Spray-assisted fabrication of photonic pigments

Abstract

A method of preparing photonic pigment particles comprising photonic structures of assembled colloidal particles, the method comprising: (a) providing a suspension of colloidal particles in a liquid medium; (b) spraying the suspension onto a solid surface with which the suspension forms a contact angle of greater than 90° to form one or more discrete droplets of the suspension on the surface; and (c) removing the liquid medium from the droplets by evaporation to provide the photonic pigment particles.

Description

[0001] SPRAY-ASSISTED FABRICATION OF PHOTONIC PIGMENTS

[0002] Field of the Invention

[0003] The present invention relates to methods for preparing photonic pigment particles and photonic pigment particles prepared from said methods.

[0004] Background of the Invention

[0005] Conventional colorants, such as dyes and pigments, whose colour is based on selective absorption of visible light, are widely used in the field of textiles, food, and cosmetics. In recent years, photonic pigments with structural coloration relying on selective light reflection are emerging and occupying an increasing market share because of their brilliant colour and the resistance of photobleaching (the loss of colour of a pigment on exposure to light). There are a variety of different materials that can be used to form photonic pigments and a variety of different processes have been investigated for their preparation.

[0006] Self-assembly of colloidal materials into photonic pigments is discussed in Goerlitzer et Al., Bioinspired Photonic Pigments from Colloidal Self-Assembly., Adv. Mater. 2018, 30, 1706654. Colloidal particles such as dispersions of nanospheres (such as silica and titanium nanospheres), nanorods (such as silica nanorods and nanofibers of certain biological polymers) can be processed by various techniques into photonic pigment particles. Biological polymers such as cellulose, and in particular cellulose nanocrystal (CNC) can be processed into photonic pigments. These materials have attracted great interest in the scientific community as a source material for the preparation of photonic pigments since cellulose is abundant, sustainable, biocompatible and biodegradable. CNC can easily be formed from cellulose by chemical treatment (for example by acid hydrolysis).

[0007] Several methods for the preparation of photonic pigments from cellulose nanocrystal are known in the art. A first method involves the preparation of coloured CNC films and then converting coloured CNC films into microflakes through mechanical grinding or sonication. This method is discussed in the papers Large-scale fabrication of structurally coloured cellulose nanocrystal films and effect pigments., Droguet et al., Nature Materials, volume 21 , March 2022, 352 - 358 and Cellulose Nanocrystal Chiral Photonic Micro-Flakes for Multilevel Anti-Counterfeiting and Identification., Chang et al., Chemical Engineering Journal., Volume 446, Part 1 , 15 October 2022, 136630. A disadvantage with the methods discussed in these papers is that due to their flake like morphology, the CNC pigment flakes show undesirable angle-dependent structural colorations.

[0008] To improve upon the method discussed above, the preparation of radially aligned CNC microspheres with angle-independent colour was carried out in WO 2018 / 033584. In the method disclosed in this patent application an emulsion-based process was carried out in which a dispersion of CNC particles in an emulsion was prepared. The dispersed phase of the emulsion was an aqueous suspension of the CNC particles and the continuous phase of the emulsion was oil. The emulsion was then dried by water removal so as to concentrate the CNC droplets present in the aqueous dispersed phase. A portion of the CNC present in the dispersed phase was in a cholesteric phase. The concentration of the CNC in the dispersed phase by water removal resulted in reduction of the cholesteric pitch of the CNC as the structure was shrunk so as to form the photonic microparticles of CNC.

[0009] A problem with this method is that a large amount of time is required in order to effectively dry and remove the water from the dispersed phase of the emulsion. Typically, time periods of around a week are required to dry the microparticles. It is also difficult to produce photonic microparticles using this method that have cholesteric pitches at the lower wavelength end of the visible light spectrum (i.e. blue / green coloured microparticles). In order to fabricate particles with cholesteric pitch this small it was found necessary to subject the microparticles to post-treatments such as heat treatment and solvent treatment (e.g. methanol treatment) to remove residual water from the microparticles and cause further shrinkage to form structures with smaller cholesteric pitches. These post-treatments are undesirable as they complicate the fabrication process and inevitably require the use of environmentally unfriendly solvents such as methanol.

[0010] There is thus a need in the art for a process of preparing photonic pigment particles that avoids one or more of the drawbacks associated with the processes discussed above.

[0011] Summary of the Invention

[0012] The present invention provides methods that can be used to provide photonic pigment particles that do not have the drawbacks discussed above. In particular, it has been found that photonic pigments may be readily formed by drying droplets of a colloidal suspension, where the droplets have a high proportion of their surface area in contact with air. As the droplets are dried, the liquid medium in the colloidal suspension evaporates and the colloidal particles in the droplet become more concentrated and assemble into photonic structures to provide photonic pigment particles.

[0013] Thus, according to a first aspect of the invention, there is provided a method of preparing photonic pigment particles comprising photonic structures of assembled colloidal particles, the method comprising:

[0014] (a) providing a suspension of colloidal particles in a liquid medium;

[0015] (b) spraying the suspension onto a solid surface with which the suspension forms a contact angle of greater than 90° to form one or more discrete droplets of the suspension on the surface; and

[0016] (c) removing the liquid medium from the droplets by evaporation to provide the photonic pigment particles.

[0017] According to a second aspect of the invention, there is provided a photonic pigment particle obtainable by a method according to the first aspect of the invention.

[0018] According to a third aspect of the invention, there is provided a textile fibre, food article, cosmetic or dye comprising a photonic pigment particle according to the second aspect of the invention.

[0019] In the process of the invention, the droplets present on the solid surface can be dried very easily and much more quickly than the droplets prepared using the emulsion process of WO 2018 / 033584. Using the process of the invention, the total fabrication time of preparing the photonic pigment particles can be reduced to less than two hours, for instance less than 1 hour, such as around 40 min. In contrast, using the emulsion process of WO 2018 / 033584, total fabrication times of around one week are required. The process of the invention is thus much more suitable for commercial scale up compared to the prior art emulsion-based processes.

[0020] Additionally, in the process of the invention, photonic pigment particles with colours throughout the visible light spectrum can be prepared easily with no need for complicated post-treatments such as heat treatment or solvent treatment such as methanol treatment. In the processes disclosed in WO 2018 / 033584, water removal and colloidal particle droplet shrinkage is harder due to the relatively weak liquid-liquid interfacial tension of water and oil present in the emulsion. The liquid-liquid interfacial tension present in the prior art emulsion process is relatively weak meaning that less droplet compression and shrinkage occurs on drying. This results in less compression of the cholesteric pitch of colloidal particles present in the droplets on drying. As a consequence, the fabrication of particles with cholesteric pitch that reflects light at the lower wavelength end of the visible light spectrum is not possible without post-treatments such as solvent or heat treatment to continuously force shrinkage of the microparticles. In contrast, in the process of the invention, the droplets of colloidal particles are shrunk by evaporation in air. The much stronger liquid-air interfacial tension present in the process of the invention results in a larger force compressing the colloidal particles in the droplets as they are shrunk on liquid removal. This larger force provides easier liquid removal and greater compression of the cholesteric pitch of the colloidal particles meaning that photonic pigments with smaller cholesteric pitches can be fabricated more easily and without any post-treatments.

[0021] Description of the Drawings

[0022] Figure 1 depicts a schematic diagram of the formation process of CNC microparticles on superamphiphobic surfaces. On the far left of the figure is depicted a microdroplet of CNC suspension in a liquid medium present upon a superamphiphobic surface. The microdroplet contains different domains of CNC cholesteric structures. As the liquid medium evaporates, the droplet shrinks causing the pitch of the CNC cholesteric structures to reduce and the different domains to fuse into a single monodomain with a constant cholesteric pitch. As evaporation continues, the microdroplet becomes kinetically arrested and buckles leading to further compression of the cholesteric pitch and an uneven outer surface to eventually provide the dry CNC microparticle.

[0023] Figure 2 (a) depicts a photograph of the fabrication process of CNC microdroplets on a superamphiphobic surface via a spraying approach. The inset is a scanning electron microscopy (SEM) image showing micro / nanostructure of a candle soot-based superamphiphobic surface. Figure 2 (b) is a photograph of CNC microdroplets on the superamphiphobic surface. The inset shows an enlarged image.

[0024] Figure 3 (a) depicts removal of dried CNC microparticles from the superamphiphobic surface by tilting of the surface. Figure 3 (b) depicts using liquid oil to remove the CNC microparticles from the superamphiphobic surface by rolling the liquid oil upon the surface.

[0025] Figure 4 depicts photographs showing blue (left), green (centre) and red (right) cellulose photonic pigment particles produced using the process of the invention suspended in ethyl cinnamate (n = 1.56). Figure 5 depicts a sequence of dark-field microscopy images of CNC microparticles produced using the process of the invention in refractive index oil (n = 1 .57) when imaged through no polarization (NP), left-circular polarisation (LCP) and right-circular polarisation (RCP) filters. CNC microparticles in the top three rows were produced from suspension of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 100 pmol g-1and tip sonication time (TS) from 0 to 6.67 s ml’1. CNC microparticles in the bottom three rows were produced from suspension of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 50 pmol g’1and TS from 0 to 8.89 s ml’1.

[0026] Figure 6 depicts the corresponding reflectance spectra of selected CNC microparticles in Figure 5a-d and Figure 5i-l produced using the process of the invention through NP (Figure 6a), LCP (Figure 6b) and RCP (Figure 6c) filters, respectively, averaged over 4+ particles. All curves were normalised against a white Lambertian diffuser coated with the same refractive index oil.

[0027] Figure 7 depicts a comparison of the optical appearance of large (average diameter: 147 pm), middle (average diameter: 115 pm), small (average diameter: 87 pm), and ultra-small (average diameter 32 pm) CNC microparticles in refractive index oil (n = 1.57) when imaged through NP, LCP and RCP filters. Also depicted are the corresponding reflectance spectra of CNC microparticles (NP), averaged over 4+ particles. All curves were normalised against a white Lambertian diffuser coated with the same refractive index oil.

[0028] Figure 8 depicts the surface morphology of multiple and single CNC microparticles, as recorded by scanning electron microscopy (SEM): blue pigments ([NaCI] / [CNC] = 100 pmol g-1, TS = 0 s ml’1), cyan-green pigments ([NaCI] / [CNC] = 100 pmol g’1, TS = 2.67 s ml’1), green pigments ([NaCI] / [CNC] = 50 pmol g’1, TS = 2.67 s ml’1), and red pigments ([NaCI] / [CNC] = 50 pmol g’1, TS = 6.67 s ml’1).

[0029] Figure 9 depicts cross-sectional SEM images of the microparticles in Figure 8, confirming that the pitch p of the helicoidal structure increases with the redshift of observed colours, p was measured from the limb of a fold, which is primarily responsible for the visual appearance.

[0030] Figure 10 (a) depicts a schematic diagram of CNC microdroplet drying on a superamphiphobic surface. On the centre left of the image is shown a spherical microdroplet of CNC suspension in a liquid medium present upon a superamphiphobic surface. The microdroplet contains different domains of CNC cholesteric structures. On the far left is shown a close up of the portion of the droplet in contact with the superamphiphobic surface where the microstructure / nanostructure of the surface is shown. On the centre right of the image is shown evaporation of the liquid medium and the compression of the droplet driven by the air-liquid interfacial tension. The droplet shrinks causing the cholesteric pitch of the CNC domains to reduce. On the far right of the image is shown the dried CNC microparticle in air with smaller volume and pitch than the domains of the CNC in the original microdroplet. Figure 10 (b) depicts the relationship between the apparent contact angle eappand the liquid-solid contact fraction f for an ideal Cassie-Baxter state at two different values of the Young’s contact angle eY. Among them, eYof 60°represents hexadecane, and hexadecane shows a 9appof 156 ± 1 °on the superamphiphobic surface. eYof 120°represents water, and water shows a eappof 165 ± 1 °on the superamphiphobic surface. Figure 10 (c) depicts the drying of a CNC microdroplet in oil. On the far left of the image is shown the CNC microdroplet in oil. In the centre is shown the shrinkage of the droplet as it is dried. Compression of the droplet is driven by liquid-liquid interfacial tension. On the far right is shown the CNC microparticle formed due to droplet shrinkage. The microparticle contains some residual water due to the less effective process of droplet shrinkage in oil. It can also be seen that the microparticle has a larger volume and pitch than the microparticle depicted in Figure 10 (a).

[0031] Figure 11 shows the diameter shrinkage ratio of CNC microdroplets with time produced by the process of the invention. After 30 hours, the diameter ratio reached 0.495 that is lower than the value of 0.567 for CNC microdroplets produced by the prior art emulsion-based method after around one-week drying in oil.

[0032] Figure 12 (a) shows dark-field optical microscope images of blue CNC microparticles produced using the process of the invention produced from the CNC suspension of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 100 pmol g-1and sonication for 0.44 s ml’1, and recorded prior to, and after methanol treatment. Figure 12 (b) shows dark-field optical microscope images of red CNC microparticles produced from the CNC suspension of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 50 pmol g_1and sonication for 6.67 s ml’1, and recorded prior to, and after methanol treatment.

[0033] Figure 13 depicts a schematic diagram of the evaporation process of CNC emulsion microdroplets using the process of the invention. On the right of the diagram is shown droplets of CNC suspension in a liquid medium present on a superamphiphobic surface. On the left of the diagram is shown an open pure water or salt solution container. The container and superamphiphobic surface were placed in a closed space to control the environmental relative humidity (RH) and cause drying of the CNC microparticles on the solid surface.

[0034] Figure 14 depicts the change in diameter of CNC microdroplets over time under different RH provided by pure water and different salt solutions.

[0035] Figure 15 (a) depicts dark-field optical microscope images of representative blue, cyangreen and red CNC microparticles drying at 34% humidity, 12% humidity, and mild vacuum. Figure 15 (b) shows the corresponding unpolarized micro-spectra averaged over a minimum of 5 microparticles and normalized against a white Lambertian diffuser coated with the same refractive index oil.

[0036] Figure 16 depicts a comparison of the appearance of a pristine superamphiphobic substrate (Figure 16 (a)) and a superamphiphobic substrate after spraying CNC microdroplets and removing CNC microparticles 30 times using the process of the invention (Figure 16 (b)).

[0037] Figure 17 depicts local SEM images of a pristine superamphiphobic substrate (Figure 17 (a)) and a superamphiphobic substrate after spraying CNC microdroplets and removing CNC microparticles 30 times using the process of the invention (Figure 17 (b)).

[0038] Figures 18 (a) to (c) are side views of a 5 pL drop of water (Figure 18 (a)), CNC suspension ([CNC] = 7.0 wt.%) (Figure 18 (b)), and hexadecane (Figure 18 (c)) on a superhydrophobic surface. Figure 18 (d) depicts dark-field optical microscope images of the blue (left image), cyan-green (central image) and red (right image) CNC microparticles that were prepared on the superhydrophobic surfaces. Corresponding micro-spectra of CNC microparticles are shown in Figure 18 (e), averaged over 4+ locations.

[0039] DETAILED DESCRIPTION OF THE INVENTION

[0040] The process of the invention comprises step (a) of providing a suspension of colloidal particles in a liquid medium. Any colloidal particles suitable for forming photonic pigment particles comprising photonic structures of assembled colloidal particles can be used. Suitable types of colloidal particles will be apparent given the benefit of the present disclosure. The colloidal particles may comprise organic colloidal particles or inorganic colloidal particles. Preferably, the colloidal particles comprise organic colloidal particles. More preferably, the colloidal particles comprise one or more biological polymers.

[0041] The colloidal particles may comprise substantially spherical colloidal particles or spherical colloidal particles such as spherical silica or spherical titania particles. For example, nanospheres such as silica nanospheres or titania nanospheres may be used as the colloidal particles. These particles are examples of inorganic colloidal particles. Other spherical particles that may be used include spherical polymer particles such as polystyrene spheres or spherical particles of a biological polymer such as spherical polysaccharide particles. For example, spherical colloidal particles comprising cellulose, starch, chitin, or derivatives thereof may be used.

[0042] Where the colloidal particles comprise spherical particles, it is preferable that the colloidal particles have some degree of uniformity to promote the formation of a highly ordered phase in the final photonic pigment particle product. This enhances the appearance of the final photonic pigment particles. Typically, spherical colloidal particles will pack to form a facecentered cubic (FCC) lattice structure. Where the colloidal particles comprise spherical particles, it is highly desired that the colloidal particles have a low degree of polydispersity (and thus a higher degree of uniformity). Typically, where the colloidal particles comprise spherical particles, the spherical particles have a polydispersity of less than 10% and preferably less than 5%. Polydispersity may be determined using standard techniques known in the art such as Dynamic Light Scattering (DLS) (ISO 22412:2017).

[0043] In other instances, the colloidal particles may comprise substantially rod-like colloidal particles or rod-like colloidal particles. Examples of rod-like colloidal particles include silica nanorods (an example of an inorganic colloidal particle) or fibers of biological polymers (organic colloidal particles) such as fibers of starch, cellulose, chitin, or derivatives thereof. Similarly to the spherical colloidal particles, it is preferred that the rod-like colloidal particles have some degree of uniformity. Where the colloidal particles comprise rod-like particles, at least a portion of the colloidal particles present in the suspension form a nematic or chiral nematic phase. As discussed above, it is preferred that the colloidal particles have some degree of uniformity. It will be understood that, whilst a mixture of spherical colloidal particles and rod-like colloidal particles may be used, this is less preferred as the colloidal particles will not have the level of uniformity that is desirable for forming the photonic structures in the photonic pigment particle product. Preferably, at least a portion, and substantially all, of the colloidal particles present in the suspension are rod-like. In particularly preferred embodiments of the present invention, the colloidal particles comprise nanocrystals of a biological polymer such as a polysaccharide. More preferably, the colloidal particles comprise nanocrystals of cellulose (CNC), chitin, or a derivative thereof. Most preferably, the colloidal particles comprise CNC, and this is preferably in the form of rod-like particles.

[0044] Where the colloidal particles comprise CNC, preferably, the nanocrystals have a length of at least 50 nm; and / or at most 1500 nm. Preferably, the nanocrystals have a length of from 50 nm to 1500 nm and more preferably from 50 nm to 1000 nm.

[0045] Where the colloidal particles comprise CNC, preferably, the nanocrystals have a width of at least 5 nm and / or at most 200 nm. Preferably, the nanocrystals have a width of from 5 nm to 200 nm and more preferably 10 nm to 50 nm.

[0046] Typically, where the colloidal particles comprise CNC, the nanocrystals have an aspect ratio of from 5 to 200 and preferably from 10 to 100.

[0047] These sizes and aspect ratios described above are also preferred for other rod-like colloidal particles.

[0048] The biological polymers discussed above such as nanocrystals of biopolymers such as cellulose may be obtained using methods known in the art or from commercial sources. For example, CNC can be obtained from acid hydrolysis or TEMPO-mediated oxidation of cellulose derived from a variety of sources such as wood pulp, paper, cotton, animal sources and bacterial sources. Similarly, other types of colloidal particle discussed above may also be obtained using techniques known in the art.

[0049] Typically, the suspension comprises water. Accordingly, it is preferred that the liquid medium is an aqueous medium. In other instances, the suspension and liquid medium may comprise an organic solvent such as an organic apolar solvent. Examples of organic solvents that may be used include cyclohexane, dimethylformamide (DMF) and toluene. In yet further instances, the liquid medium may comprise mixtures of organic solvents or mixtures of one or more organic solvents and water. However, it is preferable that the liquid medium consists essentially of water. In instances where the colloidal particles comprise the preferred CNC, it is preferred that the liquid medium comprises water and more preferred that the liquid medium consists essentially of water. Preferably, the suspension comprises the liquid medium in an amount of at least 10 %, preferably at least 50 %, and more preferably at least 90 % by mass. The colloidal particles may be present in the suspension in any suitable amount. Typically, the colloidal particles are present in the suspension in an amount of from 1% to 90% by weight; preferably from 3% to 30% by weight and more preferably from 5% to 10% by weight. The preferred amount that the colloidal particles are present in the liquid medium will be dependent on the nature of the colloidal particles and of the liquid medium. Where the colloidal particles comprise CNC and the liquid medium comprises water, it is preferred that the colloidal particles are present in the suspension in an amount of from 1% to 30% by weight; more preferably 3% to 15% by weight and most preferably 5% to 10% by weight.

[0050] It is preferred that the suspension of colloidal particles has an anisotropic phase. The presence of an anisotropic phase is preferred since it has been found that this promotes the formation of ordered structures of the colloidal particles within the liquid medium. For example, in the case of rod-like colloidal particles, a low amount of anisotropy in the suspension promotes the formation of a cholesteric (chiral nematic) phase of the colloidal particles within the liquid medium which is desirable. Thus, in preferred embodiments, the suspension comprises an anisotropic phase of the colloidal particles and a cholesteric phase of the colloidal particles. For suspensions comprising CNC and water, a low amount of anisotropy is provided by the preferred concentration ranges described above. Where the liquid medium comprises a different solvent such as an organic solvent, a low degree of anisotropy may be provided at higher colloidal particle concentrations. The concentration of colloidal particles presents in the suspension to provide a desired level of anisotropy will be dependent on the nature of the colloidal particles and the liquid medium.

[0051] The suspension may comprise one or more additional additives intended to have an effect upon the nature of the colloidal particles present in the suspension. For example, the suspension may comprise a salt, preferably a water-soluble salt, and more preferably sodium chloride. The presence of salt is particularly preferable where the colloidal particles comprise CNC since it is used to decrease the cholesteric pitch of the CNC present in the suspension. Salt may also be included for other types of colloidal particles where desired. The one or more salts can be present in the suspension in any suitable amount. Typically, the one or more salts are present in the suspension at a concentration of from 10 to 200 pmol, preferably from 20 to 150 pmol, and more preferably from 40 to 120 pmol per gram of colloidal particles. By adding salt, the pitch of the suspension may be decreased so as to be within a range such that after carrying out the process of the invention, the cholesteric pitch of the final photonic pigment particle product is similar to wavelengths of visible light. This characteristic enables the photonic particle to reflect light in the visible light spectrum and appear a certain colour. The pitch of the colloidal particle starting material will impact the cholesteric pitch of the final photonic pigment particle product. The purpose of the salt is to increase the ionic strength of the suspension which causes the pitch of the cholesteric phase to reduce. Other methods of increasing the ionic strength of the suspension to reduce the pitch include addition of acid to the suspension such as sulphuric acid. Methods of reducing the pitch of the suspension that can be used include those disclosed in WO 2018 / 033584.

[0052] The method of the invention may also comprise a step of sonicating the suspension of colloidal particles before step (b). Sonication, such as tip-sonication, of e.g. a nanocrystal suspension is typically used to improve the stability of that suspension, and the dispersion of the individual nanocrystals. Such techniques may be used to alter, such as red-shift, the pitch of the chiral nematic phase.

[0053] It is not essential that the suspension is sonicated and whether or not this is desirable will depend upon the nature of the colloidal particles and the liquid medium. Where the colloidal particles comprise CNC and the liquid medium comprises water it is preferred that the suspension is sonicated. Preferably, the sonicating energy dose is from 0.1 to 1000 joule per ml of suspension, and more preferably from 1 to 300 joule per ml of suspension. This is preferred where the colloidal particles comprise CNC and the liquid medium comprises water. For different colloidal particles and / or liquid media the sonicating energy dose that is preferred may be as described above, or different.

[0054] As discussed above, where the colloidal particles comprise CNC or other rod-like materials such as other biological polymers, the cholesteric pitch of the cellulose in the suspension may be tailored by either altering the ionic strength of the suspension to reduce the pitch or by sonicating the suspension so as to increase (red shift) the pitch. Using these techniques, the cholesteric pitch of the initial suspension may be tailored as desired. These techniques can thus tailor the cholesteric pitch of the photonic structures present in the final photonic pigment particle products. The intense colour of the CNC-based materials, including CNC particles, comes from the helicoidal structure created by self-assembly during the drying of the photonic particles as discussed in further detail below. The twisting configuration of CNCs forms a pattern that repeats over a certain distance, called pitch (cholesteric pitch). This periodic structure reflects light at a specific wavelength related to the pitch. If the pitch is changed, the colour of the photonic particle product will also change accordingly. CNC- based materials with a larger pitch reflect longer wavelengths of light, resulting in a red shift in colour, while CNC-based materials with smaller pitch reflect shorter wavelengths of light, resulting in a blue shift.

[0055] This final pitch of the product photonic particles in the method of the invention is essentially determined by the pitch in the initial CNC suspension and its evolution during the drying process.

[0056] In the process of the invention, preferably, both salt addition to the suspension and sonication of the suspension as discussed above are used. For CNC, at the preferred concentration ranges discussed above in water, the CNC suspension typically initially comprises a cholesteric phase of CNC with a pitch in the infrared region. Addition of salt to the suspension can be used to reduce the pitch so that the pitch of dried CNC pigments produced using the process of the invention can be close to the visible light spectrum thus generating visible colour. Typically, salt addition can be used to reduce the pitch to the lowest end for generating blue-coloured CNC pigments. Subsequent sonication can then be used to increase the pitch to provide suspensions generating CNC pigments with red-shifted colours.

[0057] The pitch of the suspensions comprising cholesteric phases of colloidal particles is typically from 0.5 pm to 20 pm; preferably from 1 pm to 15 pm; and more preferably from 2 pm to 10 pm directly before step (b) of spraying the suspension is carried out (i.e. after any salt addition or sonication steps). The pitch can be determined using suitable techniques known in the art such as optical microscopy. Typically, the optical microscopy is performed in transmission mode between crossed polarizers.

[0058] The process of the invention comprises step (b) of spraying the suspension onto a solid surface with which the suspension forms a contact angle of greater than 90° to form one or more discrete droplets of the suspension on the surface.

[0059] Typically, the step of spraying the suspension is carried out with a spray gun. Any suitable spray gun known in the art for spraying suspensions or solutions and forming one or more discrete droplets on a solid surface may be used. For example, the spray gun commercially available as SPARMAX SP-35 from Sparmax may be used. Similar spray guns known in the art may also be used. Typically, the spray gun is operated with a driving pressure of from 0.01 bar to 10 bar, preferably from 0.1 bar to 1 bar, and more preferably from 0.2 bar to 0.6 bar.

[0060] Typically, the spray gun is operated with a nozzle diameter of from 0.1 mm to 5 mm, preferably from 0.1 mm to 1 mm, and more preferably from 0.2 mm to 0.5 mm.

[0061] Typically, the spray gun is operated with a distance from the spray gun nozzle to the surface of from 1 cm to 100 cm, preferably from 5 cm to 30 cm, and more preferably from 15 cm to 25 cm.

[0062] Typically, the spray gun is operated with a spraying time of from 1 second to 1000 seconds, preferably from 10 seconds to 60 seconds, and more preferably from 15 seconds to 25 seconds.

[0063] Typically, the spray gun is operated with one or more of the following parameters:

[0064] (i) a driving pressure of from 0.01 bar to 10 bar, preferably from 0.1 bar to 1 bar, and more preferably from 0.2 bar to 0.6 bar;

[0065] (ii) a nozzle diameter of from 0.1 mm to 5 mm, preferably from 0.1 mm to 1 mm, and more preferably from 0.2 mm to 0.5 mm;

[0066] (iii) a distance from the spray gun nozzle to the surface of from 1 cm to 100 cm, preferably from 5 cm to 30 cm, and more preferably from 15 cm to 25 cm;

[0067] (iv) a spraying time of from 1 second to 1000 seconds, preferably from 10 seconds to 60 seconds, and more preferably from 15 seconds to 25 seconds.

[0068] Preferably, the spray gun is operated with two or more of parameters (i) to (iv) described above; more preferably three or more of parameters (i) to (iv) described above; and most preferably all of parameters (i) to (iv) described above.

[0069] More preferably, the spray gun is operated with one or more of the following parameters:

[0070] (i) a driving pressure of from 0.1 bar to 1 bar, and preferably from 0.2 bar to 0.6 bar;

[0071] (ii) a nozzle diameter of from 0.1 mm to 1 mm, and preferably from 0.2 mm to 0.5 mm;

[0072] (iii) a distance from the spray gun nozzle to the surface of from 5 cm to 30 cm, and preferably from 15 cm to 25 cm;

[0073] (iv) a spraying time of from 10 seconds to 60 seconds, and preferably from 15 seconds to 25 seconds. Preferably, the spray gun is operated with two or more of parameters (i) to (iv) described above; more preferably three or more of parameters (i) to (iv) described above; and most preferably all of parameters (i) to (iv) described above.

[0074] Most preferably, the spray gun is operated with one or more of the following parameters:

[0075] (i) a driving pressure of from 0.2 bar to 0.6 bar;

[0076] (ii) a nozzle diameter of from 0.2 mm to 0.5 mm;

[0077] (iii) a distance from the spray gun nozzle to the surface of from 15 cm to 25 cm;

[0078] (iv) a spraying time of from 15 seconds to 25 seconds.

[0079] Preferably, the spray gun is operated with two or more of parameters (i) to (iv) described above; more preferably three or more of parameters (i) to (iv) described above; and most preferably all of parameters (i) to (iv) described above.

[0080] It is desired for the droplets produced by spraying to be of substantially uniform diameter. Operating a spray gun under the parameters discussed in the paragraphs above has surprisingly been found to provide droplets with a substantially uniform diameter. A uniform diameter is desirable for a variety of reasons such as providing droplets of similar diameter than may then be shrunk to a similar size in the subsequent liquid medium removal step to yield photonic pigment particles comprising similar photonic structures that have the same or very similar colour.

[0081] It has also been found that the spraying time has an effect upon the diameter of the droplets produced on the solid surface. It has been found that longer spraying times result in droplets with a larger diameter. The spray time may thus be varied to achieve droplets of a desired diameter.

[0082] The produced droplets have a largest diameter of typically from 150 pm to 350 pm; preferably from 200 pm to 300 pm; and more preferably from 225 pm to 275 pm.

[0083] Droplets with a diameter in the abovementioned ranges can be achieved, for example, by using the spray gun operating parameters discussed in the paragraphs above. For example, in a highly preferred embodiment it was found that a spray time of 20 seconds produced droplets with a diameter of 250 pm ± 50 pm.

[0084] The solid surface used in step (b) of the process has a contact angle of greater than 90° with the sprayed suspension. The solid surface is preferably a hydrophobic surface. The term hydrophobic surface is understood in the art to refer to a surface that has a contact angle of greater than 90° to water. Preferably, the solid surface will be a superhydrophobic surface. The term superhydrophobic surface refers to a surface that has a contact angle of greater than 150° to water. The use of a hydrophobic or superhydrophobic solid surface is particularly useful when the liquid medium of the suspension comprises or consists essentially of water.

[0085] The solid surface may also be an amphiphobic surface. The term amphiphobic surface refers to a solid surface that has a contact angle of greater than 90° with hexadecane and greater than 90° with water. More preferably, the solid surface is a superamphiphobic surface. The term superamphiphobic refers to a surface that has a contact angle with water of greater than 150° and a contact angle with hexadecane of greater than 150°. The use of an amphiphobic or superamphiphobic solid surface is highly preferable where the liquid medium comprises or consists essentially of an organic solvent such as an apolar organic solvent such as cyclohexane, or where the liquid medium comprises or consists of water.

[0086] The contact angles discussed above can be determined using standard methods known in the art. Preferably, the contact angles are determined according to ASTM D7334-08 (2022).

[0087] Preferably, the solid surface used has a tilt angle of less than 10° to water or hexadecane, and preferably to both water and hexadecane. The tilt angle may be determined using the test method specified in Design of robust superhydrophobic surfaces. Nature. June 2020; 582 (7810).

[0088] Any suitable hydrophobic and amphiphobic surfaces known in the art may be used. Examples of superhydrophobic surfaces include substrates, such as glass, coated in a superhydrophobic coating, such as ULTRA EVER DRY paint (UltraTech International, Inc.). Examples of superamphiphobic surfaces that may be used include those discussed in Solvent-free synthesis of microparticles on Superamphiphobic surfaces., Deng et al., Angew. Chem. Int. Ed. 2013, 52, 11286 - 11289. As discussed in this reference, superamphiphobic layers extend the water repellency of superhydrophobic surfaces to organic liquids and aqueous solutions of surfactants. For superamphiphobicity of a surface to occur, low interfacial energy (e.g. high hydrophobicity) and microscopic protrusions with overhanging geometries are important. Microscopic pockets of air are trapped beneath the liquid. When droplets are applied to the surface, the droplets rest on top of the protrusions, and the interfacial tension of the liquid causes the drop to assume a spherical shape, much like a freely suspended or falling drop. Furthermore, because the real solid-liquid interfacial area is much smaller than the apparent contact area, adhesion of the drop to the surface is low, leading to a low roll-off angle.

[0089] Preferably, the solid surface comprises a silicon dioxide surface with surface microstructure or nanostructure. The silicon dioxide surface is used to provide a surface with high hydrophobicity. The microstructure or nanostructure of the surface create the protrusions in the surface discussed above. More preferably, the silicon dioxide surface has been treated with a halogenated alkyl silane compound, such as a fluorinated alkyl silane compound; and / or has been prepared by formation of a silicon dioxide surface on a candle soot substrate. Without being limited by theory, it is believed that the halogenated alkyl silane compound further increases the hydrophobicity of the surface. The candle soot substrate is used as a substate for forming surface microstructure and / or nanostructure. Any suitable method known in the art for forming the surfaces may be used and will be apparent given the benefit of the present disclosure.

[0090] An advantage of the use of superhydrophobic and preferably superamphiphobic surfaces is that due to the high contact angles formed between the surface and liquid medium of the suspension, a substantially spherical droplet is formed with only a very small area of contact between the liquid of the droplet and the solid surface. This has been found to provide a substantially radially symmetrical evaporation of the droplet during the subsequent step of removing the liquid medium from the droplets where the droplets maintain their spherical shape during the drying step. This is desired for the colloidal particles to assemble into three dimensional photonic structures during the drying step as discussed in further detail below. On surfaces where the droplet has a larger area of contact between the surface and the droplet, liquid flows from the centre of the droplet to the line of contact between the surface and the droplet. A superhydrophobic and preferably superamphiphobic surface thus provides a platform to minimize the adverse effects of interfacial driving force reduction caused by solid-liquid contact, and to maximize the air-liquid contact area to increase the interfacial driving force for evaporation of the liquid medium from the droplet.

[0091] The process of the invention comprises a step (c) of removing the liquid medium from the droplets by evaporation to provide the photonic particles. This step may comprise heating the one or more droplets and surface to evaporate the liquid medium from the one or more droplets. Alternatively or additionally, this step may comprise simply reducing the relative humidity of a closed environment in which the droplets are held. Preferably, step (c) of removing the liquid medium from the droplets by evaporation to provide the photonic particles comprises reducing the relative humidity of a closed environment in which the droplets are held. More preferably, reducing the relative humidity of a closed environment in which the droplets are held comprises including a saturated salt solution in the closed environment in a separate container to the solid surface and droplets. Typically, the saturated salt solution comprises a saturated salt solution of an alkali metal or an alkali earth metal. Preferably, the saturated salt solution comprises a saturated salt solution of potassium chloride, sodium chloride, magnesium chloride or lithium chloride.

[0092] Relative humidity may be reduced to up to 95 %, preferably up to 90 %, such as up to 85%, where the relative humidity for pure water is 100 %. For instance, relative humidity may be reduced to from 1 to 95 %, preferably from 3 to 90 %, and more preferably from 5 to 85 %. For instance, relative humidity may be reduced to from 5 to 50 %. Relative humidity may be determined using known methods, for instance according to ASTM E337-15 (2023).

[0093] Step (c) may be carried out for a period of from 30 minutes to 24 hours, preferably from 45 minutes to 10 hours, and more preferably from 1 to 3 hours. However, shorter durations are also possible. Thus, preferably, step (c) is carried out for a period of from 10 minutes to 24 hours, preferably from 20 minutes to 10 hours, and more preferably from 30 minutes to 1 hour.

[0094] Where step (c) of removing the liquid medium from the droplets by evaporation to provide the photonic particles comprises reducing the relative humidity of a closed environment in which the droplets are held, the droplets may be held in the closed environment for a time period of from 30 minutes to 24 hours, preferably from 45 minutes to 10 hours, and more preferably from 1 to 3 hours. Preferably, the droplets are held for a time period of from 10 minutes to 24 hours, preferably from 20 minutes to 10 hours, and more preferably from 30 minutes to 1 hour.

[0095] Step (c) may be carried out under vacuum but is preferably carried out at atmospheric pressure (about 101.3 kPa).

[0096] As discussed above, a key advantage of the present invention is that the drying time of the particles is far quicker than the necessary drying time in prior art emulsion-based processes where drying times of around a week are required. As also discussed above, a key advantage of the present invention is that the drying process can result in a greater compression of the cholesteric pitch of the colloidal particles present in the droplets (where the colloidal particles comprise a cholesteric phase such as is the case with CNC), compared with prior art emulsion-based processes. This greater pitch compression means that photonic structures with smaller pitches can be produced more easily and without any post-treatments such as solvent post-treatments. This is in contrast to the prior art emulsion-based processes which require solvent post treatment to produce photonic structures with lower cholesteric pitch such as blue and green coloured microparticles.

[0097] Accordingly, preferably, the process of the invention does not comprise carrying out any post-treatments to the produced photonic particles. Preferably, the process of the invention does not comprise carrying out any solvent addition post treatments to the produced photonic particles such as post-treatment by addition of a Ci to C8aliphatic alcohol.

[0098] Additionally, preferably, the process does not comprise heating the droplets produced in step (b) of the process, although as discussed above this step may be included as desired.

[0099] Whilst it is preferable that the process of the invention does not comprise carrying out any solvent addition post treatments to the produced photonic particles, this is not essential and this step may still be carried out in less preferred embodiments of the invention.

[0100] As discussed above, the reason why there is no need for any post-treatment in the process of the invention is because the air-liquid interface tension is larger than liquid-liquid interface tension resulting in a larger force to compress the cholesteric structures into a smaller pitch compared with emulsion-based techniques. In the process of the invention, the step of liquid medium removal can reduce the pitch of CNC microparticles to a wavelength that reflects blue, green and other short wavelengths of light. The production of such particles using the prior art emulsion-based processes is not possible without the additional post-treatment using organic solvents such as methanol.

[0101] The reduction of the cholesteric pitch of particles (such as CNC) on drying is depicted in Figure 1. As shown in Figure 1 , in the first stage of microdroplet drying, the randomly oriented cholesteric domains inside each microdroplet merge and reorganise to form a radially aligned monodomain structure (a Frank-Pryce structure). The evaporation of liquid medium at the particle surface causes a higher concentration of the colloidal particles at the particle surface. This causes a dynamic blockage at the droplet periphery resulting in the formation of a gradually hard outer shell. This outer shell then buckles due to the interplay between compressive capillary forces and the mechanical resistance of the solidifying cholesteric colloidal particle shell. Significant buckling of the radially aligned cholesteric shell enhances the pitch compression within the droplet, thereby producing visible colours.

[0102] As shown in Figure 10, enhancement of the interfacial driving force during the drying process is a potential strategy to compress to smaller pitches. In general, the maximum interfacial tension (y) value is obtained at the liquid / air interface. However, to collect the formed CNC microdroplets in a controllable manner, as discussed above, a solid surface is required to make contact with the droplets, which inevitably reduces the y value, since yLS< yLA, where yLSand yLAare interfacial tension at the interface of liquid / solid and liquid / air respectively. Compared to other solid surfaces, CNC microdroplets and superamphiphobic surfaces have the lowest solid-liquid contact area (Figure 10b). This means that the superamphiphobic surface provides a platform to minimize the adverse effects of interfacial driving force reduction caused by solid-liquid contact, i.e. maximizing the air-liquid contact area to force CNC microdroplets to be almost solely driven by yLA. Therefore, compared with CNC microdroplets that are subject to weak liquid-liquid interfacial tension (yLL) in oil (as in the prior art emulsion-based method), the much stronger yLAmakes CNC microdroplets compress into a smaller volume and pitch in the process of forming microparticles (Figure 10a, 10c).

[0103] Once the photonic pigment particles are produced, the process may further comprise removing the photonic pigment particles from the surface. This may be achieved by, for example, tilting the solid surface to remove the one or more particles and / or by applying a liquid, such as an oil, to the surface to collect the particles in the liquid. Suitable liquids for use include refractive index oil and other suitable liquids. An advantage of the use of superamphiphobic solid surfaces is that the surfaces not only repel water but also low surface energy liquids such as non-polar liquids and surfactants. Droplets on such surfaces have high contact angle (contact angle > 150°) and roll off easily (tilting angle < 10°), since the interfacial area between liquid and solid is extremely small.

[0104] The produced particles typically have a largest particle diameter of from 10 to 500 pm, preferably from 20 to 200 pm, and more preferably from 30 pm to 150 pm. The term largest diameter is used to refer to the largest dimension of each particle. Larger diameter particles will typically reflect a larger intensity of light. The largest diameter of the particles may be affected by for example, the droplet size produced in the spraying step and the concentration of the colloidal particles in the solution. The photonic pigment particles are preferably substantially dried in the drying step. Accordingly, typically, the water content of the particles is 50% v / v or less, preferably 20% v / v or less, more preferably 10% v / v or less, still more preferably 5% v / v or less, and most preferably 2% v / v or less.

[0105] Alternatively, the content of the particles may be expressed as the nanocrystal content, where high nanocrystal contents are preferred. The nanocrystal content may be expressed as a weight percentage of the total weight of the particle. The nanocrystal content of the particle may typically be at least 50 wt%, at least 60 wt%, at least 70 wt%, at least 80 wt%, at least 90 wt%, at least 95 wt% or at least 98 wt%.

[0106] The photonic particles produced comprise photonic structures of assembled colloidal particles. A discussed above, the colloidal particles self-assemble during the drying step into photonic structures. The term photonic structures as used herein is used to refer to a nanostructure or microstructure with a refractive index that changes periodically. The photonic structures thus contain regularly repeating regions of different refractive index. The variation of the refractive index occurs on the scale of the wavelength of visible light. This causes the photonic structures to reflect visible light of certain wavelength. Photonic structures reflect light in the visible light spectrum (380 nm to 700 nm) and thus macroscopically appear to be coloured.

[0107] The photonic structures can be classified by the type of morphological modulation of their refractive index. For example, in the case of spherical colloidal particles, the photonic structures may be perfectly periodic (crystalline) or have domains of periodicity (polycrystalline). The photonic structures may alternatively be arranged with short range order (so-called photonic glasses) or even random (which produces white particles).

[0108] In the case of rod-like particles such as biological polymers such as CNC, the photonic structures can be classed as e.g. nematic (where the nanoparticles are oriented in parallel but not arranged in well-defined planes), or chiral nematic (otherwise known as cholesteric) where the nanoparticles align locally along a helicoidal structure.

[0109] Preferably, the photonic structures have a chiral nematic phase, as is observed with fiber or rod-shaped particles such as CNC.

[0110] Accordingly, the photonic structures present in photonic pigment particles produced by the process of the invention may comprise ordered (crystalline), polycrystalline, short-range correlated, or random structures. These structures can interact with light in the visible light spectrum.

[0111] More preferably, the photonic pigment particles comprise a polymer nanocrystal (such as CNC) having a radially aligned cholesteric phase. Typically, these photonic pigment particles show a series of concentric rings in the polarized optical microscopy image of the particles; or the photonic pigment particles show a Maltese cross pattern in the polarized optical microscopy image of the particle.

[0112] The radial alignment of a particle may be determined from the polarization micrograph for example when the particle is viewed through cross-polarizers, optionally with a first-order tint plate. The chiral nematic phase may extend throughout the particle, and is not confined to a shell of material. The chiral nematic phase is also preserved during shrinkage, for example where water is at least partially removed from the system.

[0113] Where the pigment particles comprise photonic structures with a chiral nematic phase, the pitch of the chiral nematic phase is typically from 0.05 to 5 pm; preferably 0.05 to 4 pm; more preferably from 0.1 to 2 pm; still more preferably 0.1 to 1 pm; and most preferably 0.2 to 0.5 pm. For example, the pitch of the particles may be from 0.2 pm to 0.7 pm; 0.2 pm to 0.5 pm; 0.3 pm to 0.7 pm or 0.3 pm to 0.5 pm; 0.4 pm to 0.7 pm or 0.5 pm to 0.7 pm. The pitch of the chiral nematic phase can be determined by suitable methods known in the art. For example, the pitch of the chiral nematic phase can be determined by scanning electron microscopy (SEM), and preferably scanning electron microscopy (SEM). Typically, in carrying out this method the particles are broken and cross-sectional scanning electron microscopy (SEM) can be used to determine the pitch.

[0114] The photonic pigment particles have structural colour. The colour may be ultraviolet colour, visible colour or infrared colour, and it is preferably visible colour. Visible colour refers to a colour with a wavelength in the range 380 to 750 nm. Infrared colour refers to a colour with a wavelength in the range 750 to 1 mm, most preferably 750 to 5 pm. Ultraviolet colour refers to a colour with a wavelength in the range 100 to 380 nm, most preferably 200 to 380 nm, such as 300 to 380 nm.

[0115] A reference to structural colour is a reference to the wavelength of light having a maximum reflectivity when normal incident light is directed onto the particle. The term particle as used herein is used to be distinguishable from a film. A particle typically has a spherical structure, a substantially spherical structure, a collapsed spherical structure, an elliptical structure, a substantially elliptical structure, or a collapsed elliptical structure. In contrast, a film has a planar morphology.

[0116] According to a second aspect of the invention, there is provided a photonic pigment particle obtainable by the method of the first aspect of the invention.

[0117] The pigment particles produced by the method of the invention inherently comprise lower amounts of contaminants than photonic particles produced by prior art emulsion-based methods. For example, particles produced using the process of the invention will comprise lower amounts of oil and solvents such as methanol as it is not necessary to use these materials in the manufacturing process of the invention, in contrast to the emulsion-based processes known in the art.

[0118] For example, photonic pigment particles of the invention typically comprises less than 5% by weight of a total weight of oil and organic solvents; preferably less than 2% by weight of a total weight of oil and organic solvents; more preferably less than 1% by weight of a total weight of oil and organic solvents; still more preferably less than 0.5% by weight of a total weight of oil and organic solvents; and most preferably less than 0.1% by weight of a total weight of oil and organic solvents.

[0119] According to a third aspect of the invention, there is provided a photonic pigment particle comprising photonic structures of assembled colloidal particles; wherein the photonic pigment particle comprises less than 5% by weight of a total weight of oil and organic solvents. Preferably, the photonic pigment particle comprises less than 2% by weight of a total weight of oil and organic solvents; more preferably less than 1% by weight of a total weight of oil and organic solvents; still more preferably less than 0.5% by weight of a total weight of oil and organic solvents; and most preferably less than 0.1% by weight of a total weight of oil and organic solvents. The oil content of the particles can be determined by suitable methods known in the art. For example, thermogravimetric analysis (TGA) can be used to determine the oil content of the particles.

[0120] The pigment particles produced using the method of the invention may be used in any known application for pigment particles and in particular any known application for photonic pigment particles. For example, the particles may be used to provide colour to clothes and other textile fibers, paper or card products, food articles, cosmetic articles or other consumer products, or any other type of dye. The photonic pigment particles may be included in dye compositions. The dye compositions may contain additional agents such as solvents. The photonic pigment particles may also be used in paint compositions such as compositions for painting vehicles.

[0121] According to a third aspect of the invention, there is thus provided a textile fiber, food article, cosmetic or dye comprising the photonic pigment particle of the second aspect of the invention.

[0122] The present invention will now be illustrated by the following non-limiting examples.

[0123] Examples

[0124] Cellulose nanocrystal suspension preparation

[0125] An aqueous CNC slurry was purchased from the Process Development Center of the University of Maine (batch no. 2021-FPL-170, [CNC] = 10.6%, pH-neutralized form, 1.1 wt% sulfur content).

[0126] The as-received CNC suspension was diluted with Milli-Q water and 0.1 mol L’1NaCI solution to obtain two types of suspensions with a CNC concentration of 7% and [NaCI] / [CNC] ratios of 50 pmol and 100 pmol, respectively. Twelve suspension samples were prepared. 45 ml of each suspension was treated with a tip sonicator (Fisherbrand 505 Sonic Dismembrator 500 W, amplitude = 40%, tip diameter = 12.7 mm) for a time interval between 0 to 400 s, with the aim of adjusting the strength of the chiral interactions to regulate the cholesteric pitch and thus control the microstructure and colour of the final CNC microparticles. Then, the suspensions were allowed to stand for a few days for phase separation until a clear and stable stratification was observed. The suspensions separated into an upper layer isotropic phase, and a bottom layer anisotropic phase. The bottom layer was used for fabricating CNC microparticles.

[0127] The sonication and salt concentration parameters of the twelve prepared CNC suspensions are shown in Table 1. The letter (a to I) shown in Table 1 for the suspensions corresponds to the images of particles produced from each suspension shown in Figure 5. Table 1 - Parameters of different CNC suspensions

[0128] Fabrication of superamphiphobic surfaces using candle soot as a template

[0129] Initially, a uniform layer of black candle soot was generated on a glass slide by carefully holding and moving the slide in the flame of a burning candle for approximately 3 minutes. Next, the soot-coated glass slides were placed in a desiccator along with two open glass vessels, each containing 4 ml of tetraethoxysilane (Sigma-Aldrich) and aqueous ammonia solution (Sigma-Aldrich), respectively. The Stober reaction was initiated on the surface of candle soot on the glass slide through vacuuming the desiccator and allowing chemical vapor deposition. The reaction lasted for more than 24 hours. Subsequently, the samples were subjected to air plasma treatment for 5 minutes using a plasma cleaner (Diener Electronic Atto-BLS), followed by exposure to an open glass vessel of 300 pL of 1H,1 H,2H,2H-perfluorodecyltrichlorosilane (Sigma-Aldrich) in a vacuumed desiccator for 2 hours at the ambient temperature. The superamphiphobic surfaces were successfully fabricated.

[0130] Fabrication of cellulose nanocrystal microparticles

[0131] The prefabricated CNC suspensions were sprayed onto the superamphiphobic surfaces by a spray gun (SPARMAX SP-35) to generate CNC microdroplets. A spray gun driving pressure of 0.4 bar; a spray gun nozzle of 0.35 mm; and about 18 cm as a spray distance were used. The size of the microdroplets was primarily determined by the spraying time, with longer spraying times resulting in larger droplets. As an example, a spraying time of 20 seconds typically produced most microdroplets with a diameter of approximately 250 ± 50 pm.

[0132] Figure 2 (a) depicts the spraying of a CNC suspension on to the superamphiphobic surface to provide spherical droplets of the suspension upon the surface. The spherical droplets had a contact angle of greater than 150° on the surface. Also shown in the Figure is a scanning electron microscopy (SEM) image showing the microstructure / nanostructure of the candle soot-based superamphiphobic surface. Figure 2(b) shows photographs of CNC microdroplets on the superamphiphobic surface.

[0133] Next, the superamphiphobic substrate (25 mm x 60 mm) loaded with CNC microdroplets and a small disk (diameter x height = 40 mm x 5 mm) containing Milli-Q water or aqueous solution were rapidly placed under a petri dish (diameter x height = 90 mm x 7 mm), effectively isolating them from the external environment.

[0134] Pure water and various saturated salt aqueous solutions were utilized to control the relative humidity (RH) within the petri dish, thereby regulating the drying speed of CNC microdroplets. The corresponding relative humidity at 20 °C for pure water, saturated NaCI solution, saturated MgCI2solution and saturated LiCI solution is reported to be 100%, 76%, 34% and 12%, respectively.

[0135] A schematic diagram of the droplet drying process is shown in Figure 13. The figure shows a saturated salt solution (10) placed in an enclosed chamber (12) to control the relative humidity (20) in the chamber. CNC microdroplets (14) are disposed and dried on a superamphiphobic surface layer (16) on a substrate (18). (The white box depicted in the superamphiphobic layer is present solely to better show the numbering “16”.)

[0136] Figure 14 shows the change in diameter of CNC microdroplets over time under different relative humidities.

[0137] Decreasing environmental relative humidity (RH) can effectively accelerate the drying rate of CNC microdroplets, thereby significantly improving the productivity of CNC microparticles. Saturated solutions of NaCI, MgCI2, and LiCI were used to reduce the RH to 76%, 34%, and 12%, respectively. The corresponding evaporation time from CNC microdroplets to CNC microparticles was about 20 hours, 2 hours, and 40 minutes, respectively. As shown in Figure 15, when the RH is 12%, the dried particles still have bright and uniform colour and reflection peak intensity, and the drying time of 40 minutes is far less than the time required by the prior art emulsion-based processes of more than one week. The process of the invention thus greatly improves the production efficiency of CNC microparticles when compared to prior art processes. While a drying time of 40 min did not compromise the optical quality of the particles, reducing the drying time further to approximately 10 min by applying mild vacuum resulted in an observable change in appearance (Figure 15, in particular Figure 15 (b)). After the drying process, the large petri dish was removed, and CNC photonic pigments were generated on the superamphiphobic surfaces. Removal of particles from the substrate

[0138] The CNC microparticles were removed from the superhydrophobic substrates in two ways. The first method involved tilting the substrate to release the microparticles, which were then collected in a dry state. The second method involved collecting the microparticles within droplets by rolling a refractive index oil or other suitable liquids across the superhydrophobic surface.

[0139] These methods of removing the microparticles from the solid surface are depicted in Figure 3.

[0140] Photographs of blue, green and red CNC microparticles prepared by the methods described above are shown in Figure 4. In the photographs, the particles are dispersed in ethyl cinnamate. The change of the generated colour was based on the adjustment of the salt content and sonication time of initial CNC suspensions.

[0141] The process steps described above were repeated thirty times. After thirty cycles of use, the superamphiphobic coating used to produce CNC microparticles still retained its intact appearance and surface micro / nano structure after thirty repeated spraying and removal cycles, indicating ideal durability. Figures 16 and 17 depict the appearance and local SEM images, respectively, of a pristine superamphiphobic substrate (Figures 16 and 17 (a)) and the same superamphiphobic substrate after spraying CNC microdroplets and producing and removing CNC microparticles thirty times (Figures 16 and 17 (b)).

[0142] Optical characterization of CNC microparticles

[0143] To reduce the broadband scattering from the particle-air interface resulting from the surface buckling, the CNC microparticles were dispersed in refractive index-matching oil (Cargille Series A, nj)5= 1.5700) during the imaging.

[0144] A customised Zeiss Axio scope A1 microscope with a CMOS camera (Pixelink PL-D725CU- T, calibrated with a white diffuser) was used for collecting reflection optical micrographs. A Halogen lamp (ZEISS, HAL100) was used as a light source in Koehler illumination. Darkfield images of CNC microparticles were taken with a Zeiss EC Epiplan-Apochromat objective (*20, NA 0.6), with configuration of the microscope that the numerical aperture (NA) in illumination was limited by the NA of the objective. The reflected light could also be filtered with a quarter wave plate and a linear polariser mounted at different orientations to distinguish between left- or right-handed circularly polarised light. During the micro - spectroscopy, the microscope was coupled to a spectrometer (Avantes, AvaSpec - HS2048) using an optical fibre (Avantes, FC - UV200 - 2 - SR, 200 pm core size) in confocal configuration. The reflectance spectra were normalised in dark-field against a white diffuser (Labsphere SRS - 99 - 010) coated with the same refractive index oil. The timelapse series of drying CNC microdroplets were recorded on the same microscope, using a Zeiss EC Epiplan-Neofluar objective (*5, NA 0.13) under a bright field, and imaged with a CMOS camera (Pixelink PL-D725CU-T). Photographs of vials containing microparticles dispersed in ethyl cinnamate (Sigma-Aldrich, = 1.558) were recorded under diffuse illumination (i.e., fluorescent ceiling light) with an iPhone 13 smartphone.

[0145] Figure 5 depicts dark-field microscopy images of CNC microparticles in refractive index oil (n = 1.57) when imaged through no polarization (NP), left-circular polarisation (LCP) and rightcircular polarisation (RCP) filters, showing the red shift in optical appearance as the sonication times of the commercial CNC suspensions increases.

[0146] Figure 6 depicts the corresponding reflectance spectra of selected CNC microparticles from Figure 5, through NP, LCP and RCP filters, respectively, averaged over 4+ particles. All curves were normalised against a white Lambertian diffuser coated with the same refractive index oil.

[0147] Figure 7 shows a comparison of the optical appearance of large (average diameter: 147 pm), middle (average diameter: 115 pm), small (average diameter: 87 pm), and ultra-small (average diameter: 32 pm) CNC microparticles in refractive index oil (n = 1 .57) when imaged through NP, LCP and RCP filters. Corresponding reflectance spectra of CNC microparticles (NP), averaged over 4+ particles are shown below the optical depiction. All curves were normalised against a white Lambertian diffuser coated with the same refractive index oil.

[0148] As shown in Figure 5a and 5g, the CNC microparticles generated by the two suspensions that had not been sonicated but where the suspension had NaCI added appear bright blue.

[0149] In order to further redshift to obtain a series of CNC microparticles covering the entire visible wavelength, ultrasonic treatment of the initial CNC suspensions was used to weaken the chiral interaction between CNCs. After sonication, five different 7.0 wt.% CNC suspensions with [NaCI] / [CNC] = 100 pmol g-1at different sonication stages (0.44, 1.56, 2.67, 4.89, and 6.67 s ml-1) were used to produce CNC microparticles under the same conditions. The significant red shift caused by the increase in sonication time can be observed, and ideal blue and green CNC microparticles were obtained (Figure 5b-5f). When the sonication time exceeds 6.67 s ml’1, there is no longer an obvious redshift phenomenon. Reducing the salt concentration of the CNC suspension appropriately is an effective strategy for further redshift and obtaining red CNC microparticles.

[0150] When the [NaCI] / [CNC] of the 7.0 wt.% CNC suspension is decreased to 50 pmol g-1and the sonication time exceeds 4.89 s ml’1, the produced CNC microparticles begin to appear bright red (as shown in Figure 5j-5l). The corresponding reflectance spectra confirm that based on these two standard CNC suspensions, supplemented by ultrasonic treatment within the time range of 0 to 8.89 s ml’1, CNC microparticles covering the entire visible-wavelength can be achieved, with typical blue, green, and red CNC microparticles produced from suspensions of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 100 pmol g-1and TS = 0.44 s ml’1(Figure 5b), [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 100 pmol g’1and TS = 2.67 s ml’1(Figure 5d), and [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 50 pmol g’1and TS = 6.67 s ml’1(Figure 5k), respectively.

[0151] In addition, by controlling the spraying parameters, it was possible to manufacture a series of CNC microparticles with different sizes (Figure 7a-7d). The reflected light intensity of CNC microparticles is directly proportional to their diameter (Figure 7e).

[0152] The diameter changes during the drying process of CNC microdroplets on the superamphiphobic surface under an optical microscope were observed. The results are depicted in Figure 11. The results showed that the final formed CNC microparticles were able to shrink to 0.495 of the original diameters of microdroplets. This was significantly lower than the diameter shrinkage ratio of 0.569 of CNC microparticles dried in oil (as in the prior art emulsion-based process). The diameter shrinkage ratio of 0.569 is similar to the diameter shrinkage ratio of CNC microparticles dried using the prior art emulsion-based process after methanol-treatment. It is thus demonstrated that the process of the invention can achieve similar droplet shrinkage without solvent post-treatment to prior art emulsion-based processes where post-treatment is used. This is also why the microdroplets generated by the same suspension of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 100 pmol g’1appear blue with a smaller pitch after drying on the superamphiphobic surface, while red with a larger pitch after drying in oil.

[0153] In a further experiment, methanol post-treatment was carried out on CNC microparticles produced using the method of the invention. Dark-field microscopy images of the microparticles before and after solvent treatment are shown in Figure 12. It can be seen in Figure 12 that the dark-field microscopy images of blue and red CNC microparticles before and after methanol-treatment basically remained unchanged (Figure 12a, 12b). This indicates that polar solvents do not cause further compression and buckling, further confirming that CNC microparticles produced using the process of the invention whose compression is driven by yLAare in a dry state with almost no residual water present (even without methanol post-treatment).

[0154] Morphology characterization of CNC microparticles by scanning electron microscopy (SEM)

[0155] In order to observe the external surface morphology of the CNC microparticles, conductive carbon tape was employed to transfer them onto flat stubs, which was subsequently coated with a 10 nm thick layer of platinum using a sputter coater (Quorum, Q150T ES). For the purpose of imaging the interior of the microparticles, they were embedded in cellulose butyrate acetate resin, followed by freeze-drying in liquid nitrogen for 5 min and mechanically crushing. The crushed fragments were transferred to vertical stubs using a conductive tape and coated with a 10 nm thick layer of platinum.

[0156] Figure 8 shows surface morphology of multiple and single CNC microparticles, as recorded by scanning electron microscopy (SEM): blue (produced from suspension of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 100 pmol g’1and TS = 0.44 s ml’1), cyan-green (produced from suspension of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 100 pmol g-1and TS = 2.67 s ml’1), green (produced from suspension of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 50 pmol g’1and TS = 2.67 s ml’1) and red (produced from suspension of [CNC] = 7.0 wt.%, [NaCI] / [CNC] = 50 pmol g’1and TS = 6.67 s ml’1) CNC microparticles.

[0157] Figure 9 shows cross-sectional SEM images of the microparticles in Figure 8, confirming that the pitch p of the helicoidal structure increases with the redshift of observed colours, p was measured from the limb of a fold, which is primarily responsible for the visual appearance.

[0158] As expected from the optical analysis, SEM top views of CNC microparticles show the surface buckled morphology caused by compression (Figure 8). SEM cross-sections of CNC microparticles revealed that the cholesteric domain remains locally well-aligned to the highly buckled surface of the microparticles (Figure 9). The pitch (p, mainly responsible for visual appearance) are consistent across multiple regions and microparticles (Figure 9). The average p of representative CNC microparticles, blue, cyan-green, green and red, increased from 250 ± 26 nm to 299 ± 28 nm, 341 ± 32 nm and 397 ± 28 nm respectively as the red shift, which is basically consistent with the corresponding optical reflectance peak wavelengths.

[0159] Preparation of CNC microparticles on a superhydrophobic surface

[0160] A superhydrophobic surface was prepared to demonstrate that the preparation method of CNC photonic pigments can be carried out on such surfaces.

[0161] The superhydrophobic surface was prepared by spraying commercial ULTRA EVER DRY paint (UltraTech International, Inc.) onto glass slides following the supplier’s guidance notes.

[0162] The properties of the surface were assessed. A high apparent contact angle for a 5 pl drop of water (eapp= 156 ± 1 °) and CNC suspension (6app= 154 ± 1 °) confirmed that this surface has a good superhydrophobicity - this is shown in Figures 18 (a) and (b), respectively. Figure 18 (c) shows that the hexadecane wetted the surface, with the middle dark line being the needle for adding droplets. Since the surface could not repel hexadecane, it is a superhydrophobic but not superamphiphobic surface.

[0163] CNC suspensions were sprayed onto this superhydrophobic surface following the same procedures as outlined in the previous examples. After drying at a relative humidity (RH) of 12%, achieved using saturated LiCI solutions, CNC microparticles were obtained.

[0164] As with the previous examples, the CNC microparticles were dispersed in an index matching oil (Cargille Series A , = 1 .5700) for imaging by dark-field microscopy. These images are shown in Figure 18 (d) with blue CNC pigments on the left, cyan-green CNC pigments in the central image, and red CNC pigments on the right. Corresponding reflectance spectra were obtained using micro-spectroscopy. The microscope images and the corresponding microspectra were collected without a filter.

[0165] As shown in Figures 18 (d) and (e), the CNC microparticles prepared on the superhydrophobic surface exhibited a similar colour and micro-spectra as the ones prepared on the superamphiphobic surface. The method for preparing CNC photonic pigments of the present invention is not restricted to superamphiphobic surfaces and can be extended to other superhydrophobic surfaces. This demonstrates the commercial feasibility of the method of the present invention.

Claims

CLAIMS1. A method of preparing photonic pigment particles comprising photonic structures of assembled colloidal particles, the method comprising:(a) providing a suspension of colloidal particles in a liquid medium;(b) spraying the suspension onto a solid surface with which the suspension forms a contact angle of greater than 90° to form one or more discrete droplets of the suspension on the surface; and(c) removing the liquid medium from the droplets by evaporation to provide the photonic pigment particles.

2. The method of Claim 1 , wherein the colloidal particles comprise inorganic or organic colloidal particles.

3. The method of Claim 1 or Claim 2, wherein the colloidal particles comprise nanocrystals of a biological polymer, such as a polysaccharide.

4. The method of Claim 3, wherein the polymer nanocrystals comprise nanocrystals of cellulose, chitin, or a derivative thereof; preferably wherein the polymer nanocrystals comprise cellulose nanocrystals (CNC).

5. The method of any preceding claim, wherein the suspension comprises water; preferably wherein the suspension comprises water in an amount of at least 10 %, preferably at least 50 %, and more preferably at least 90 % by mass.

6. The method of any preceding claim, wherein the suspension comprises a salt, preferably a water-soluble salt, and more preferably sodium chloride.

7. The method of any preceding claim, wherein the colloidal particles are present in the suspension in an amount of from 1 to 90 %, preferably from 3 to 30%, and more preferably from 5 to 10 % by weight of the suspension.

8. The method of any preceding claim, wherein the process further comprises a step of sonicating the suspension of colloidal particles before step (b); preferably, wherein the sonicating energy dose is from 0.1 to 1000 joule per ml of suspension, and more preferably from 1 to 300 joule per ml of suspension.

9. The method of any preceding claim, wherein a spray gun is used for spraying the suspension in step (b), wherein the spray gun is operated with one or more of the following parameters:(i) a driving pressure of from 0.01 bar to 10 bar, preferably from 0.1 bar to 1 bar, and more preferably from 0.2 bar to 0.6 bar;(ii) a nozzle diameter of from 0.1 mm to 5 mm, preferably from 0.1 mm to 1 mm, and more preferably from 0.2 mm to 0.5 mm;(iii) a distance from the spray gun nozzle to the surface of from 1 cm to 100 cm, preferably from 5 cm to 30 cm, and preferably from 15 cm to 25 cm;(iv) a spraying time of from 1 second to 1000 seconds, preferably from 10 seconds to 60 seconds, and more preferably from 15 seconds to 25 seconds.

10. The method of any preceding claim, wherein a spray gun is used for spraying the suspension in step (b), wherein the spray gun is operated with one or more of the following parameters:(i) a driving pressure of from 0.1 bar to 1 bar, and preferably from 0.2 bar to 0.6 bar;(ii) a nozzle diameter of from 0.1 mm to 1 mm, and preferably from 0.2 mm to 0.5 mm;(iii) a distance from the spray gun nozzle to the surface of from 5 cm to 30 cm, and preferably from 15 cm to 25 cm;(iv) a spraying time of from 10 seconds to 60 seconds, and preferably from 15 seconds to 25 seconds.

11. The method of any preceding claim, wherein a spray gun is used for spraying the suspension in step (b), wherein the spray gun is operated with one or more of the following parameters:(i) a driving pressure of from 0.2 bar to 0.6 bar;(ii) a nozzle diameter of from 0.2 mm to 0.5 mm;(iii) a distance from the spray gun nozzle to the surface of from 15 cm to 25 cm;(iv) a spraying time of from 15 seconds to 25 seconds.

12. The method of any preceding claim, wherein the surface is a hydrophobic surface, preferably a superhydrophobic surface, more preferably an amphiphobic surface, and still more preferably a superamphiphobic surface.

13. The method of any preceding claim, wherein the surface has a tilt angle of less than 10° to water or hexadecane, and preferably to both water and hexadecane.

14. The method of any preceding claim, wherein the surface comprises a silicon dioxide surface with surface microstructure or nanostructure, and preferably wherein the silicon dioxide surface: has been treated with a halogenated alkyl silane compound, such as a fluorinated alkyl silane compound; and / or has been prepared by formation of a silicon dioxide surface on a candle soot substrate.

15. The method of any preceding claim, wherein step (c) comprises: heating the one or more droplets and surface to evaporate the liquid medium from the one or more droplets; and / or reducing the relative humidity of a closed environment in which the droplets are held, for instance to from 5 to 50 %.

16. The method of any preceding claim, wherein step (c) is carried out for a period of from 10 minutes to 24 hours, preferably from 20 minutes to 10 hours, and more preferably from 30 minutes to 1 hours.

17. The method of any preceding claim, wherein step (c) is carried out at atmospheric pressure.

18. The method of any preceding claim, wherein the process further comprises removing the photonic pigment particles from the surface, preferably by tilting the solid surface to remove the one or more particles and / or by applying a liquid, such as an oil, to the surface to collect the particles in the liquid.

19. The method of any preceding claim, wherein the particles have a largest diameter of from 10 to 500 pm, preferably from 20 to 200 pm, and more preferably from 30 pm to 150 pm.

20. The method of any preceding claim, wherein the particle comprises a polymer nanocrystal having a radially aligned cholesteric phase.21 . A photonic pigment particle obtainable by the method of any preceding claim.

22. A textile fibre, food article, cosmetic or dye comprising the photonic pigment particle of Claim 21.

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