A method for droplet transport based on microstructured surfaces

Abstract

The application provides a droplet transport method based on a microstructure surface; a plurality of nested triangle microstructure units composed of isosceles right triangles are prepared on a substrate surface, and a low surface energy functional layer is coated on the whole domain of the triangular microgrooves and the triangular ridges to obtain a directional hydrophobic microstructure surface; after a droplet to be transported collides, the droplet spreads to form a liquid film; the nested triangle microstructure units cause the first contact line area and the second contact line area to have differential pinning resistance, the droplet undergoes asynchronous retraction, generates a net tangential momentum along a preset transport direction, and directional movement of the droplet can be realized without the driving of external fields such as electric fields, magnetic fields and temperature gradients. The application relies on a multilayer nested structure to amplify the retraction phase difference, and cooperates with a low-adhesion hydrophobic layer in the whole domain to solve the problems of droplet splashing, retention and uncontrollable movement direction under high-speed impact conditions, and the process is suitable for aviation metal substrates and has strong engineering practicability.

Description

Technical Field

[0001] This invention belongs to the fields of functional surface manufacturing and interface fluid control technology, and is particularly applicable to scenarios such as pre-drainage of condensate on aircraft surfaces, anti-icing, self-cleaning of heat exchange equipment, spray cooling, and microfluidic directional transport. Specifically, it relates to a droplet directional transport method without external field drive. Background Technology

[0002] In the high-altitude, low-temperature environment of aircraft, raindrops and condensate are easily trapped and iced upon impact with the skin. Current de-icing methods mostly rely on electric heating or active hot gas heating, which consumes a lot of energy. If impacting droplets could be autonomously and directionally discharged, ice formation could be suppressed at its source. Current droplet-directed transport technologies fall into two categories:

[0003] (1) External field driving scheme: Electric field, magnetic field, temperature gradient and mechanical vibration drive the movement of droplets. It requires a power supply, heating and vibration mechanism. The equipment is complex and energy consumption is high, which is difficult to adapt to the aviation lightweight scenario.

[0004] (2) Pure microstructure passive scheme: single-layer oblique groove, ratchet-type asymmetric texture, relying only on a single geometric asymmetry to generate weak lateral force. When high-speed droplets collide, liquid film breakage and satellite droplet splashing are likely to occur, resulting in a large amount of kinetic energy dissipation; moreover, the adhesion force between the groove and the substrate interface is large, and the droplet retraction amplitude is insufficient, so only local displacement can occur, and long-distance overall directional movement cannot be achieved.

[0005] Existing technologies only design microstructures or perform hydrophobic modifications separately, without synergistic coupling between the two, and without utilizing multi-layered nested triangular structures to amplify the droplet retraction phase difference. As a result, the industry pain point of droplet retention and uncontrollable motion direction under high-speed impact conditions has remained unresolved for a long time.

[0006] The existing single triangular groove microstructures are only arranged in a single layer, which cannot amplify the phase difference; at the same time, the existing hydrophobic microstructures are only simple coatings and are not coupled with multi-layer nested triangular geometry, which cannot achieve stable directional transport without external field under high-speed droplet impact. Summary of the Invention

[0007] This invention overcomes the shortcomings of existing droplet transport methods, such as reliance on external drive, easy breakage and stagnation under high-speed impact, and poor directional movement. It provides a droplet transport method based on a nested triangular microstructure surface, which relies solely on the energy of droplet impact and capillary retraction, without the need for external fields such as electric fields, magnetic fields, and temperature gradients. This method achieves stable and controllable directional transport of droplets and is suitable for engineering scenarios such as aviation anti-icing and condensate drainage.

[0008] This invention is achieved through the following technical solution:

[0009] This invention relates to a droplet transport method based on a microstructured surface, comprising substrate pretreatment, microgroove construction, low-energy surface modification, and droplet impaction processes. It achieves directional droplet transport without external field by relying on the differentiated contact line resistance of a multi-layered nested triangular microstructure. The specific steps are as follows:

[0010] Step (a), Substrate pretreatment: Provide substrate 1, clean and dry the surface of substrate 1 to be processed to obtain a clean processing surface;

[0011] Step (b), directional microstructure construction: at least one set of nested triangular microstructure units 2 are prepared on the surface of substrate 1 using a femtosecond laser; wherein, the nested triangular microstructure unit 2 includes multiple layers of unidirectional nested isosceles right-angled triangular microgrooves 3, and triangular ridges 4 are formed between adjacent triangular microgrooves 3;

[0012] Step (c), Surface low-energy treatment: The triangular microgrooves 3 and triangular ridges 4 are modified in their entirety by using a low surface energy modifier to form a low surface energy functional layer 5, thereby obtaining a directional hydrophobic microstructure surface 6.

[0013] Step (d), droplet impact: control the droplet 7 to be transported to impact the center of the nested triangular microstructure unit 2 with a preset Weber number, and the droplet inertial spreads to form a liquid film;

[0014] Step (e), directional transport: the nested triangular microstructure unit 2 generates differential pinning resistance on the three-phase contact lines on both sides of the droplet, causing the droplet to retract asynchronously and generate net tangential momentum along the preset direction of the triangle, driving the overall displacement of the droplet.

[0015] The multi-layer isosceles right-angled triangular microgrooves 3 and the global low surface energy functional layer 5 work together to generate net tangential transport momentum, and neither can be omitted.

[0016] Preferably, in step (b), the interior angles of the triangular microgrooves 3 are 45°, 45°, and 90°, respectively; the spacing between adjacent groove features is 200μm to 800μm; and the top width of the triangular ridge 4 is 30μm to 200μm.

[0017] Preferably, in step (b), the parameters of the femtosecond laser are: pulse width of 100 fs to 500 fs, repetition frequency of 100 kHz to 1000 kHz, average power of 5 W to 30 W, and scanning speed of 5 mm / s to 100 mm / s; the substrate 1 is 7075 aluminum alloy, titanium alloy, stainless steel, or nickel-based alloy.

[0018] Preferably, in step (c), the low surface energy modifier is prepared from 99 parts of anhydrous ethanol and 1 part of silane; wherein the silane is FAS-17 perfluorosilane; the surface low energy treatment specifically involves immersing the substrate 1 with nested triangular microstructure units 2 on the surface at a temperature of 50-70°C for 8-24 h, and then heating and curing it at a temperature of 100-150°C for 1-4 h.

[0019] Preferably, in step (d), the droplet 7 to be transported is a water droplet with a volume of 3μL to 8μL and an impact Weber number of 30 to 110; the impact point is located at the geometric center of the nested triangular microstructure unit 2.

[0020] Preferably, in step (c), the static water contact angle of the directional hydrophobic microstructure surface 6 is 110° to 165°; in step (d), the ambient temperature during the droplet impact and retraction process is controlled at 15°C to 35°C.

[0021] Preferably, the method satisfies the transport conditions by adjusting the groove spacing, ridge width, and droplet impact velocity: the droplets have sufficient retraction amplitude after spreading, there is no large-area liquid film rupture, and the capillary restoring forces on both sides cannot be completely canceled out.

[0022] Preferably, the method uses a high-speed imaging device 9 to record the entire process of droplet motion, with a frame rate of 2000 fps to 20000 fps, and evaluates the transport effect through spreading factor, retraction time, and droplet centroid displacement.

[0023] Preferably, in step (d), the droplet 7 to be transported moves directionally along the projection direction of the angle bisector, right-angle side, or hypotenuse of the nested triangular microstructure unit 2.

[0024] Preferably, this method is applied to scenarios such as aircraft de-icing, self-draining of condensate from heat exchange equipment, self-cleaning of material surfaces, spray cooling, and microfluidic directional delivery.

[0025] The core technology of this invention:

[0026] (1) Multi-layered nested isosceles right-angled triangular microstructure (structural innovation)

[0027] The system employs a multi-layered, isosceles right-angled triangle arrangement nested in the same direction from the outside in. The right-angled and hypotenuse grooves create differentiated pinning resistance on the three-phase contact lines of the droplet. The multi-layered nesting amplifies the timing difference of the unpinning of the contact lines on both sides, stably generating a unidirectional net tangential capillary force. This differs from existing single-layer asymmetric grooves and solves the problem of droplets only shifting locally and not displacing as a whole under high-speed impact.

[0028] (2) Synergistic coupling of micro-geometric trenches and global low surface energy modification (innovative synergistic mechanism)

[0029] A low surface energy functional layer 5 is used to cover the entire triangular microgrooves 3 and triangular ridges 4: the low energy layer reduces solid-liquid adhesion and ensures sufficient droplet retraction amplitude; the nested triangular microstructures provide differentiated pinning and retardation, and both are indispensable. If there is only hydrophobicity without microstructures, the droplet's retraction forces in all directions will cancel each other out and bounce in place; if there is only groove without hydrophobic coating, there will be excessive adhesion and liquid residue. The two work together to achieve purely passive directional transport without external field.

[0030] (3) Impact spreading-asynchronous retraction momentum rectification mechanism (engineering control innovation)

[0031] After the droplet spreads out due to impact inertia, the first contact line region 19 and the second contact line region 20 retract asynchronously, converting the surface tension into directional translational momentum along the preset transport direction 18. A multi-parameter control system for groove spacing, ridge width, droplet Weber number, and surface contact angle is established, and quantitative detection is performed using a high-speed imaging device 9 to solve the problems of droplet breakage, random retention, and uncontrollable transport direction under high-speed impact.

[0032] The present invention has the following advantages:

[0033] (1) No external field drive, eliminating the need for droplet release device 10, temperature control, heating and vibration equipment, lightweight and low energy consumption, suitable for the lightweight anti-icing needs of drones, civil aircraft and other aviation aircraft;

[0034] (2) The multi-layer nested triangular structure amplifies the phase difference of the contact line and can still avoid liquid film splashing and liquid residue under high Weber number and high-speed impact conditions, thus achieving long-distance stable directional drainage and reducing surface condensation from the source.

[0035] (3) The triangular microgrooves 3 and the triangular ridges 4 are fully covered with the low surface energy functional layer 5, which takes into account both the low adhesion retraction capability and the geometric directional blocking effect. A single structure cannot achieve the synergy of the two, resulting in an unexpected directional transport effect.

[0036] (4) Femtosecond lasers can be directly processed on the surface of aerospace metal substrates 1. The process is compatible, the parameters are adjustable, and it is suitable for multiple fields such as condensation heat exchange, self-cleaning, and microfluidics.

[0037] (5) The entire testing system consists of a high-speed imaging device 9, a temperature control platform 11, a sealed test chamber 12, and a dry gas supply device 13. It can standardize and quantify the entire process of droplet spreading, retraction, and displacement, and has strong engineering feasibility.

[0038] (6) This invention couples the three elements of multi-layered nested triangular micro-geometry, global low-adhesion hydrophobic layer, and impact momentum rectification mechanism. No single structure can achieve high-speed stable transport of droplets without external field, resulting in unexpected composite technical advantages. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the overall surface of the nested triangular microstructure;

[0040] Figure 2 This is a schematic diagram of the droplet impact and spreading process based on the microstructure surface;

[0041] Figure 3 This is a schematic diagram of the asynchronous retraction and directional transport process of droplets;

[0042] Figure 4 This is a schematic diagram of the pinning resistance and capillary restoring force distribution of the droplet contact line;

[0043] Figure 5 This is a schematic diagram of a droplet impact and high-speed imaging testing system;

[0044] Figure captions: 1: Substrate; 2: Nested triangular microstructure unit; 3: Triangular microgroove; 4: Triangular ridge; 5: Low surface energy functional layer; 6: Directional hydrophobic microstructure surface; 7: Droplet to be transported; 8: Liquid film; 9: High-speed imaging device; 10: Droplet release device; 11: Temperature control platform; 12: Sealed test chamber; 13: Dry gas supply device; 14: Initial position of droplet; 15: Maximum spread area of ​​droplet; 16: Droplet retraction area; 17: Droplet transport termination position; 18: Preset transport direction; 19: First contact line area; 20: Second contact line area. Detailed Implementation

[0045] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are merely further illustrations of the present invention, but the scope of protection of the present invention is not limited to the following embodiments.

[0046] Example 1

[0047] Standard optimal working condition

[0048] 1. Sample and equipment preparation: Substrate 1 is a 30 mm × 30 mm × 2 mm 7075 aerospace aluminum alloy sheet; the complete testing system is as follows: Figure 5 As shown, it includes a high-speed imaging device 9, a droplet release device 10, a temperature control platform 11, a sealed test chamber 12, and a dry gas supply device 13.

[0049] 2. Substrate pretreatment: Substrate 1 is placed in anhydrous ethanol and deionized water for ultrasonic cleaning for 10 min each, and then dried with drying gas to remove surface oil and oxide debris.

[0050] 3. Fabrication of nested triangular microstructure unit 2: Referring to Figure 1, a total of 5 layers of nested isosceles right-angled triangles in the same direction are processed on the surface of substrate 1 using a femtosecond laser; the spacing between adjacent triangular microgrooves 3 is 450 μm, and the top width of the triangular ridge 4 is 100 μm; laser parameters: pulse width 200 fs, repetition frequency 500 kHz, average power 12W, scanning speed 30 mm / s; after processing, the molten residue in the grooves is removed again by ultrasonic cleaning.

[0051] 4. Global Low Surface Energy Modification: FAS-17 and anhydrous ethanol were mixed at a volume ratio of 1:99 and stirred for 1 hour to prepare a modification solution; the substrate 1 was completely immersed and soaked in a 60°C water bath for 12 hours; after removal, it was cured in a 120°C oven for 2 hours, and a low surface energy functional layer 5 was uniformly coated on the surface of the triangular microgrooves 3 and triangular ridges 4 to obtain the substrate. Figure 1 The directional hydrophobic microstructure surface 6 shown has a static water contact angle of 138°.

[0052] 5. Closed environment test: The sample is fixed on the temperature control platform 11, the ambient temperature inside the closed test chamber 12 is 25℃, and the dry gas supply device 13 introduces dry air to control humidity; the droplet release device 10 generates 5μL droplets 7 to be transported, which are vertically impacted from the initial position 14 of the droplet to the geometric center of the nested triangular microstructure unit 2, matching the impact Weber number 42; the high-speed imaging device 9 acquires the entire process at 8000 fps.

[0053] 6. Experimental phenomena: During the impact spreading stage, as shown in Figure 2, the droplet 7 to be transported spreads inertia to form a liquid film 8, covering the maximum spreading area 15 of the droplet; the retraction sequence is shown in Figure 3, where the liquid film 8 forms the droplet retraction area 16; the force mechanism is shown in Figure 4, where the first contact line area 19 has high pinning resistance and delayed retraction, while the second contact line area 20 contracts rapidly, generating a net tangential force along the preset transport direction 18; the droplet moves smoothly from the initial position 14 to the droplet transport termination position 17 without splashing or leaving grooves, forming stable directional transport.

[0054] 7. Effect Description: This embodiment also uses the attached... Figure 1 The diagram shows a multi-layered nested triangular geometry combined with a globally low surface energy coating. Figure 4 The differentiated contact line resistance mechanism and the synergistic effect of triple innovation make it impossible for a single structure to achieve this stable transport effect.

[0055] Example 2

[0056] This embodiment involves a high Weber number high-altitude raindrop simulation condition.

[0057] Substrate 1 Material, Attachment Figure 1The microstructure dimensions, laser processing parameters, and low surface modification process are shown. Figure 5 The entire testing system configuration is exactly the same as in Example 1; only the height of the droplet release device 10 is adjusted to increase the impact speed, so that the impact Weber number of the droplet 7 to be transported reaches 89.

[0058] Test process: The droplet impact and spreading pattern is shown in Figure 2. The liquid film 8 spreads over a larger area and becomes thinner, with a small amount of micro-liquid filaments splashing at the edge. The retraction process is shown in Figure 3. The multi-layer triangular micro-grooves 3 maintain the retraction phase difference between the first contact line region 19 and the second contact line region 20 in Figure 4, and still generate net tangential momentum along the preset transport direction 18. The main body of the transported droplet 7 moves completely from the initial position 14 to the droplet transport termination position 17.

[0059] Effect description: (attached) Figure 1 The multi-layered nested triangular structure can adapt to high-altitude, high-speed raindrop impact conditions, meeting the real anti-icing and de-icing requirements of aircraft.

[0060] Comparative Example 1

[0061] Comparative Example 1 involves a coating with no microstructure and only low surface energy throughout the entire surface area;

[0062] Base material 1 uses the same 7075 aluminum alloy, without any processing or attachments. Figure 1 The nested triangular microstructure unit 2 shown is cleaned, silane modified, and so on. Figure 5 The testing system and droplet parameters are completely consistent with those in Example 1.

[0063] Test phenomenon: Droplet 7 to be transported spreads and forms Figure 2 After liquid film 8, there is no differential hindrance caused by triangular grooves; Figure 4 The first contact line region 19 and the second contact line region 20 retract synchronously, and the capillary restoring forces cancel each other out; the droplet only retracts in place or bounces vertically at its initial position 14, without... Figure 3 The directional displacement shown is toward the droplet transport termination position 17.

[0064] Conclusion: Appendix Figure 1 The multi-layered, unidirectional nested isosceles right-angled triangular microstructure unit 2 is an essential core structure for realizing directional transport.

[0065] Comparative Example 2

[0066] Comparative Example 2 involves a single-layer triangular trench with full-area drainage;

[0067] A single layer of isosceles right-angled triangular microgrooves 3 are machined only on the surface of substrate 1, without any attachments. Figure 1 Multi-layer nested structure; low surface modification; Figure 5 The testing system and droplet operating conditions are the same as in Example 1.

[0068] Test phenomena: When the liquid film 8 retracts, the phase difference between the first contact line region 19 and the second contact line region 20 in Figure 4 is extremely small, and the lateral force is weak; the droplet 7 to be transported only undergoes a slight local shift, and most of the liquid remains at the initial position 14 of the droplet, making it impossible to complete the transport. Figure 3 The example shown is long-distance directional transport.

[0069] Conclusion: The multi-layered, unidirectional nested design in Figure 1 amplifies the resistance difference between the contact lines on both sides, which is a key innovation to ensure stable transport. With the same 5μL droplet impact and a Weber number of 42, the droplet displacement of this invention is ≥3.2mm; in Comparative Example 1, there is no horizontal displacement, only vertical rebound.

[0070] Comparative Example 3

[0071] Comparative Example 3 involves a multi-layered nested triangular microstructure without a low surface energy functional layer;

[0072] Substrate 1 Processing Figure 1 Complete multi-layer nested triangular microstructure unit 2, omitting the silane low surface modification process, the substrate 1 retains the original metal surface; Figure 5 The test system and droplet parameters remain unchanged.

[0073] Test phenomena: The solid-liquid interface adhesion is extremely strong, the liquid film 8 shrinkage is minimal, and a large amount of liquid remains inside the triangular microgroove 3, with no... Figure 3 The overall displacement to the droplet transport termination position 17 is shown.

[0074] Conclusion: Appendix Figure 1 The triangular microgrooves 3 and the triangular ridges 4, which fully cover the low surface energy functional layer 5, are necessary synergistic features. Only by working together can sufficient retraction and directional transport be achieved.

[0075] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A droplet transport method based on a microstructured surface, comprising substrate pretreatment, microgroove construction, low-energy surface modification, and droplet impaction steps, characterized in that, The directional transport of droplets without external field is achieved by relying on the differentiated contact line resistance of a multi-layered nested triangular microstructure. The specific steps are as follows: Step (a), Substrate pretreatment: Provide substrate (1), clean and dry the surface of substrate (1) to be processed to obtain a clean processing surface; Step (b), directional microstructure construction: at least one set of nested triangular microstructure units (2) are prepared on the surface of the substrate (1) using a femtosecond laser; wherein, the nested triangular microstructure unit (2) includes multiple layers of isosceles right-angled triangular microgrooves (3) nested in the same direction, and a triangular ridge (4) is formed between adjacent triangular microgrooves (3). Step (c), Surface low-energy treatment: The triangular micro-groove (3) and triangular ridge (4) are modified in their entirety by using a low surface energy modifier to form a low surface energy functional layer (5) and obtain a directional hydrophobic microstructure surface (6). Step (d), droplet impact: control the droplet to be transported (7) to impact the center of the nested triangular microstructure unit (2) with a preset Weber number, and the droplet inertial spreads to form a liquid film; Step (e), directional transport: the nested triangular microstructure unit (2) generates differential pinning resistance on the three-phase contact line on both sides of the droplet, causing the droplet to retract asynchronously and generate net tangential momentum along the preset direction of the triangle, driving the overall displacement of the droplet.

2. The droplet transport method based on microstructured surfaces as described in claim 1, characterized in that, In step (b), the interior angles of the triangular microgrooves (3) are 45°, 45°, and 90°, respectively; the spacing between adjacent groove features is 200μm to 800μm; and the top width of the triangular ridge (4) is 30μm to 200μm.

3. The droplet transport method based on microstructured surfaces as described in claim 1, characterized in that, In step (b), the parameters of the femtosecond laser are: pulse width 100 fs to 500 fs, repetition frequency 100 kHz to 1000 kHz, average power 5 W to 30 W, and scanning speed 5 mm / s to 100 mm / s; the substrate (1) is 7075 aluminum alloy, titanium alloy, stainless steel or nickel-based alloy.

4. The droplet transport method based on microstructured surfaces as described in claim 1, characterized in that, In step (c), the low surface energy modifier is prepared from 99 parts anhydrous ethanol and 1 part silane; wherein the silane is FAS-17 perfluorosilane; the surface low energy treatment is specifically as follows: the substrate (1) with nested triangular microstructure units (2) on the surface is immersed at 50-70°C for 8-24 h and then heated at 100-150°C for 1-4 h to cure.

5. The droplet transport method based on microstructured surfaces as described in claim 1, characterized in that, In step (d), the droplet to be transported (7) is a water droplet with a volume of 3μL to 8μL and an impact Weber number of 30 to 110; the impact point is located at the geometric center of the nested triangular microstructure unit (2).

6. The droplet transport method based on microstructured surfaces as described in claim 1, characterized in that, In step (c), the static water contact angle of the directional hydrophobic microstructure surface (6) is 110° to 165°; in step (d), the ambient temperature during the droplet impact and retraction process is controlled at 15°C to 35°C.

7. The droplet transport method based on microstructured surfaces as described in claim 1, characterized in that, By adjusting the spacing of the triangular microgrooves (3), the width of the triangular ridge (4), and the droplet impact speed, the transport conditions are met: the droplet has sufficient retraction amplitude after spreading, there is no large-area liquid film rupture, and the capillary recovery forces on both sides cannot be completely offset.

8. The droplet transport method based on microstructured surfaces as described in claim 1, characterized in that, A high-speed imaging device (9) was used to record the entire process of droplet motion, with a frame rate of 2000 fps to 20000 fps. The transport effect was evaluated by the spreading factor, retraction time, and droplet centroid displacement.

9. The droplet transport method based on microstructured surfaces as described in claim 1, characterized in that, In step (d), the droplet (7) to be transported moves directionally along the angle bisector, right-angle side or hypotenuse projection direction of the nested triangular microstructure unit (2).

10. The droplet transport method based on microstructured surfaces as described in claim 1, characterized in that, It is applied to scenarios such as aircraft de-icing, condensate self-drainage in heat exchange equipment, material surface self-cleaning, spray cooling, and microfluidic directional delivery.

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