An electrowetting coalescence droplet anti-sticking surface and a manufacturing process thereof

Abstract

This invention discloses an anti-adhesion surface for electrowetting and coalescing droplets and its manufacturing process, relating to the field of microfluidics. The anti-adhesion surface comprises a transparent support substrate, an electrode composite array, a transparent dielectric layer, and a hydrophobic layer arranged sequentially. The electrode composite array includes alternating lateral trapping electrodes and a central guiding electrode. The boundaries of adjacent lateral trapping electrodes and central guiding electrodes are nested with curvature matching and maintain a substantially constant lateral gap to form a non-uniform electric field gradient between adjacent electrodes. Furthermore, a necking structure is formed in the central guiding electrode. The curvature-matched nesting structure and the necking structure work synergistically to induce instability and detachment of the three-phase contact line at the droplet's trailing edge during droplet coalescence, achieving continuous directional migration and detachment after droplet coalescence. This invention solves the problem of motion hysteresis in traditional strip or rectangular gradient electrodes after droplet coalescence, improving the anti-adhesion effect of the transparent substrate surface in complex humidity environments.

Description

Technical Field

[0001] This invention belongs to the field of microfluidics technology, and specifically relates to an electrowetting and anti-sticking surface for coalescing droplets and its manufacturing process. Background Technology

[0002] In numerous industrial sectors and daily life, the adhesion of liquid droplets to solid surfaces often leads to a series of problems, such as bacterial growth and cross-infection on medical device surfaces, contamination and performance degradation of precision optical lenses, fouling and reduced heat transfer efficiency on heat exchanger surfaces, and short-circuit risks caused by the accumulation of condensed droplets inside electronic products. Therefore, developing efficient and reliable non-stick surfaces is of great significance for improving equipment performance, ensuring safety and hygiene, and saving energy and maintenance costs.

[0003] Traditional passive anti-stick surfaces, such as superhydrophobic surfaces, rely on low surface energy chemical modifications and micro / nano rough structures to allow droplets to roll off easily at high contact angles (≥150°). However, these surfaces have inherent drawbacks such as poor mechanical durability and difficulty in removing small or sticky droplets. More importantly, they completely lack active control capabilities: once droplets evaporate, become contaminated, or are structurally damaged, the surface itself cannot intervene in any way, resulting in poor adaptability.

[0004] In recent years, electrowetting technology has been increasingly applied to anti-stick surfaces due to its advantages such as fast response speed, low energy consumption, and strong controllability. It dynamically regulates droplet behavior by applying voltage to change the solid-liquid interfacial tension. Existing solutions often employ simple strip or rectangular electrodes to drive droplet movement or coalescence by constructing a wettability gradient. However, this type of design suffers from a common technical bottleneck: droplets often stagnate after coalescence. This is because the surface energy released during coalescence is largely lost through droplet oscillations and heat dissipation, while driving the overall movement of large droplets requires overcoming their greater trailing edge contact line pinning force. Simple gradient electric fields cannot provide sufficient and precise force to drive the droplets after coalescence, resulting in delayed droplet removal and significantly reduced anti-stick efficiency, especially in transparent devices where rapid restoration of visual clarity is crucial.

[0005] Therefore, there is an urgent need for an innovative electrode structure design that can not only efficiently capture and coalesce droplets, but also effectively convert coalescence energy and actively drive the coalesced droplets to detach rapidly, thereby achieving active and efficient anti-sticking throughout the entire process. Summary of the Invention

[0006] The present invention aims to at least partially solve one of the aforementioned technical problems in the prior art. To this end, the present invention provides an electrowetting and coalescing droplet anti-sticking surface with active control function and its manufacturing process, which can actively control droplet coalescence and drive its sliding, thereby improving the anti-sticking effect of the target surface.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: An electrowetting and non-sticking surface for coalescing droplets includes a transparent support substrate, an electrode composite array, a transparent dielectric layer, and a hydrophobic layer arranged sequentially. The electrode composite array includes alternating lateral trapping electrodes and central guiding electrodes. The boundaries of adjacent lateral trapping electrodes and central guiding electrodes are nested with matching curvature and maintain a basically constant lateral gap to form a non-uniform electric field gradient between adjacent electrodes. Furthermore, the central guiding electrode has a constricted structure. The nested structure with matching curvature and the constricted structure work together to induce instability and detachment of the three-phase contact line at the trailing edge of the droplet during droplet coalescence, thereby achieving continuous directional migration and detachment of the droplet after coalescence.

[0008] In optional or preferred embodiments, the transparent electrodes consist of at least two sets of electrodes arranged coplanarly, with the width of the electrodes being non-uniformly distributed.

[0009] In optional or preferred embodiments, the central angle between the first and second arc boundaries is 90°-160°, and the lateral gap is 20μm-80μm.

[0010] In an optional or preferred embodiment, the central guide electrode forms a longitudinally constricted structure between two adjacent arcuate recessed areas.

[0011] In optional or preferred embodiments, the horizontal width of the necking structure varies from 100 μm to 300 μm to form an unstable contact line region at the trailing edge of the droplet and trigger pin detachment behavior.

[0012] In optional or preferred embodiments, the widths of the central guiding electrode and the lateral capturing electrode vary from 100 μm to 800 μm to form a continuously varying electric field driving force.

[0013] In optional or preferred embodiments, the lateral capture electrode and the central guide electrode are transparent conductive films.

[0014] In an optional or preferred embodiment, the central guiding electrode and the lateral trapping electrode are indium tin oxide (ITO) electrodes with a thickness of 200 nm to 800 nm.

[0015] In optional or preferred embodiments, the dielectric layer is made of SU8 polymer photoresist or parylene, with a thickness of 1μm-10μm and a dielectric constant greater than 2.5.

[0016] In optional or preferred embodiments, the hydrophobic layer is made of a fluorinated polymer material (such as Cytop), with a thickness of 0.5 μm-2 μm and an initial contact angle of ≥110°.

[0017] In optional or preferred embodiments, both the dielectric layer and the hydrophobic layer are obtained by spin coating at a speed of 1000 rpm to 5000 rpm and a time of 30 s to 60 s, and are then dried at 80 ℃ to 220 ℃ for 50 min to 180 min.

[0018] Based on the above technical solution, the embodiments of the present invention have at least the following beneficial effects: By designing an electrode composite array with a concentric circular arc boundary nesting structure and a necking structure, precise and coordinated control of the surface electric field distribution and the droplet interface energy state was achieved without significantly increasing process complexity and manufacturing cost. Specifically: 1. The concentric and equidistant circular arc electrode boundaries generate a strong electric field gradient with curvature matching, which efficiently and directionally captures and converges dispersed micro-droplets (such as condensed droplets) to the central region, greatly improving the collection efficiency and aggregation speed of the initial droplets.

[0019] 2. The innovative necking structure not only guides the droplet shape through geometric constraints, but more importantly, it actively promotes unstable detachment of the trailing edge contact line at the trailing edge of the droplet, fundamentally solving the core problem of the lag in the movement of large droplets after aggregation, and realizing the continuity and efficiency of the droplet aggregation and sliding process.

[0020] 3. Since the indium tin oxide (ITO) electrode, polymer dielectric layer (such as SU-8), and fluorinated polymer hydrophobic layer (such as Cytop) used are all highly transparent materials, this anti-stick surface can maintain the original light transmittance of the substrate (such as glass) to the maximum extent while achieving active anti-stick function. It is particularly suitable for scenarios with stringent requirements for visual clarity, such as camera lenses, optical sensor windows, precision instrument panels, automotive glass, and building curtain walls.

[0021] 4. This solution enhances the driving force through structural innovation rather than simply increasing the driving voltage, which helps reduce the risk of dielectric layer breakdown and charge injection, improving the long-term electrical stability and lifespan of the device. The entire manufacturing process is based on mature planar micromachining technologies (such as laser processing, photolithography, and spin coating), with simple steps, strong controllability, and high compatibility with the needs of large-area, mass production, showing good prospects for industrial application. Attached Figure Description

[0022] Figure 1 This is a three-dimensional structural layered diagram of the droplet anti-stick surface in an embodiment of the present invention; Figure 2 This is a top view schematic diagram of the electrode configuration in an embodiment of the present invention; Figure 3 This is a scanning electron microscope image of a single composite electrode in an embodiment of the present invention; Figure 4 This is a cross-sectional scanning electron microscope image of the droplet anti-stick surface in an embodiment of the present invention; Figure 5 This is a schematic diagram of the cross-sectional layered structure of the droplet anti-stick surface in an embodiment of the present invention; Figure 6 This is a diagram illustrating the mechanism of the droplet anti-sticking principle in an embodiment of the present invention; Figure 7 This is a schematic diagram of a partial structure of the central guiding electrode in an embodiment of the present invention; Figure 8 These are before-and-after comparison images of the droplet anti-sticking effect in embodiments of the present invention.

[0023] In this diagram, 01 is a transparent support substrate, 02 is a lateral trapping electrode, 03 is a central guiding electrode, 031 is a recessed crescent-shaped boundary on the central guiding electrode, 032 is a necking structure formed on the central guiding electrode, 04 is a dielectric layer, and 05 is a hydrophobic layer. Detailed Implementation

[0024] This section will describe specific embodiments of the present invention in detail with reference to the accompanying drawings. Preferred embodiments of the present invention are shown in the drawings, which are used to exemplarily illustrate the technical solutions of the present invention so that those skilled in the art can intuitively and clearly understand the various technical features and overall structure of the present invention, but should not be construed as limiting the scope of protection of the present invention.

[0025] To better illustrate this embodiment, some components in the accompanying drawings may be enlarged or reduced in size, and do not represent the actual dimensions; certain omissions in the drawings will be understood by those skilled in the art.

[0026] The present invention provides an electrowetting and anti-adhesion surface for coalescing droplets, which can actively induce the coalescing of droplets by utilizing the electrowetting effect, thereby achieving a functional surface that is highly efficient in preventing droplet adhesion, self-cleaning, or directional transport.

[0027] Reference Figure 1 A three-dimensional layered schematic diagram of a non-stick surface for agglomerated droplets based on alternating current wetting is presented. Figure 1 This section mainly describes the complete structure of the embodiments of the present invention. The anti-stick surface includes a transparent support substrate 01, an electrode composite array, a dielectric layer 04, and a hydrophobic layer 05 that are stacked in sequence. The electrode composite array includes alternating lateral trapping electrodes 02 and central guiding electrodes 03.

[0028] The lateral capture electrode 02 and the center guiding electrode 03 are disposed on the upper surface of the transparent support substrate 01, the dielectric layer 04 covers the lateral capture electrode 02 and the center guiding electrode 03, and the hydrophobic layer 05 covers the upper surface of the dielectric layer 04.

[0029] Reference Figure 2 A top view schematic diagram of the electrode configuration for an electrowetting, agglomerated droplet anti-sticking surface is presented, showing the key components for achieving efficient and stable anti-sticking function, including a lateral trapping electrode 02 and a central guiding electrode 03. (Refer to...) Figure 2 and Figure 7 The lateral capture electrode 02 includes multiple lateral capture electrode units arranged laterally, and the central guide electrode 03 includes multiple central guide electrode units arranged laterally, with the lateral capture electrode units and central guide electrode units alternately arranged. Each lateral capture electrode unit includes two symmetrically arranged first arcuate boundaries, and the first arcuate boundary includes multiple arcuate segments arranged longitudinally and protruding towards adjacent central guide electrode units; each central guide electrode unit includes a symmetrically arranged second arcuate boundary, and the second arcuate boundary includes multiple concave crescent-shaped boundaries 031 arranged longitudinally, with a necking structure 032 formed between two opposing concave crescent-shaped boundaries 031 of the two second arcuate boundaries. In the longitudinal direction, the multiple concave crescent-shaped boundaries 031 and the multiple arcuate segments are arranged one-to-one, and adjacent first arcuate boundaries and second arcuate boundaries are nested with matching curvature and maintain a basically constant lateral gap to form a non-uniform electric field gradient between adjacent electrodes. The concave crescent-shaped boundary 031 forms a nested arc structure with curvature matching with the first arc boundary on the lateral capture electrode 02, which is used to construct a non-uniform electric field gradient at the electrode gap; the necking structure 032 is used to form a local geometric constraint region after the droplets coalesce, thereby inducing the droplet trailing edge contact line to detach.

[0030] In one embodiment, the transparent support substrate 01 can be made of glass, quartz glass, or a transparent polymer material, used to support the structure of each functional layer and ensure the overall light transmittance of the device.

[0031] In one embodiment, both the dielectric layer 04 and the hydrophobic layer 05 are obtained by spin coating.

[0032] The lateral capture electrode 02 and the central guide electrode 03 are a pair of coplanar electrodes, both distributed on the transparent support substrate 01. (Refer to...) Figure 3 Scanning electron microscope (SEM) images of the electrodes are presented to characterize the actual fabrication morphology of the electrode structure in the embodiments of the present invention. Figure 3 As can be seen, a nested arc structure with curvature matching characteristics is formed between the lateral capture electrode 02 and the central guiding electrode 03. This nested arc structure can ensure the continuous distribution of the electric field while facilitating the detachment and migration of droplets after coalescence. Scanning electron microscopy results show that the electrode boundaries in this embodiment are clear and the structure is complete, which can meet the requirements of droplet driving for electrode accuracy and uniformity.

[0033] Reference Figure 4A cross-sectional scanning electron microscope (SEM) image of an electrowetting, agglomerated droplet anti-sticking surface is presented to characterize the actual stacking structure of each functional layer in the embodiments of the present invention. Figure 4 As can be seen, a dielectric layer 04 and a hydrophobic layer 05 are uniformly and sequentially covered on the transparent support substrate 01, with continuous interfaces and tight bonding between the layers. The dielectric layer 04 effectively isolates the direct conduction between the droplet and the electrode, while ensuring the transmission of the electric field required for droplet actuation; the hydrophobic layer 05 reduces the adhesion between the droplet and the surface, thereby improving droplet migration and desorption efficiency. Cross-sectional scanning electron microscopy results show that the functional interlayer structure prepared in this embodiment is stable and meets the application requirements of the device.

[0034] Reference Figure 5 A cross-sectional layered structure diagram of an electrowetting and droplet-preventing surface is shown, including a transparent support substrate 01, a lateral trapping electrode 02, a central guiding electrode 03, a dielectric layer 04, and a hydrophobic layer 05. When energized, droplets aggregate and move in the hydrophobic layer 05.

[0035] In one embodiment, the lateral capture electrode unit and the central guide electrode unit are disposed on the same plane and are arranged alternately in the transverse direction to form an electrode composite array. The electrode unit has a structure with continuously varying width along a preset direction, and the electrode is an irregular electrode. The width variation of the electrode is monotonically or periodically varied, and the width variation of the electrode is formed by the arc boundary. Unlike the existing equal-width strip electrode, in the embodiment of the present invention, the electrode unit has a non-constant electrode width along its extension direction, that is, the width of the electrode unit varies along the length direction, and different width segments are alternately or continuously distributed along the extension direction of the electrode unit.

[0036] In one embodiment, the lateral trapping electrode 02 and the center guiding electrode 03 are made of indium tin oxide (ITO), with an electrode width ranging from 100 μm to 800 μm, a central angle between the first and second arc boundaries of 90° to 160°, a lateral gap between adjacent electrodes of 20 μm to 80 μm, and an electrode thickness of 200 nm to 800 nm; the dielectric layer 04 is made of SU-8 polymer photoresist with a thickness of 1 μm to 10 μm and a dielectric constant greater than 2.5; the hydrophobic layer 05 is made of fluorinated polymer Cytop with a thickness of 0.5 μm to 2 μm and an initial contact angle ≥ 110°.

[0037] The working process and principle of the non-stick surface provided in this embodiment of the invention: When tiny, discrete droplets adhere to the anti-stick surface due to environmental condensation, the lateral trapping electrode 02 and the central guiding electrode 03 are located on the surface of the transparent support substrate 01. The dielectric layer 04 and the hydrophobic layer 05 sequentially cover the structural electrodes, isolating the droplets from the structural electrodes through the dielectric layer 04 and the hydrophobic layer 05. An AC driving voltage is applied between the lateral trapping electrode 02 and the central guiding electrode 03. Due to the unique arc-shaped nested structure of the electrode composite array, [refer to...] Figure 6 A strong electric field gradient with matching curvature, pointing towards the central recessed region, is generated at the transverse gap. This gradient field exerts an effective electrowetting force on the droplet, driving the discrete micro-droplets on both sides to rapidly migrate, aggregate, and coalesce towards the central region, forming a large droplet located within the crescent-shaped boundary 031 of the central guiding electrode 03. As the volume of the central droplet increases, part of its trailing edge contact line extends into the necking structure 032 region. The sharp narrowing of the necking structure 032 region causes a sharp increase in the curvature of the solid-liquid-gas three-phase contact line of the droplet at this location, generating significant local geometric constraints and causing the trailing contact line to be in an unstable state. Under the continuous driving force of the electric field and the internal pressure of the droplet, the trailing edge of the droplet becomes unstable at the neck and is forcibly detached. This process greatly releases the trailing edge anchoring force that hinders the movement of the droplet. Subsequently, under the combined action of the overall electrowetting driving force and the release of surface energy, the droplet gains sufficient kinetic energy and rapidly accelerates forward (towards the next large electrode recessed region), sliding away from its original position and eventually being cleared from the surface.

[0038] Under the combined action of periodic AC voltage and gradually widened electrodes, the electric field changes over time, preventing droplets from remaining in a single location for extended periods. This promotes continuous droplet movement and aggregation, thereby enhancing the overall stability of the anti-sticking effect. Furthermore, the required voltage is low, ranging from 100V to 300V, and the current is only tens of microamps, offering advantages such as low power consumption and high response speed. Operation is also simple; when droplets adhere to the anti-sticking surface, refer to... Figure 8 By simply applying an AC voltage, droplets can be coalesced to prevent sticking, increasing the dry area rate from 56% to 95%.

[0039] In this invention, driven by an AC signal, the electrode structure utilizes curvature matching to achieve efficient capture and coalescence of microdroplets. Furthermore, through the modulation of the energy barrier caused by necking, it counteracts the anchoring force at the droplet's trailing edge, enabling rapid, non-adhesive sliding of large droplets. This invention solves the motion hysteresis problem of traditional strip or rectangular gradient electrodes after droplet coalescence, significantly improving the anti-sticking effect on transparent substrate surfaces in complex humidity environments.

[0040] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A surface for preventing electrowetting and coalescing droplets from sticking, characterized in that, It includes a transparent support substrate (01), an electrode composite array, a transparent dielectric layer (04), and a hydrophobic layer (05) arranged sequentially. The electrode composite array includes alternating lateral trapping electrodes (02) and central guiding electrodes (03). The boundaries of adjacent lateral trapping electrodes (02) and central guiding electrodes (03) are nested with curvature matching and maintain a basically constant lateral gap to form a non-uniform electric field gradient between adjacent electrodes. Furthermore, a necking structure (032) is formed in the central guiding electrode (03). The curvature matching nested structure and the necking structure (032) work together to induce instability and detachment of the three-phase contact line at the trailing edge of the droplet during the droplet coalescence process, thereby achieving continuous directional migration and detachment of the droplet after coalescence.

2. The electrowetting and anti-sticking surface for coalescing droplets according to claim 1, characterized in that, The lateral capture electrode (02) includes multiple lateral capture electrode units arranged laterally, and the central guide electrode (03) includes multiple central guide electrode units arranged laterally. The lateral capture electrode units and the central guide electrode units are alternately arranged. Each lateral capture electrode unit includes two symmetrically arranged first arc boundaries, and the first arc boundaries include multiple arc segments arranged longitudinally that protrude towards the adjacent central guide electrode units. Each central guide electrode unit includes symmetrically arranged second arc boundaries, and the second arc boundaries include multiple concave crescent-shaped boundaries (031) arranged longitudinally. A necking structure (032) is formed between two opposite concave crescent-shaped boundaries (031) of the two second arc boundaries. The adjacent first arc boundaries and second arc boundaries are nested in a curvature-matched arrangement.

3. The electrowetting and anti-sticking surface for coalescing droplets according to claim 2, characterized in that, The central angle between the first and second circular arc boundaries is 90°-160°, and the lateral gap is 20μm-80μm, so as to form an electric field gradient that drives the droplet migration.

4. The electrowetting and anti-sticking surface for coalescing droplets according to claim 2, characterized in that, The horizontal width of the necking structure (032) is 100μm-300μm.

5. The electrowetting and anti-sticking surface for coalescing droplets according to claim 1, characterized in that, The widths of the central guiding electrode (03) and the lateral capturing electrode (02) vary non-uniformly along the longitudinal direction, ranging from 100μm to 800μm.

6. The electrowetting and anti-sticking surface for coalescing droplets according to claim 1, characterized in that, The lateral capture electrode (02) and the center guide electrode (03) are transparent conductive films.

7. The electrowetting and anti-sticking surface for coalescing droplets according to any one of claims 1-6, characterized in that, The electrode composite array forms a time-varying electric field under AC voltage driving to suppress droplet long-term residence in local areas and promote continuous droplet migration.

8. A manufacturing process for the electrowetting and anti-sticking surface for coalescing droplets as described in any one of claims 1-7, characterized in that, Includes the following steps: An electrode composite array is formed on the surface of a transparent support substrate (01); A dielectric layer (04) is formed on the electrode composite array. A hydrophobic layer (05) is formed on the dielectric layer (04); The electrode composite array includes alternating lateral capture electrodes (02) and central guide electrodes (03), with the boundaries of adjacent lateral capture electrodes (02) and central guide electrodes (03) nested in a curvature-matched manner and maintaining a substantially constant lateral gap; and a necking structure (032) is formed in the central guide electrode (03).

9. The manufacturing process according to claim 8, characterized in that, The electrode composite array is formed by femtosecond laser processing or photolithography, wherein the femtosecond laser frequency is 180kHz-250kHz and the output power is 1.2W-3.8W.

10. The manufacturing process according to claim 8, characterized in that, The dielectric layer (04) and the hydrophobic layer (05) are formed by spin coating at a speed of 1000rpm-5000rpm and a time of 30s-60s, and are then dried at 80℃-220℃ for 50min-180min.

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