at least one of the two stimulus-dependent states to another of the at least two stimulus-dependent states by controlling a level or absence or presence of the stimulus. In embodiments, the first coating comprises a thermotropic liquid crystal that displays a reversable change in orientation and surface morphology upon application of the stimulus, wherein the stimulus comprises heat, electrical stimulus, or a combination thereof.

[0030] In embodiments, the first portion of the substrate comprises an electrode, the first coating comprises a film disposed over the electrode, and the controller for applying the stimulus comprises a power source configured to apply an electrical field to the electrode.

[0031] In embodiments, the electrode comprises a patterned electrode.

[0032] Another aspect of the invention relates to a method for producing a tactile actuator. The method includes preparing a Polymer Network-Stabilized Liquid Crystal (PSLC) film on one or more predetermined portions of a substrate, the PSLC film having a variable surface morphology based upon external stimulus conditions; applying a first set of external stimulus conditions that causes the PSLC film to display a first surface morphology; applying a second set of external stimulus conditions that causes the PSLC film to display a second surface morphology; and conveying tactile information based on the first surface morphology, the second surface morphology, or a combination thereof.

[0033] In embodiments, the external stimulus conditions comprise temperature or electrical field conditions.

[0034] Another aspect of the invention relates to a method for producing a tactile actuator. The method includes preparing a Polymer Network-Stabilized Liquid Crystal (PSLC) film having a predetermined alignment having a variable surface morphology based upon stimulus conditions, preparing the PSLC film comprising the steps of creating an LC- monomer homogenous mixture by mixing nematic liquid crystal 4-Cyano-4’ -pentylbiphenyl (5CB) with liquid crystalline photoreactive monomer l,4-Bis[4-(3-acryloyloxypropoxy) benzoyloxy]-2-methylbenzene (RM257) at a predetermined ratio at a predetermined temperature for a predetermined amount of time until a resulting solution is clear; applying a PVA or OTS monolayer to each facing surface of opposing substrates; positioning the substrates to form a cell gap and filling the cell gap with the LC-monomer homogenous mixture; heating the LC-monomer homogenous mixture to first temperature for a first duration of time sufficient to erase flow marks; cooling the LC-monomer homogenous mixture to a second temperature and a nematic phase state; exposing the LC-monomer homogenous mixture to ultraviolet (UV) light; and removing one of the opposing substrates via a thermal release process, leaving the PSLC film disposed on the other of the opposing substrates. The method for producing a tactile actuator further includes applying a first set of external stimulus conditions that causes the PSLC film to display a first surface morphology; applying a second set of stimulus conditions to one or more portions of the PSLC film, thereby modifying the first surface morphology to a second surface morphology in the one or more portions; and conveying tactile information based on different surface morphologies in the one or more portions.

[0035] Another aspect of the invention relates to a tactile information device comprising a substrate having a surface and a first instance of the surface having at least a first touch- perceptible attribute defining a difference from a second touch-perceptible attribute of a second instance of the surface, the difference defining a predetermined information signal. The difference between the first instance and the second instance are defined by at least one of a presence of a first coating in a first portion of the substrate and an absence of the first coating in a second portion of the substrate; the first coating having a first chemical composition in the first portion and the second coating having a second chemical composition different from the first chemical composition in the second portion; the first coating having a first state in the first portion and a second state in the second portion; or the first coating having a first state in the first portion of the substrate during a first time instance and having a second state in the first portion of the substrate during a second time instance. As used herein, the term “instance” refers to a coating in a specific location at a specific point in time. Thus, reference to first and second instances may relate to different locations or the same location at different times. The difference between the first touch- perceptible attribute and the second touch-perceptible attribute may comprise a degree of slipperiness or stickiness.

[0036] In embodiments, the tactile information device may comprise a tactile actuator, wherein the first coating has at least two stimulus-dependent states responsive to a stimulus. In a tactile information system comprising the tactile actuator, the system further comprises a controller in communication with the actuator for applying and controlling the stimulus to dynamically modify the coating from at least one of the two stimulus-dependent states to another of the at least two stimulus-dependent states by controlling a level or absence or presence of the stimulus. The first coating may comprise a thermotropic liquid crystal that displays a reversable change in orientation and surface morphology upon application of the stimulus, wherein the stimulus comprises heat, electrical stimulus, or a combination thereof. In embodiments, the first portion of the substrate comprises an electrode, the first coating comprises a film disposed over the electrode, and the controller for applying the stimulus comprises a power source configured to apply an electrical field to the electrode.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 depicts a method of identifying surfaces based on chemical coatings, according to aspects of the invention.

[0039] FIG. 2 depicts a method of LCP film fabrication and POM images to confirm bulk alignment, according to aspects of the invention.

[0040] FIGs. 3A-3C depict molecular stimuli to modulate frictional forces for fine touch, according to aspects of the invention.

[0041] FIGs. 4A-4C depict a tactile actuator that generates distinctive sensations based on changes in molecular orientation of LCP films, according to an aspect of the invention.

[0042] FIGs. 5 Al -5 A3 depict patterns for PA, VA, and isotropic LCP films, according to aspects of the invention.

[0043] FIG. 5B depicts film topography and surface chemistry of LCP films at different temperatures, according to aspects of the invention.

[0044] FIG. 5C depicts AFM phase images illustrating surface features of LCP films, according to aspects of the invention.

[0045] FIGs. 5D1-5D3 depict plots of Power Spectrum Density (PSD) of height images on a log (PSD) versus k spatial frequency PA, VA, and isotropic LCP films, according to aspects of the invention.

[0046] FIGs. 6A-6B depict disruption of molecular alignment of LCP films with heat stimuli, according to aspects of the invention.

[0047] FIGs. 7A-7D depict mechanical testing set-up and friction traces of LCP films, according to aspects of the invention. FIG. 8 depicts results from human participants testing. FIG. 9 depicts a linear regression fit of human performance with mechanical testing data across combinations of silanes. Stars and circles indicate pairs of silanes which human subjects could and could not distinguish, respectively. FIG. 8 depicts heat actuation results from human testing participants.

[0048] FIG. 9 depicts a linear regression fit of human performance with mechanical testing data across combinations of silanes. Stars and circles indicate pairs of silanes which human subjects could and could not distinguish, respectively.

[0049] FIG. 10 is a schematic diagram depicting an exemplary patterned electrode, according to aspects of the invention.

[0050] FIG. 11 depicts a plot of exemplary actuator response time measured by plotting an applied signal to a patterned electrode against the responding pixel brightness collected in a video recording using a crossed POM camera.

[0051] DETAILED DESCRIPTION OF THE INVENTION

[0052] Tactile aids are an important medium for spatial information for people who are visually impaired or blind. However, lines, bumps, and physical textures traditionally used in tactile aids can quickly become cluttered.

[0053] As an alternative to physical textures, the present inventors explored the concept that chemical coatings can deliver distinctive tactile textures, as tested in a card identification task with braille-literate blind participants, for example. Humans are capable of perceiving not only fine textures with roughness as low as RMS ~13nm), but the molecular properties of materials. This perception derives from the frictional and adhesion forces produced between human fingertips and a surface, providing pressures and vibrations interpreted by 1000s of mechanosensory neurons located in human fingertips.

[0054] Most objects have surfaces that are suitable for application of chemical coatings onto them. The experiments described herein demonstrated that people can differentiate surfaces by touch based on the surfaces’ chemistry, not just by physical features like bumps or roughness. The methods described herein involve the deposition, and binding of, chemical molecules or macromolecules of different coatings to the surface or portions of the surface of tactile aids or other objects to convey tactile information. Depending on their chemical composition, these molecules can provide different touch-perceptible attributes (e.g., feel) on the user’s finger. For example, two or more different coatings can be used on a surface of an object (e.g., a substrate) to provide touch-discriminable or distinguishable by touch tactile information. Different textures or tactile sensations can be tuned to be perceived as sticky, slippery, or other touch-perceptible attributes, all of which are derived from the chemical properties of the surface, not the physical shape of the surface.

[0055] The present inventors have demonstrated that several combinations of different materials are easily discriminable by sense of touch, including by testing with low-vision and blind study participants. Traditionally, purely physical features are used to convey tactile information (e.g., bumps, braille), but this approach on its own places a significant spatial limitation on the efficiency and effectiveness by which the conveyed information can be interpreted. The use of chemical coatings to convey this tactile information, or to augment physical features doing so, adds dimensionality to the information for the user to be able to perceive more complex information than traditionally possible, with higher efficiency and effectiveness of interpretation. The addition of a parallel method of conveying information by touch can also allow the spatial footprint of the conveyed information to be significantly decreased while still retaining current or better efficiency. For instance, braille characters coated in discriminable chemical coatings can reduce the space required to effectively convey information by reducing the amount of “dead” space required to be able to differentiate characters that all feel the same. Braille users can gather information at a more similar pace, density, and complexity as compared to visual means.

[0056] The methods described herein investigate the chemical effects on tactile friction and perception which can influence frictional forces and tactile distinction through means beyond traditional physical bumps or asperities, and instead by molecular difference. The present inventors have developed sets of different materials, such as silanes and low- roughness polymer films, whose chemical composition alone enables the materials to be distinguished by sense of touch. This feature allows smooth and otherwise previously indistinguishable object surfaces to provide different tactile feedback to convey information to the user or for pleasantness or complexity of its feel. The methods described herein can be applied in parallel to physical features to increase discriminability between features and to add dimensionality to the information or sensation made available to the user.

[0057] The chemical coatings described herein are easier to manufacture and apply to objects and products, more broadly applicable, less spatially limited, and provide added richness to products and experiences as compared to physical features (e.g., bumps, braille) traditionally used to convey tactile information. Turning now to FIG. 1, example objects, such as playing cards 100, for example, were coated with one of two types of silanes: red cards 100a were silanized with pentyltri chorosilane (C5) and black cards 100b were silanized with n- butylaminopropyltriethoxysilane (C4-APTMS). The abbreviation used herein refers to the function groups that remain on the surface after silanization. The coatings were applied by plasma treating the surfaces of the cards and vacuum depositing the liquid silane in a dessicator.

[0058] Only numbered cards were used in the experiments. After materials characterization by atomic force microscopy, x-ray photoelectron spectroscopy, FTIR, and mechanical testing, a number “n” of blind and braille literate participants (e.g., n = 10, aged 20-55) were asked to perform a 2-altemative forced choice task (2-AFC) where two cards of the same number, one black and one red, were presented and the participants were asked to identify which of the two cards is black by touch. The results showed that participants, on average, could identify cards based on feel statistically significantly above chance (p < 1 x 10’5), with two of the five subjects correctly identifying 10 out of 10 cards.

[0059] The tests demonstrated that the ability to discriminate a surface is not a descriptive outcome, but rather allows for more quantitative relationships between physical and chemical properties of the cards and human responses. The demonstrated ability to design surfaces that are differentiable to fine touch with molecular-scale differences in monolayers can provide a new route for creating tactile sensations without using physical features.

[0060] The experiments with playing cards demonstrate that different chemical surface coatings can be used to identify objects. The advantage of using a coating is that, unlike with a bump or a physical tactile identifier, the entire surface of an object (e.g., a substrate) can be coated and the process of coating objects is inherently suited to scaling up in manufacture.

[0061] The methods disclosed herein aim to provide fine-touch high-fidelity tactile sensations through the inherent molecular alignment and ordering of liquid crystal (LC) films. Material properties beyond physical roughness influence frictional and adhesion forces (e.g., phase energy, chemical functionality, microstructure). Experiments described herein measure the dynamic friction force on a mock finger at conditions relevant to human touch and evaluate the entire friction force trace (e.g., evolution) and how the mechanics change depends on a condition characterized on a microscale.

[0062] The examples described herein demonstrate the mechanics and useability of a Polymer Network-Stabilized Liquid Crystal (PSLC) film for actuating fine touch sensations. The inherent molecular switch ability of LCs can be used to produce fine touch sensation.

[0063] Liquid crystals (LCs) can be fabricated into liquid crystal polymer network (LCP) films by aligning the LCs in their nematic state and subsequentially polymerizing a reactive group. Heat or applied electrical field can disrupt this ordering achieved during alignment. Upon release of the stimulus, the LCs return to an aligned ordered state forming a mechanically reversible material useful for different applications. The materials explored for touch are typically used to induce different scales of roughness and to create surfaces with controllable physical patterns, such as braille displays, for example.

[0064] While the PSLC film orientation alone has a small effect on fine-touch discrimination, the phase separation and network morphologies due to the fabrication method disclosed herein can result in varying degrees of liquid crystal and network composition exposed at the surface leading to difference in fine-touch sensation. For example, upon heating, films can undergo changes in orientation and surface morphology, resulting in changes in surface mechanics and friction trace features, especially for a planar aligned film, and ultimately turning off fine-touch distinction between planar aligned films versus a control unaligned film, and turning on fine touch distinction between planar aligned films versus vertically aligned films.

[0065] The planar aligned film disclosed herein can be fabricated onto a patterned comb electrode to test for molecular actuation via electrical stimuli. Users can distinguish an off and on state of the haptic actuator through molecular reorientation, in particular under an external stimulus, such as heat or electrical field, for example.

[0066] To explore the influence of molecular alignment of liquid crystals on touch, the present inventors fabricated both planar aligned PSLC films and vertically aligned PSLC films, as well as an isotopic, control film that was polymerized in an isotropic phase, and upon cooling had no directed alignment. One way to fabricate PSLC films is by using acrylate functionalized LC monomers in a mixture with non-functionalized liquid crystal to first, have a supportive network capable of alignment, and second, free LC capable of mobility and reversible reorientation. Then, polymerizing in aligned, nematic state can reversibly alter alignment with external stimulus, in such as temperature stimulus (e.g., UV light initiated), for example.

[0067] Fabrication of PSLC films

[0068] Exemplary polymer network stabilized liquid crystal (PSLC) films were prepared by first mixing nematic liquid crystal 4-Cyano-4’ -pentylbiphenyl (5CB, TCI Chemicals) with liquid crystalline photoreactive monomer l,4-Bis[4-(3-acryloyloxypropoxy) benzoyloxy]-2- methylbenzene (RM257, TCI Chemicals) at a 5CB: RM257 ratio of 73 wt. %. Materials were mixed at 80°C until solution became clear, producing a homogenous nematic liquid mixture with a nematic-isotropic Transition Temperature (TNI) at 56°C. A high LC: monomer ratio exceeding -60% is necessary for reorientation of liquid crystal anchored between polymer grains, but must be low enough to form a solid film network.

[0069] Turning now to FIG. 2, exemplary PSLC films were fabricated on low roughness quartz wafers 200 which were surface treated to provide different LC alignments. A mechanically buffed Polyvinyl alcohol (PVA) alignment layer 202 was used to achieve homogenous alignment (e.g., planar alignment), where the average orientation of the liquid crystal molecules is parallel to the substrate. A hydrophobic octyltrichlorsilane (OTS) monolayer 204 was used to achieve homeotropic alignment, where the average orientation of the liquid crystal molecules are perpendicular to the substrate.

[0070] For planar alignment, the quartz wafers were spincast with a 7% PVA, MW solution at 2300 rpm for 45 seconds and then baked at 120°C for 2 hours. PVA surfaces were then mechanically buffed with a microfiber cloth in anti-parallel directions. For vertical alignment, quartz wafers were exposed to air plasma, such as a Glow Plasma System, for example, for one minute to introduce reactive hydroxyl surfaces onto the surface. Wafers were immediately transferred to vacuum desiccators containing -50 uL of OTS on a glass slide. Desiccators were than evacuated and held under static vacuum for 48 hours at 90°C. For the isotropic film, untreated quartz glass was used as the underlying substrate.

[0071] Surface treated wafers were pressed together to form a cell gap which was filled with the LC-monomer homogenous mixture 206 through capillary action, the mixture comprising LC monomers 208 and non-functionalized liquid crystal 209, as described herein above. LC-monomer containing cells were heat cycled at 65°C for 1 minute to erase flow marks and then cooled back to room temperature and a nematic phase state. Once alignment was formed and confirmed by crossed polarizers, the cell was exposed to UV light (365 nm) with intensity 7=10 mW / cm2 for 30 minutes. The reaction was done at room temperature for the planar aligned (PA) nematic sample and vertical aligned (VA) nematic sample, and at a curing temperature of 65°C for the isotropic sample. The top glass substrate was removed via a thermal release process and a solidified liquid crystal polymer network (LCP) film with a thickness ~45 um remained. The LCP film was rinsed with Deionized (DI) water to remove any unanchored liquid crystal and left to dry.

[0072] A shown in FIG. 2, polarized optical microscopy confirms uniaxial alignment for PA and VA films. Under crossed polarizers, homogeneously, or planar aligned LCPs exhibit dark state because the alignment completely disrupted the passing polarized light. As the LCP film is rotated however, the sample demonstrated a bright state. Homeotropic, or vertical alignment, of LCPs also displays a dark state through a similar mechanism, and remains in dark state upon rotation. Unaligned liquid crystals will not display macroscopic anisotropy, because the direction of orientation in not well defined. As a result, the unaligned LC molecules demonstrated an isotopic display of both bright and dark state.

[0073] Fabrication of Electrical PSLC films

[0074] For electrically responsive configuration, PSLC films were fabricated onto glass substrates overcoated with an interdigitated, comb gold electrode pattern with a width and gap of 5 pm. The electrode surface was used as the bottom substrate in fabrication. The top substrate was quartz glass spincast with a thin PVA buffed polymer layer rubbed along the same direction as the electrode stripes. As actuation of the PSLC film occurs from the presence of an electrical field, direct electrical contact between the film and electrode is not necessary. Afterwards, the cell gap was filled with LC / monomer mixture and exposed to UV light on the side of the top substrate for 30 min at room temperature. Successful planar alignment was checked with crossed polarized optical microscope (POM).

[0075] Surface Characterization

[0076] Grazing Incidence Wide Angle X-Ray Scattering. (GIWAXS, Xenocs

[0077] S AXS / W AXS) 2D scattering patterns were obtained at room temperature and with a wavelength = 0.154 nm and incident angle of 0.2°. GIWAXS scattering profiles were used to confirm uniaxial alignment and quantify the orientation order parameter (S) of the aligned films. Wide Angle X-ray scattering was also collected on the PA film in the upright orientation module where the beam penetrated through the entire film. 2D patterns were collected under this confirmation at both room temperature and under heat to confirm the disruption of anisotropic molecular ordering.

[0078] Differential Scanning Calorimetry. (DSC, TA instruments) experiments were performed on TA Instrument Discovery series to characterize the phase transition temperatures and properties (reversible nematic to isotropic transition) of the final LCP network films after fabrication. All films were tested at a heating rate of 10 °C / min from -80 to 80 °C under N2 atmosphere and observed for crystallization and melt peaks.

[0079] Atomic Force Microscopy. (AFM, Bruker Multimode, analyzed with Gwyddion software) Height and phase AFM images were collected on all three alignments (PA, VA, Iso) at both room temperature and at 40 °C using a temperature module powered by a NanoScope 6 controller. AFM height images were collected to characterize surface roughness and topography, and phase images were collected to characterize material chemistry and composition present at the surface. Surface images were obtained through tapping mode over a scar are of 1 pm x 1 pm area at a scan rate of 1 Hz and drive frequency of 300 kHz with RTESPA-300 Burker AFM tips.

[0080] FIGs. 3A-3C illustrate molecular stimuli to modulate frictional forces for fine touch, according to aspects of the invention. Static tactile aids rely on large-scale physical textures or asperities, such as roughness, lines, or bumps, for example, to generate mechanical forces on the user’s finger and provide human tactile distinguishability. Self-assembled silane monolayers (FIG. 3 A) can be used to predict fine touch. For example, silicon wafer (hydrophilic) versus fluorinated silane (hydrophobic) having roughness less than 10 nm are distinctive. The friction difference is due to surface energy. Silanes can be deposited via chemical vapor deposition to physically smooth silicon wafers hydroxylated by oxygen plasma. Molecular ordering can impact friction via intermolecular forces. Molecular control, e.g., chain length (N>7) or amine group functionality, can impact friction differences.

[0081] The present inventors have demonstrated that humans can distinguish between two isosteric silanes which differ only by a single nitrogen-for-carbon substitution. The mechanism of tactile contrast originates from a difference in monolayer ordering, as quantified by atomic force microscopy, which was replicated in two alkylsilanes with a three-carbon difference in length. This approach may be generalizable to other materials and lead to new tactile sensations derived from materials chemistry.

[0082] The present inventors used mechanical testing and human psychophysics to design tactile surfaces based on molecular phenomena originating from silane-derived monolayers on relatively smooth surfaces. While the mock finger used in mechanical testing differs from a real human finger, it sufficiently captured friction phenomena to predict human behavior which reduces the need for human or animal testing. By sliding their fingers across these surfaces, humans can perceive single atom substitution of a carbon-to- nitrogen between isosteric alkyl and aminosilanes, or alkylsilanes with a three-carbon difference in chain length. The mechanism of tactile contrast came from transitions in monolayer ordering, as quantified by the Hurst exponent, which generated sufficiently noticeable differences in friction and represents a new route for creating tactile sensations without using physical features like bumps or textures. Discovering that humans are sensitive to molecular effects as small as a single site substitution opens the door to chemical and materials approaches for the rationale design of new tactile sensations.

[0083] In addition, the present inventors developed a method to establish structure-property relationships for tactile materials by connecting materials phenomena to human tactile performance via mesoscale friction, i.e., a structure-dynamics- property relationship. This methodology can be applied to repurpose existing material platforms, such as stimuli- responsive polymers and liquid crystal elastomers, to discover new tactile sensations. The tactile materials described herein, and more broadly, the methodology to discover new material systems, can lead to richer and more accurate tactile sensations for human-machine interfaces, higher quality tactile graphics for the blind, enable no-power tactile diagnostics of cognitive function, and modernize tools to investigate fundamental aspects of touch by providing sufficiently precise tactile interfaces to study sensory integration and perception.

[0084] While silanes are a good model system due to monolayer formation and systematic variation in functional groups, polymers are ubiquitous in industrial manufacturing and are already present in haptic interfaces and industrial surface coatings which are applied to most household appliances and consumer goods. It is common to tune the bulk properties, such as stiffness, for example, of a polymer for desired mechanical properties, but polymer films have not been used to directly influence fine touch via nanoscale friction. Therefore, the present inventors investigated polystyrenes with the same molecular formulas and similar number averaged molecular weights, which varied primarily in their tacticity, i.e., the relative stereochemistry of monomer units, and degree of polymer crystallinity, i.e., the regular packing of polymer chain segments into a molecularly ordered conformation. While increasing the degree of crystallinity is commonly known to increase bulk elastic modulus, increasing crystallinity also lowers frictional forces. The present inventors have demonstrated that materials with the same chemical structure but with different tacticity and degree of crystallinity, could be a new route to generate tactile sensations in haptic interfaces.

[0085] Dynamic screens or haptic devices rely on electrostatic forces, ultrasonic vibrations, or haptic feedback to modulate friction for low fidelity sensation. Crystalline microstructure of rigid, semi-crystalline polymers (FIG. 3B) can be used to generate tactile sensations. Polystyrene films of varying morphology were processed through differences in tacticity and isothermal annealing (Tm: Melting Temperature , Tc: Crystallization Temperature), evaluated with mechanical testing to form predictions, and then validated with human psychophysical testing. The present inventors spin-coated amorphous (atactic) and semicrystalline (isostatic) polystyrene stereoisomers with similar molecular weights to generate thin and smooth (relative to human tactile sensitivity) films. The degree of crystallinity can be further tuned through isothermal annealing at the crystallization temperature (Tc). As humans use friction to discriminate between surfaces, how much difference in macroscopic friction is present between the different surfaces can be quantified through mechanical testing with a mock finger and custom analysis. To account for human variability, mechanical testing was performed at a range of applied forces and sliding velocities which encompasses typical human exploration. As two surfaces which show larger differences in friction across a relevant range of masses and velocities are likely to be easier for subjects to discriminate, mechanical testing can be used to form predictions of human performance, which is validated with human psychophysical testing. Differences below the established physical limit in fine touch ARa range about ~3 nm. The human response can be predicted with stick-slip friction features phase map. Semi-crystalline polymer films can be used to control fine touch because the crystalline structure reduces frictional forces and wear in polymer films, decreases energy dissipation, and increases yield strength, thereby connecting crystallinity and mesoscale friction to fine touch. Crystallinity leads to condition-dependent friction (segment mobility). Material selection of amorphous and semicrystalline polymers can include atactic Polystyrene (aPS) 302 and isotactic Polystyrene (iPS) 304. The degree of crystallinity can be controlled through thermal annealing and tacticity (Tc = 175°C).

[0086] Thermotropic LCs are capable of producing large mechanical strains through their intrinsic ordering phases smectic (crystalline ordering), nematic (liquid crystal), and isotropic (disordered) which can be controlled through heat or electric field. Changes in the molecular orientation of LC crystal monomers 208 and non-functionalized liquid crystals 209 (FIG. 3C) generate distinctive frictional sensations. Disruption of the anisotropic alignment of LC crystal monomers 208 and non-functionalized liquid crystals 209 to set a direction of the orientation can be used for actuation. The molecular orientation and phase separation of liquid crystal films can be used for fine touch actuation, without physical bumps, providing distinctive touch-perceptible attributes and differences in friction perceptible as degrees of slipperiness (e.g., “polished-like” feel) or stickiness (e.g.. “tacky” feel).

[0087] The present inventors explored stimuli-responsive liquid crystal networks as a potential tactile actuator with changes in sensation derived solely from the inherent changes in molecular orientation and exposed chemistry, as opposed to actuation of roughness or formation of a bump. The molecular ordering of liquid crystal materials can not only influence bulk mechanics and viscoelastic adhesion phenomena, but also surface adhesion shown in electrowetting applications. The present inventors polymerized liquid crystal into solid films under three different molecular orientations and demonstrated that human participants can feel different molecular orientation when actuated by heat or by an electric field. Thermotropic liquid crystals can be fabricated into a solid polymeric networkby embedding liquid crystals into a liquid crystalline network. Liquid crystal alignment and actuation was confirmed with polarized optical microscopy (POM), wide angle x-ray scattering (WAXS), differential scanning calorimetry (DSC), and AFM.

[0088] FIGs. 4A-4C illustrate a tactile actuator that generates distinctive sensations based on changes in molecular orientation of the LCP films. The anisotropic alignment of LC monomers 208 and non-functionalized liquid crystal 209 can be based on the fabrication methods. The degree of orientation is defined by an order parameter, S which can be, for example, Planar: S = 0.51, Vertical: S = 0.62, Isotropic: S = 0. Heat or temperature differences AT or applied electrical field or difference in voltage values AV can disrupt this ordering achieved during alignment. Depending on their molecular orientation, the LCP films can provide a different perceptible feel on the user’s finger, with difference in voltage values AV ranging between 0V and 100V (feeling of ON or OFF). A voltage of 100V is sufficient to cause molecular reorientation of the film shown visibly with cross polarizers in FIG. 4C.

[0089] LCP Film Characterization

[0090] Grazing Incidence Wide Angle X-Ray Scattering (GIWAXS) 2D patterns confirmed the molecular anisotropy and uniaxial alignment of the liquid crystal films. The pattern for the isotropic film (FIG. 5A3) is a diffuse, isotropic scattering ring indicating a molecularly disordered film lacking orientation, while the patterns for PA (FIG. 5A1) and VA films (FIG. 5A2) show specific regions of scattering peak intensities indicating anisotropic alignment of the liquid crystal mesogens through TI -TI stacking. For the VA films, the TI -TI stacking crystalline peak appears along the direction parallel to the substrate along the x- axis, indicating vertical alignment of the liquid crystal mesogens in the film network. For the PA film, the TI -TI stacking crystalline peak is seen perpendicular to the substrate, indicating planar alignment of the liquid crystal mesogens.

[0091] The degree of molecular alignment was quantified by the orientation order parameter (5) and obtained from the azimuthal intensity of the primary scattering peaks. The azimuth angle orientation represents scattering due to the mesogenic orientation in reference to the substrate. The 2D patterns provided a corresponding ID azimuthal intensity distribution, and S can be obtained from integrating this intensity at the primary scattering peaks ( = 80° for PA and = 18° for VA) against the isotropic background. 5 = 0 represents no preferred orientation or ordering and S = 1 represents perfect ordering. S = 0.51 was calculated for PA films, S = 0.62 for VA films, and S was set to 0 in Iso films as an isotropic baseline. These S values are typical for aligned liquid crystal polymer networks retaining the nematic phase after polymerization. Thus, the GIWAXS confirms that the aligned films retain different liquid crystal alignments after polymerization based on the surface alignment technique. This includes the bulk Iso film, which was formed while heated, and which remains isotropic upon cooling to room temperature due to formation of the network.

[0092] The liquid crystal ordering parameter S indicates the magnitude of molecular order that can be disrupted, i.e., degree of actuation, by external stimuli, such as heat or electricity, for example. This change in order with heat was demonstrated with the PA film alignment. WAXS Scattering patterns also showed that the anisotropic alignment was effectively disrupted with heat.

[0093] Differential Scanning Calorimetry. Turning now to FIGS. 6A and 6B, Differential Scanning Calorimetry (DSC) heating and cooling curves show a crystallization peak upon cooling and a melt peak upon heating for all films at ~36-37°C. The bulk mixture of 5CB liquid crystal and liquid crystal monomer before polymerization had a temperature TNI ~ 56° as confirmed by POM, however no crystallization or melt peak appears at this temperature. The crystallization peak at the TNI for neat 5CB liquid crystal is ~37°C, thus the DSC curves show that only the anchored liquid crystals are mobile enough to undergo the reversible phase transition from liquid crystalline phase (nematic) to disordered phase (isotropic), and not the polymerized matrix. This result is consistent with similar liquid crystal networks where the film is a polymerized solid, and the higher crosslinked network precludes the material from undergoing thermotropic phase transitions. However, the overall LC network can undergo thermal expansions and strains (~5%) with heat and this is dependent on the crosslinking density, as well as the temperature at which the films are polymerized. In summary, DSC shows that the polymerized films retain reversible molecular reorientation from liquid crystalline to isotropic phases at ~37°C due to the anchored liquid crystal component.

[0094] Atomic force Microscopy. The methods described herein investigate the potential of liquid crystal films to acuate fine touch sensations through their inherent molecular alignment and ordering, and decouple these effects from secondary actuation of topographical changes like physical bumps and roughness. Therefore, the surface roughness and topography of the LCP films was characterized with AFM height images collected at room temperature and at 40°C as shown in FIG. 5B, for example. The average roughness parameter, Ra, of the LCP films was calculated from the height images using Gwyddion software. All films had low roughness with an Ra = 1.086 nm for the PA film, Ra = 0.5785 nm for the VA film, and an Ra = 1.475 nm for the Iso film. The difference in Ras were all less than the perceivable limit to discriminate surfaces purely by roughness alone (A Ra > 7 nm). Upon heating, all films resulted in an increase in average roughness, with the VA film having the largest increase to an Ra = 1.076 nm, PA film with an Ra = 1.139 nm, and the Iso film with an Ra = 1.693 nm. Although the films become rougher with heat, these differences were all still below the perceivable limit, thus discrimination between the films would likely not be driven by an apparent physical change.

[0095] The surface features of the films can also be seen in the AFM height images. Both the PA and VA films show phase separated domains with regular spacing. The VA film has a rougher structure pointing outwards from the film, and smaller domain spacings compared to the PA film. However, the Iso film lacks regular domain spacing. Upon heating, both the PA and VA films show phase separation with larger domain spacings.

[0096] These differences in characteristic dimensions of the surface can be quantified by the Power Spectrum Density (PSD) of the height images on a log (PSD) versus log (kspatial frequency) plot shown in FIG.s 5A-5C for the PA film (Fig. 5D1), VA film (Fig. 5D2\ and Iso film (Rig. 5D3), respectively. Surface domains of regular spacing or different roughness are revealed by a peak or “knee” of two linear regions on the PSD plot. In contrast, randomly rough surfaces with self-affine characteristics will show a straight line with a constant slope. The spatial frequency is inversely related to the real space wavelength of surface features.

[0097] From the PSD plot shown in FIG. 5D1, the PA film at room temperature shows a distinct intercept with a corresponding average domain size of ~50 nm. The VA film at room temperature shows a more pronounced intercept, and a much smaller corresponding domain size at ~30 nm. The isotropic film however lacks a regular domain spacing at room temperature as evidenced by the lack of a clear intercept in the PSD plot shown in FIG. 5D3. Upon heating, for both the PA film (FIG. 5D1) and the VA film (FIG. 5D2), the intercepts of the PSD shifts to lower spatial frequencies, indicating larger characteristic domain sizes, 90 nm and 40 nm respectively, and larger phase separation. In both cases, the surface domains become less repeated with larger spacings under heat. These phase- separated domains at the film surface represent the liquid crystalline domains in the polymer network which have different roughness than the film network itself. The changes in the film surface features can be further illustrated with the AFM phase images shown in FIG. 5C. The contrast differences in the AFM phase images represent difference in material properties, such as viscoelasticity and adhesion, for example, and can be used to visualize the composite film morphology. All phase images were scaled to the same range around their individual mean phase value for clarity. The PA and VA films at room temperature show heterogeneity in phase across the surface. This is likely due to material property differences between the polymer network (RM257) and anchored liquid crystals (5CB). The isotropic film, however, does not show this same material heterogeneity as the aligned films, but rather a relatively homogenous material surface with a low phase, suggesting a rigid, non-adhesive network at the surface. This is likely because the network was polymerized while the liquid crystals were in an isotropic phase, and upon cooling did not undergo the same phase separation morphology as those polymerized in the nematic phase. Instead, the isotropic films formed a distinct, less phase- separated network. Upon heating, however, the isotropic film morphology becomes much more chemically heterogenous and more similar to the PA and VA films under heat, as seen by the contrast in the phase images.

[0098] While AFM height images revealed subtle difference in roughness with heat, phase images reveal that the material properties and film morphology at the surface undergo significant changes with heat. The average phase for all films increased with heat, and this difference was most significant for the VA film increasing from an average phase of -13° to ~ 83°. This suggests that the VA film network undergoes significant changes in surface morphology due to the alignment strategy, and this structural property change under heat is most pronounced in the VA films compared to the PA or Iso films. Liquid crystal networks are able to undergo slight thermal expansions with heat, preferentially expanding perpendicular to the long axis of the molecule, inducing phase separation. In the PSLC films here, this manifests as exposing liquid crystalline domains at the surface and the magnitude of exposure of these domains depends on the alignment strategy.

[0099] In conclusion, surface characterization reveals that the PSLC films have differences not only in liquid crystalline alignment confirmed by WAXS and POM, but also different surface morphologies which are influenced by heat, especially for the VA film. All films have two components: one, a liquid crystalline component which is anchored at a different orientation depending on the alignment strategy and can be disrupted with heat, and second, a network component which influences the surface morphology and phase separations and differs depending on the surface alignment technique. While the network does not phase transition from a liquid crystalline to isotropic state with heat, the bulk film does undergo a phase transition with heat, changing the phase separation of the film and the surface chemistry exposed. Thus, both the surface chemistry and morphology of the LCPs is different amongst the films and is influenced by heat.

[0100] Mechanical Testing.

[0101] As will be shown herein, it is possible for two surfaces which exhibit larger differences in material properties to have comparatively smaller differences in friction. To rationally determine which surfaces would generate the largest tactile actuation between the on and off state, or to determine which film morphology is contributing the most to tactile actuation, the present inventors measured the mesoscale friction as friction is ultimately the tactile stimulus felt by humans.

[0102] Turning now to FIGs. 7A-7D, a custom mechanical testing setup was used to measure the mesoscale friction between a mock finger and the liquid crystal polymer films at both room temperature and under 40 °C. In the approach described herein, a soft, elastic mock finger that mimics key parameters of a real human finger was used to predict tactile stimuli typically available to subjects, while keeping the mechanical testing within a feasible parameter space. The human finger is a multi-layered structure consisting of a rigid bone surrounded by soft tissue layers, with an overall effective modulus of -100 kPa. While soft, it is not sticky and sebum excretion makes the skin of the finger moderately hydrophilic (-60° by water contact angle). While the shape of a finger is rounded, in a sliding motion the interface of the finger during contact rapidly approaches saturation in contact area (at -1 N force 90% contact area of the fingertip is achieved), providing an approximately planar, opposed to spherical interface. This straight edge interface is consistent with residue patterns left by human subjects. Finally, while a human finger typically possess fingerprints, these fingerprints vary from person to person. Prior work showed that while fingerprints amplify differences in friction, surfaces which contained differences in friction for patterned mock fingers were qualitatively similar to flat fingers.

[0103] In the exemplary experimental setup, to recreate macroscopic sliding friction conditions during human fine touch, a mock finger was prepared by curing a soft polydimethylsiloxane (PDMS) slab with dimensions of 1 cm x 1 cm x 5 cm around a 3-D printed acrylic cylindrical “bone” at 60°C for one hour providing a finger with a dead weight of ~5 - 6g to mimic soft tissue surrounding rigid bone in a human finger. The rectangular geometry ensures consistent nominal contact area even under different loading conditions, and human subjects tend to leave residues with consistent width and straight edges even at low loads (~1 N). The 3D printed bone provides mechanical rigidity and mechanical stiffness similar to the distal phalange within a human finger, and the elastic modulus of the PDMS can be controlled by the ratio of base to crosslinker (30: 1) to achieve 100 kPA, similar to the effective modulus of human fingertips. The Effective Modulus of the PDMS was tuned to lOOkPa (using 30: 1 base:crosslinker) to mimic the properties of a human finger. A contact area for the finger of 1 cm x 1 cm was also used to mimic the human finger, as well as various volcities and applied masses within the ranges of typical human tactile finger exploration (e.g. 5 - 45 mm / s and O-lOOg, respectively). The invention is not limited to any particular testing set-up, however.

[0104] The mock finger was slid across each surface at 16 different sliding conditions (4 velocities and 4 applied masses) relevant to human touch exploration to analyze the unique stick-slip friction present in the friction traces. Stick slip friction originates from the real friction that is produced dynamically as a finger slide across a surface due to transient trapping of the soft finger (stick) and subsequent energetic releases (slip) on the mesoscale, and is dependent on the applied mass, velocity, and temperature of the surface.

[0105] FIG. 7B shows representative friction traces and differences in stick-slip friction amongst the differently aligned PSLC films. The PA film shows the highest magnitude stiction spike followed by higher amplitude and slower oscillations compared to the VA and Iso films. However, this stick-slip friction behavior changes upon heating, with all films reducing in stiction, but significantly for the PA film. The different oscillations seen in the stick slip friction traces are at amplitudes perceivable to touch (p = F / Fn > 0.035) and must be considered for fine touch perception.

[0106] At room temperature, PA in all case tended to show a more adhesive surface producing the largest stiction spike at most conditions. In general, large stiction spikes indicate an adhesive contact, and have been seen in sliding friction of nematic liquid crystal networks. Liquid crystals in isotropic phase have shown to reduce stiction, or static friction, compared to their more adhesive nematic state.

[0107] To combine all the friction collected from the experimental conditions into a predictor of tactile distinction, the similarity or dissimilarity of the stick slip friction traces of each film was quantified through cross-correlation analysis as shown in FIG. 7C, of an analysis method used in previous studies with reliable predictability. A comparison that generated similar friction traces results in high correlation with large, symmetric triangular cross-correlation traces and are likely harder for humans to discriminate by fine touch, therefore are denoted by red (R) and predicted to be difficult to discriminate. Comparisons that generated distinctive friction traces result in low correlations with small, or irregularly shaped correlation traces and are denoted with green (G) and predicted to be more discriminable. Similarity was quantified by cross-correlating the friction traces of each film at every condition, and then cross-correlation was normalized by the absolute magnitude of both input vectors and calculated by the equation 1 : where a and b are the time-series friction traces measurements and a is the mean value of the vector. The cross-correlation trace can be parametrized into a single value by the area under the curve, i.e., average correlation shown in Fig. 7C, but can also be further parametrized into a single value of skew or kurtosis. This analysis condenses all the stick slip friction traces from a comparison of two surfaces or conditions into a single value on the scale of “similar” to “discriminable” (red-to-green scale, from dark Red (D) to Red (R) to Orange (O) to Yellow (Y) to light Green (L) to dark Green (G) (FIG. 7D). In the case shown in FIG. 7D, cross-correlation values were calculated to compare the friction differences within the same film at room temperature versus heat.

[0108] Discriminability matrices shown in Fig. 7D combine the parametrized crosscorrelation values at each experimental condition to summarize the differences in friction across the experimental space. In this case, the discriminability matrices compare differences in friction within a single film at room temperature versus under heat. Matrices which show more green regions indicate more conditions in which a PSLC surface shows distinct differences in friction traces and is likely easier to discriminate under a wider range of exploration conditions. The skew cross-correlation parameter, which has also served as a good predictor in previous studies, also predicted that both the PA and VA film undergo mesoscale friction changes with heat. The discriminability matrices shown suggest that the PA film undergoes the largest changes in mesoscale friction with heat. Interestingly, this film showed the smallest changes in roughness and phase as characterized in AFM.

[0109] In summary, the PSLC films show differences in stick-slip friction, particularly differences in stiction, and this friction changes with conditions, as well as with heat, at varying magnitudes depending on the film’s alignment strategy. These changes are predicted with cross-correlation analysis, showing that the PA film has the largest changes in mesoscale friction and would most likely be the strongest actuator between its on versus off state, which can be verified with human testing.

[0110] Human Testing.

[0111] Heat Actuation.

[0112] To verify if humans could distinguish differences in the PSLC films’ changes in mesoscale stick-slip friction with heat, the present inventors first performed human psychophysical testing on all films with heat actuation, systematically looking at alignment and heat to verify differences in friction are perceivable. A single unheated film versus a heated film could not simply be compared because subjects would recognize the films due to the temperature difference. Instead, a three-alternative forced choice task, or “odd-man out” test, was used comparing the PSLC films at both room temperature and with heat to measure how tactile discrimination differs between the two films (e.g., between the room temperature versus heated condition). Each participant performed each comparison for 5 trials. The first set of comparisons was performed at room temperature conditions. As the Iso film has a different visual appearance from the VA and PA films, subjects were blindfolded for all of the room temperature comparisons. In the heated case, films were heated to 40°C on a hot plate. The invention is not limited to any specific controller for controlling heat received by the film, however, and any heat sources and systems (e.g. thermostats) for controlling the degree or presence or absent of the heat may be provided. For these experiments, participants’ vision was occluded by a black box, but participants were not blindfolded out of safety precautions. However, at 40°C, all films became visually identical. Human testing results show that VA and PA, although completely opposite orientations, were slightly but not significantly distinctive through touch (accuracy = 44.4%, z = 1.47, p > 0.05). However, VA versus Iso films were slightly more distinctive at room temperature (accuracy = 46.7%, z = 1.791, p < 0.05) and the PA films versus the Iso films were significantly distinctive through touch (accuracy = 55.6%, z = 3.06, p < 0.01). These results indicate that opposite alignments (e.g., planar versus vertical) have less of an influence on tactile perception compared to aligned versus isotropic alignment.

[0113] All three comparisons were then tested at 40 °C. As shown in FIG. 8, PA versus Iso films under heat became significantly less distinctive under heat and very close to chance (accuracy = 37.8%, z = 0.523, p > 0.05). This difference in perception at room temperature versus under heat was statistically significant determined by a z-score test of 2 population proportions (z = 1.69, p < 0.05). However, VA versus Iso films under heat became slightly more distinctive under heat (accuracy = 53.3%, z = 2.742, p < 0.05), although not significantly (z = 0.6325, p > 0.05). This is an opposite trend compared to the PA versus Iso film comparison. Interestingly, when comparing PA versus VA films under heat, although now both less ordered with their characteristic alignment disrupted, and hypothesized to become more similar, became significantly distinctive (accuracy = 66.7%, z = 4.64, p < 0.01).

[0114] This is evidence that simply disruption in orientation (e.g., order parameter) is likely not the only predictor of fine touch distinction as PA and VA films had higher differences in alignments at room temperature compared to their heated state, yet to humans, PA and VA became more distinctive with heat. Rather, the overall network structure formed during synthesis due to alignment technique and polymerization conditions altered the change in surface morphology of the materials under heat, exposing different surface adhesive properties as the bulk film was disordered. The VA network did indeed undergo the largest network change in terms of topography and phase shown in AFM.

[0115] Correlating mechanical testing with human performance

[0116] Prior to insights from human testing, a discriminability matrix based on skew was a convenient starting point for predicting human performance on different materials via friction. To better explain human performance, results from human testing were fitted to several predictors based on the friction traces with a stepwise linear regression. Preforming a regression required identification of predictors. Predictors were identified by summing a parametric value across all masses and velocities for a given pair of silanes from the crosscorrelation analysis. (These parametric values were the average, variance, skew, and kurtosis, of the cross-correlation.). “Zones of discriminability” were also considered as predictors. Zones of discriminability counted the number of green regions, i.e. high discriminability, in Figure 7D, which are not apparent when taking an average value of discriminability. This was quantified by counting the number of experimental conditions in the top quartile of each parametric category (e.g., skew, variance) across every surface. FIG. 9 shows the results of the linear regression model, wherein stars and circles indicate pairs of silanes which human subjects could and could not distinguish, respectively. The silanes tested are listed in Table 1.

[0117] TABLE 1

[0118] The linear model, shown in Equation 2, yielded a correct predictive response on each of the five different silanes tested. (t(variance) = -6.28 and p < 0.01, t(nQ4 skew)) =

[0119] 11.83 and p < 0.01, F = 65.7, p < 0.001, r2= 0.71 for skew alone, r2= 0.96 with both terms).

[0120] Thus, a distinguishability score at least above 0.5, preferably above 0.6, and more preferably above 0.7, 0.8, and 0.9, in respective order of preference, may be used for identification of pairs of different coatings that may provide acceptable levels of tactile differentiability to a human user to use in connection with embodiments of the invention. Q4 refers to the number of experimental conditions in the fourth quartile of the skew data set across every surface. Here, “distinguishability” was defined on a scale from 0 to 1 denoting pairs of materials and was not the same scale as “discriminability” used for comparing friction traces.

[0121] The total variance from mechanical testing was negatively correlated with human performance which suggests that a wide distribution of different friction forces was not helpful for discriminating surfaces. Rather, a smaller (but presumably distinct) distribution of amplitudes in the oscillations of friction contributes to the ability of subjects to discriminable samples. The importance of zones of discriminability based on skew offer a clearer mechanistic origin for the human ability to discriminate between silanes: unlike averaging the entire experimental space, the zones of discriminability are traceable to discrete experimental conditions. Skew represents asymmetry between the number of large amplitudes versus small amplitudes, relative to the mean amplitude.

[0122] The Hurst exponent had a moderate correlation (r2=0.64) with subject performance which was expected because the sets of silanes which had a high success rate with subjects testing had one silane with a high degree of monolayer ordering and another silane with low degree of monolayer ordering. To reemphasize, the Hurst exponent here was a method to quantify monolayer ordering, as the magnitude of roughness remained below the physical limits of detection for human subjects. In the experiments, the Hurst exponent was correlated with successful discrimination by human subjects only because the mechanism is through differences in friction caused by monolayer ordering. Therefore, taking differences in the Hurst exponent without the context of the experimental system is unlikely to be predictive of human performance. Despite the ideal scenario presented here, the correlation between Hurst exponent and human performance was not as high as the correlation based on mechanical testing in Eqn. 2. Therefore, human performance is better predicted by measurements of mesoscale friction and supports the use of mechanical testing.

[0123] Consequently, to create better tactile contrast in materials, the model in Eqn. 2 suggests materials where one surface generates large amplitude oscillations in friction forces and the other surface generates low amplitude oscillations at a given applied mass and sliding velocity. Another route for improved tactile contrast may emphasize optimizing on a few, but distinct, experimental conditions over creating surfaces with moderate differences across multiple conditions. In exemplary material systems studied, high skew surfaces were generated by combining the effects of a short and long chain length with the effects of hydrogen bonding to create an abrupt and complete phase transition. Even in relatively simple alkylsilanes, another potential route of tactile contrast, the odd-even effect of silanes, is suggested by the higher model prediction on C7 than C8 compared to APTMS.

[0124] Electrical Actuation.

[0125] Human testing results suggest that the PA films are a good candidate for a tactile actuator through molecular reorientation because human discrimination accuracy changes significantly between the room temperature and heated conditions. This trend is also supported by PA films exhibiting the largest change in mesoscale friction with heat. However, heat is not the most ideal stimuli for most application actuators. To test if the PSLC film could also acuate fine touch under electrical stimuli, human testing was also performed on a PA film fabricated onto a comb gold electrode, with the alignment parallel to the electrodes shown in FIG. 10. Alignment was verified with POM where under crosspolarizers, the film showed dark state. Upon applying a DC electrical field across the film with a power source, not shown, the PSLC film switches to bright state and light scattering in a pattern based on the comb electrode dimensions due to molecular reorientation in the film. A voltage of 100V is sufficient to cause molecular reorientation of the film shown visibly with cross polarizers. The invention is not limited to any particular type (AC or DC), amount (V or W) of power supplied to the electrode, type of power source, or controller for the power supplied to the electrode.

[0126] For testing of electrical actuation, a three-alternative forced choice task was employed again, but this time the participant was presented three films of the same PA films, with one of the films under electrical field at 100 V. As before, the location and the alternate were randomly selected every trial. Each participant completed the task for 5 trials.

[0127] The average accuracy for identifying the “on” from “off’ PA film was 72%, and significantly above chance (p < 0.01), where all participants were able to discriminate the surface above chance. Participants described the difference as changing from a “tacky-like” feel (off) to a “smoother, polished-like” feel (on). Interestingly, the PA film “on” versus the PA film “off’ was the most discriminable comparison amongst all the comparisons done under heat. This may have been because with heat actuation, both films were changing in a way that it masked the switch ability. Another possibility is that the patterning of the molecular reorientation caused by the patterned electrodes actually made the actuation better than when molecular orientation was disrupted across the films. Regardless, this experiment demonstrates that the PA PSLC films can effectively actuate fine touch with electrical field through molecular orientation. This is a promising result indicating the potential for thin film geometry and a molecular switch in thin film or flat screen electronic devices. As used herein and as understood by those of skill in the art, the term “thin film” refers to a layer of materials ranging from fractions of a nanometer (i.e., a monolayer) to several micrometers in thickness.

[0128] To quantify the actuator response time of the PSLC film, an electrical field was generated through an electrical bias of 100V as applied in a repeated cycle between no field (0V) and 100V with a driving frequency of 7 Hz (-0.14 s). The film was recorded under crossed polarized microscopy (POM) to measure the speed of actuation, which showed that, film switched from a dark state to a bright state with a rapid response to electrical stimuli. The difference in peak times between the current signal and pixel brightness were calculated to be lower than the recorded frame rate of the camera, 1 frame / 17 ms or - 60 frames / s, as depicted in FIG. 11. Therefore, the response time was limited to the camera frame rate and is faster than - 60 Hz. This fast response is also evidence that the mechanism of switching is molecular reorientation to the electrical field, not based on joule heating (i.e. from resistance across the electrode). The mechanical properties of the PSLC likely facilitated this rapid response through low viscoelasticity. At 0V, the PSLC film is aligned with the electrodes and the axis of the polarizer. At, 100 V, light transmission increases due to the molecular reorientation in the film induced by the electric field.

[0129] Extrapolating what is influencing differentiation of the films with heat stimuli is somewhat confounding, as there are two properties changing at the same time, network morphology and molecular orientation, and these properties are changing differently with each material due to the alignment strategy. The present inventors used heat to systematically disrupt the order in different film orientations and established that the mechanism of actuation can be attributed to both the liquid crystal orientation and the network morphology. In comparing the PA v Iso films, application of heat decreased accuracy, which was attributed to the disruption of the PA nematic phase into a more isotropic phase, thus resulting in a film that is more difficult to tell apart from the Iso film due to liquid crystal orientation. However, in comparing VA vs Iso films, application of heat increased distinction. Atomic force microscopy showed that the topography of the film changed the most for VA morphology, thus increased accuracy under heat can be attributed due to significant changes in the VA network morphology changing the phase separation and surface composition, overriding the loss of orientation in the liquid crystals as the sole drive of actuation.

[0130] Vertical and planar films both have phase separation and similarly exposed amount of nematic LC at the surface verified by AFM phase, with PA heterogenous surface properties verified by AFM phase, with PA having larger phase domain sizes than VA due to planar orientation. However, the Iso film, due to the polymerization process, shows a less phase separated network at room temperature verified by the homogenous AFM phase. In isotropic state (upon heating), the networks relax, soften, and surface morphology changes exposing isotropic LC at the surface, so that the Iso film now has more similar surface properties to VA and PA films (AFM phase). PSLC networks can undergo thermal expansions with heat and this is dependent on their network density, as well as the temperature at which they were polymerized. For both PA and VA films, polymerization was done at room temperature, far from the bulk TNI which typically leads to a larger thermomechanical response than when polymerized near the TNI, which was the case for the Iso film. The preferential expansion direction is perpendicular to the long axis of the molecule, as the expansion is dominated by increasing intramolecular distances. Therefore, the network structure of the VA film would undergo expansion parallel to the substrate, the inverse direction as the PA film, drastically shifting the phase separation and surface morphology and likely impacting the surface mechanics and fine touch distinction.

[0131] The methods of designing liquid crystal materials for fine touch sensations described herein can be optimized in different ways. For example, while controlling alignment and orientation alone may not be enough for notable effects, the matrix and phase separation of LCP can be used to control the level of exposed surface energy of the surface by constraining what molecules are present at the surface. Designing a matrix in such a way can largely influence the difference in what is exposed at the surface. Properties, such as the crosslink density and heterogeneity of the network, for example, likely affect mechanical properties of the films, as well as the mechanical properties under stimuli.

[0132] Skew as predictor

[0133] Material properties do not necessarily predict touch perception. While both material components of the PSLC composite contribute to tactile distinction, the more robust predictor of touch perception is the resulting mesoscale friction as this serves as the cue for touch. For example, the PA film had the largest change in mesoscale fiction, followed by the VA film; however, the PA film showed the smallest topological change (AFM). To better predict the results from human testing, accuracy from each comparison was fitted to the cross-correlation parameters derived from the friction traces with a stepwise linear regression. The cross-correlation traces can also be parametrized into a single value of skew. Skew predicts that both the PA and VA films undergo mesoscale friction changes with heat. These discriminability matrices demonstrate that the PA film undergoes the largest changes in mesoscale friction with heat. The present inventors considered skew as a predictor for human accuracy by taking the average skew value at all conditions and fitting to the average human accuracy in participant testing with linear regression analysis. Skew had a low, but positive correlation (r2= 21%) to human accuracy. Skew represents the asymmetry between the number of large amplitudes versus small amplitudes, relative to the mean amplitude of the cross-correlation curve between two different frictional traces. It is possible that this parameter captures the large differences in stiction spikes between the films.

[0134] Polymer-stabilized liquid crystals and their inherent molecular switchability can be used to control touch perceptibility, without inducing large topographical changes. However, it is likely not as simple as considering molecular reorientation in their design, but rather the network morphology around is an important factor in surface design. Ultimately, the methods describe herein delivered something that changes texture rapidly through liquid crystal reorientation in a solid-state, transparent film. This actuation was through inherent molecular properties, rather than physical textures induced by molecular actuation which is more amenable to thin film geometry or devices and instant electrical actuation. Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.

Claims

WHAT IS CLAIMED IS:

1. A method for conveying information by touch, the method comprising: providing at least one substrate having a surface; disposing one or more coatings on the surface or one or more portions of the surface of the at least substrate; and conveying with the one or more coatings tactile information in the form of different touch-perceptible attributes due to chemical properties of the one or more coatings.

2. The method of claim 1, wherein the different touch-perceptible attributes include differences in friction perceptible as degrees of slipperiness or stickiness.

3. The method of claim 1 or claim 2, wherein the one or more coatings comprises at least two coatings, each having a different chemical structure.

4. The method of claim 1 or claim 2, wherein the one or more coatings comprises a single coating having among its chemical properties at least two stimulus-dependent states, wherein conveying the tactile information comprises dynamically modifying the coating from at least one of the two states to another of the at least two states by controlling a level or absence or presence of the stimulus.

5. The method of claim 4, wherein the single coating comprises a thermotropic liquid crystal that displays a reversable change in orientation and surface morphology upon application of heat and / or electrical stimulus.

6. The method of claim 3, wherein each of the at least two coatings comprises a macromolecular structure that is different from the at least two coatings, the method further comprises depositing and binding the at least two coatings to respective surfaces or surface portions of the at least one substrate.

7. The method of claim 6, further comprising coating braille characters in discriminable chemical coatings to differentiate characters that otherwise feel the same.

8. The method of claim 6, wherein the at least two coatings are applied to the surfaces of tactile aids or objects by plasma treating a target surface to receive the coating and vacuum depositing of a liquid silane on the plasma treated target surface.

9. The method of claim 6, wherein a first coating of the at least two coatings comprises pentyltrichlorislane (C5) and a second coating of the at least two coatings comprises n- butylaminopropyltriethoxysilane (C4-APTMS).

10. The method of claim 6, further comprising identifying objects based on the conveyed tactile information.

11. The method of claim 5, wherein the single coating comprises a Polymer Network- Stabilized Liquid Crystal (PSLC) film applied to the at least one substrate, the PSLC film configured to display reversable changes in orientation and surface morphology based upon application of heat and / or electrical stimulus.

12. The method of claim 11, wherein the at least one substrate comprises a surface- treated, low-roughness quartz wafer substrate.

13. The method of claim 11, wherein the at least one substrate comprises a glass substrate overcoated with a patterned electrode.

14. The method of claim 13, wherein the patterned electrode comprises an interdigitated, comb gold electrode pattern.

15. The method of claim 11, wherein the PSLC film comprises a mixture of a nematic liquid crystal 4-Cyano-4’ -pentylbiphenyl (5CB) and a liquid crystalline photoreactive monomer l,4-Bis[4-(3-acryloyloxypropoxy) benzoyloxy]-2-methylbenzene (RM257).

16. A tactile information device comprising: a substrate having a surface; a first instance of the surface having at least a first touch-perceptible attribute defining a difference from a second touch-perceptible attribute of a second instance of the surface, the difference defining a predetermined information signal; the difference between the first instance and the second instance defined by at least one of: a presence of a first coating in a first portion of the surface and an absence of the first coating in the second portion of the surface;the first coating having a first chemical composition in the first portion of the surface and a second coating having a second chemical composition different from the first chemical composition in the second portion of the surface; the first coating having a first state in the first portion of the surface and a second state in the second portion of the surface; and the first coating having a first state in the first portion of the surface during a first time instance and a second state in the first portion of the surface during a second time instance.

17. The tactile information device of claim 16, wherein the difference between the first touch-perceptible attribute and the second touch-perceptible attribute comprises a degree of slipperiness or stickiness.

18. A tactile actuator comprising the tactile information device of claim 16 or 17, wherein the first coating has at least two stimulus-dependent states responsive to a stimulus and the tactile actuator is configured to receive the stimulus, the stimulus configured to dynamically modify the coating from at least one of the two stimulus-dependent states to another of the at least two stimulus-dependent states.

19. A tactile information system comprising the tactile actuator of claim 18, the system further comprising a controller in communication with the tactile actuator for applying and controlling the stimulus by controlling a level or absence or presence of the stimulus.

20. The tactile actuator of claim 18, wherein the first coating comprises a thermotropic liquid crystal that displays a reversable change in orientation and surface morphology upon application of the stimulus, wherein the stimulus comprises heat, electrical stimulus, or a combination thereof.

21. The tactile information system of claim 19, wherein the first portion of the substrate comprises an electrode, the first coating comprises a film disposed over the electrode, and the controller for applying the stimulus comprises a power source configured to apply an electrical field to the electrode.

22. The tactile information system of claim 21, wherein the electrode comprises a patterned electrode.

23. The tactile information device of claim 16, wherein the information signal is definedby a pattern of differences between a plurality of first instances and a plurality of second instances.

24. The tactile information device of claim 21, wherein the pattern forms one or more characters.

25. The tactile information device of claim 24, wherein the one or more characters comprise braille characters.

26. A method for producing the tactile actuator of claim 18, the method comprising: preparing a Polymer Network-Stabilized Liquid Crystal (PSLC) film on one or more predetermined portions of a substrate, the PSLC film having a variable surface morphology based upon external stimulus conditions; applying a first set of external stimulus conditions that causes the PSLC film to display a first surface morphology; applying a second set of external stimulus conditions that causes the PSLC film to display a second surface morphology; and conveying tactile information based on the first surface morphology, the second surface morphology, or a combination thereof.

27. The method of claim 26, wherein the external stimulus conditions comprise temperature or electrical field conditions.

28. A method for producing the tactile actuator of claim 18, the method comprising: preparing a Polymer Network-Stabilized Liquid Crystal (PSLC) film having a predetermined alignment having a variable surface morphology based upon stimulus conditions, preparing the PSLC film comprising the steps of: creating an LC-monomer homogenous mixture by mixing nematic liquid crystal 4-Cyano-4’ -pentylbiphenyl (5CB) with liquid crystalline photoreactive monomer l,4-Bis[4-(3-acryloyloxypropoxy) benzoyloxy]-2-methylbenzene (RM257) at a predetermined ratio at a predetermined temperature for a predetermined amount of time until a resulting solution is clear; applying a PVA or OTS monolayer to each facing surface of opposing substrates;positioning the substrates to form a cell gap and filling the cell gap with the LC-monomer homogenous mixture; heating the LC-monomer homogenous mixture to first temperature for a first duration of time sufficient to erase flow marks; cooling the LC-monomer homogenous mixture to a second temperature and a nematic phase state; exposing the LC-monomer homogenous mixture to ultraviolet (UV) light; and removing one of the opposing substrates via a thermal release process, leaving the PSLC film disposed on the other of the opposing substrates; applying a first set of external stimulus conditions that causes the PSLC film to display a first surface morphology; applying a second set of stimulus conditions to one or more portions of the PSLC film, thereby modifying the first surface morphology to a second surface morphology in the one or more portions; and conveying tactile information based on different surface morphologies in the one or more portions.

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