Liquid Superspreading on Surface with Microhexagonal Structure Inspired by Rock‐Climbing Fish

The dynamic spreading mechanism of liquid on a specific surface is vital for understanding interface wetting and antifouling. Whereas, how to control the spreading process and accelerate the spreading speed is a major challenge. The rock‐climbing fish is characterized by its alepidote feature that lives in stream habitats dominated by strong currents. The mucus on its body surface plays a vital role in its adherence and maintenance of antifouling and antibacterial properties. However, the rapid, uniform, and efficient spreading mechanism of mucus on the fish body surface remains largely unknown. Herein, it is revealed that the surface of the rock‐climbing fish is overlaid fully by the microhexagonal texture structure. This hexagonal structure shows a superspreading effect on liquid diffusion, resulting from testing with bionic microfabrication inspired by the rock‐climbing fish. It is demonstrated that the microhexagonal‐textured surface can enhance liquid spreading quickly and evenly on the surface by regulating the moving contact line of the liquid. This kind of superspreading mechanism has great potential applications in the antifouling, electroencephalogram electrode interfaces, flexible skin sensors, and interfacial lubrication of underwater surfaces.


Introduction
There are various functional surfaces in nature due to mechanical demands or adaptive selection of biological evolution.These surfaces usually exist as two-phase interfaces, such as solidliquid or solid-gas interfaces, with their structures affecting The dynamic spreading mechanism of liquid on a specific surface is vital for understanding interface wetting and antifouling.Whereas, how to control the spreading process and accelerate the spreading speed is a major challenge.The rock-climbing fish is characterized by its alepidote feature that lives in stream habitats dominated by strong currents.The mucus on its body surface plays a vital role in its adherence and maintenance of antifouling and antibacterial properties.However, the rapid, uniform, and efficient spreading mechanism of mucus on the fish body surface remains largely unknown.Herein, it is revealed that the surface of the rock-climbing fish is overlaid fully by the microhexagonal texture structure.This hexagonal structure shows a superspreading effect on liquid diffusion, resulting from testing with bionic microfabrication inspired by the rock-climbing fish.It is demonstrated that the microhexagonal-textured surface can enhance liquid spreading quickly and evenly on the surface by regulating the moving contact line of the liquid.This kind of superspreading mechanism has great potential applications in the antifouling, electroencephalogram electrode interfaces, flexible skin sensors, and interfacial lubrication of underwater surfaces.
antifouling surfaces, such as creams, cosmetics, and lubricant fluids and completely wetting applications, such as electroencephalogram (EEG) electrode interfaces, and flexible skin sensors for enhancing signal-to-noise ratio by spreading and maintaining conductive liquid. [17,18]he wetting and spreading behavior of liquid on the whole solid surface is controlled mainly by the chemical properties of the surface [19] or the surface morphology. [20,21]Chemical protocols are usually involved in the formation of hydrophilic or hydrophobic coatings on the surface.Therefore, the hydrophilic or hydrophobic gradient of the surface can be modified to control the propagation direction of liquid droplets.However, this coating has poor chemical stability or is prone to mechanical damage.The surface morphology can be adjusted with local geometric changes in the roughness, [22,23] patterned surface, [24] or global regulation, such as in the form of spheres, [25] or cylinders/ fibers. [26]In those global morphology regulations, the fiber structure has been studied in more detail.Nevertheless, fibers are prone to self-adhesion, which affects the overall wetting effect. [27]he microhexagonal texture structure was observed on the surface of the rock-climbing fish, and this patterned structure covered the entire surface.The wettability of the whole surface and the spreading speed of the liquid through the patterned surface will have better stability and practicability.
In this article, the microhexagonal texture structure on the body surface of rock-climbing fish was described.Based on the characteristics of the patterned surface, the silicon substrates were etched with femtosecond laser processing to achieve the patterned groove of an equilateral triangle, equilateral quadrilateral, and equilateral hexagon.After casting and curing Sylgard 184 onto these silicon masters, three texture structure polydimethylsiloxane (PDMS) films were obtained.By comparing the moving contact line (MCL) and spreading speed of droplets on these three patterned surfaces, it was observed that the hexagonal-textured surface could accelerate the spreading of liquid.The mechanism of the hexagonal texture structure accelerating liquid diffusion was analyzed in the following.

Microhexagonal Structure and Function in Rock-Climbing Fish
Under the optical microscope, closely arranged hexagonal structures on the surface of the rock-climbing fish could be observed.To obtain their 3D images, the samples were scanned with atomic force microscopy (AFM), and the resulted microphotographs were shown in Figure 1.There were dense hexagonal units on the skin surface and 10-20 μm lengths of their sides for the units; the heights of these sides were 0.8-1.2μm.The hexagonal unit was consisted internally of narrow elaboration wrinkles, with grooves of 100 nm depth located between the units to form the boundaries.The heights of wrinkle stripes were approximately 180-200 nm, the widths 100-200 nm, and the inter-stripe spaces 700-800 nm.
A similar hexagonal structure was found in the suction disk area of the lumpfish with adhesion capacity.The hexagonal structure was created by epidermal cells, as reported in the literature, to disperse and maintain mucus for adhesion.Therefore, we proposed the following conjecture about the functions of the hexagonal texture structures of the rock-climbing fish: rapidly dispersing the mucus secreted by the mucus gland holes and Figure 1.The microhexagonal structure on the rock-climbing fish and manufactured bionic microhexagonal textures.A) the rock-binding fish; B) the atomic force microscopy (AFM) photograph of the hexagonal texture structure on the body surface of the fish; C) the textures within the hexagonal unit; D) processing and manufacturing method: femtosecond laser etches of the silicon substrate, and then PDMS obtained from reversing the mold; and E) the bionic microhexagon structure.
keeping the mucus on the skin surface longly.Mucus is capable of antifouling and antibacterial functions, which can keep the fish skin clean and then the microscopic surface structures free from environmental pollution and damage.

Bionic Design and Fabrication of the Microstructure
To verify the function of the hexagonal structures and the internal convex textures, we designed three units of microstructure surfaces, as shown in Figure 2. According to the tessellation principle: regular triangles, regular quadrilaterals, and regular hexagons can be used separately for dense translation paving; that is, by copying and translating these three units, the whole surface can be paved.The first unit shape was a triangle and filled with equidistant triangular convex stripes; the second one was a square and filled with equidistant square convex textures; and the third one was an equidistant hexagon and filled with regular hexagon convex configurations with equal space.The circumscribed circle diameter of these three units was set to 100 μm, and their boundaries were edgeshared.The filled inter-fringe space was 6 μm, with 2 μm fringe width and height.The designed patterned structures were etched on the silicon substrate with a femtosecond laser, and through the pouring and mold turning of PDMS, three textured surfaces with a circumference of 100 μm, a stripe spacing of 4-6 μm, a stripe width of 1-2 μm, and a stripe height of 1-2 μm were achieved.As a control, the nonstructured PDMS surface was produced by turning the mold on the substrate without etching grooves.

Spreading Effectiveness of Liquids on a Hexagonal-Textured Surface
To study the spreading phenomenon of liquid droplets on four patterned surfaces, the hydrophobic PDMS obtained by turning over the mold was treated with oxygen plasma to produce superhydrophobic films (Figure 3A-D) with contact angles of 1°-2°at this moment.The liquid droplet of 1 μL was dropped on four patterned surfaces and completely infiltrated the grooves and wrinkles, with the spreading areas of the liquid droplets as 0.52 AE 0.02 mm 2 for the nontextured structures; 0.57 AE 0.03 mm 2 for the triangle ones; 0.56 AE 0.01 mm 2 for the square ones; and 0.96 AE 0.02 mm 2 for the hexagon ones, respectively.The droplet spreading radius within 4 s was recorded: on the nontextured surfaces 161 AE 29 μm; on the triangle ones 316 AE 29 μm; on the square ones 241 AE 20 μm; and on the hexagon ones 450 AE 23 μm, respectively.The fastest spreading speed and biggest spreading area could be observed in the hexagonal structure, for example, 180% higher speed and 85% larger area than those on the nontextured surface (Movies S1-S4, Supporting Information).

MCL of Liquid on Four Surface Patterns
To study the physical mechanisms of spreading enhancement on the hexagonal structure, we placed the PDMS films treated with oxygen plasma in the air for 1 h to increase the contact angle and weaken their hydrophilicity, as shown in Figure 3E-H with the contact angles of 28°-30°at this time.The spreading speed of the droplets on the treated surfaces decreased.The spreading process of the droplets on the surfaces was clearly observed by continuously dropping the droplets on the surfaces at a constant speed.As shown in Figure 3E, when the droplet expanded on the surface without a microstructure, the MCL was close to circular.During the spreading of the droplet, the MCL was pinned at the triangle edges to form a polygon composed of several triangle units (Figure 3F).As shown in Figure 3G for the square surface, the MCL of the droplets was pinned at the borders of the unit to nearly form a square pattern.In Figure 3H, the MCL formed a roughly circular pattern on the hexagonal-textured surfaces.From the previous experiments, it could be observed that during liquid droplet spreading, the MCLs were always nailed along the straight line formed by the pattern.The intersection of the triangle and square elements formed long straight lines, while the intersection of the hexagonal elements always created an included angle of 120°.The long straight line has more resistance to liquid expansion and is less conducive to the spread of liquid droplets.

Mechanisms of Liquid Spreading on a
Hexagonal-Textured Surface

Texture Structure for Liquid Spreading
To analyze the physical mechanism of droplet spreading on these patterned surfaces, the explanatory schematic diagrams were established (Figure 4).The droplet used in the experiment was %1 μL.According to the droplet sizes, we assumed that the influence of linear tension could be ignored. [28,29]We also assumed that the diameter of a single cavity was significantly smaller than the capillary length so that the surface tension, rather than gravity, dominated the static and dynamic characteristics of the system.In addition, according to the geometric sizes in this experiment, the liquid was completely filled in the texture cavities; therefore, in this situation, the interface wetting state was the Wenzel state.
The contact angle of a droplet on a solid surface in the air was described by Young γ lg cosθ 0 ¼ γ gs À γ ls (1)   where γ lg , γ gs , and γ ls are the surface tensions of the liquid-gas, gas-solid, and liquid-solid interfaces, respectively.θ 0 is the contact angle of the droplet.Before the droplet spread, as shown in Figure 4a, the total surface/interface energy of the system was E init where A c,0 is the droplet liquid-gas interface area and A lsðmaxÞ is the real area of the textured surface.Liquid spreading could be seen as a process of thermodynamic equilibrium.When the liquid-spreading process stopped, the system reached a thermodynamic steady state.In the final state (Figure 4b), the droplet contacted the textured surface at an angle of θ.Then, the system energy was defined as E fin The energy change was defined as ΔE, and the energy reduced in the process was equal to that required for liquid spreading  and D) on the regular hexagonal-textured surface, respectively; the shape of the droplet during spreading E) on the nontextured surface close to circular, F) on the regular triangular-textured surface constricted to the shape with the triangle edge as the contact line, G) on the square-textured surface limited to the quasi-square shape with the straight edge as the contact line, and H) on the regular hexagonal-textured surface similar to the circle.
As Figure 4c shows, in this experiment, the liquid was completely filled in the texture cavities.By Cassie's equation, [30] the Cassie's angle θ satisfies the following where f a and f b are the area factors of surface A and surface B; cosθ a and cosθ b represent the percentage of the raised edges and grooves in the total surface area, respectively, as shown in Figure 4D-F.Due to its complete wetting when the droplet contacted the textured surface, it could be considered that the droplet spread on two surfaces with different contact angles: liquid surface A above the groove with a contact angle θ a and convex solid surface B with a contact angle θ b , θ a ¼ 0 and θ b equal to the solid-phase contact angle θ 0 .θ 0 indicates the inherent contact angle of this material on a flat surface.In our experiments with f a < 1 and f b < 1, in the final state, the contact angle θ < θ 0 was achieved, and the wettability of the textured surface was enhanced.

Mechanism of the Hexagonal Unit for Liquid Spreading
The topological structure of the internal angle can lead to significant changes in the wettability of the surface. [31]As early as in the 18th century, the mathematician Brook Taylor (1712) proposed the famous "Taylor conjecture."That is, the liquid surface at the internal angle is paraboloid when it spreads forward.In 1969, Concus and Finn proposed the Concus-Finn condition by the mathematical method: for an open angle of 2α, the inner corner of θ 0 > π 2 À α, the droplet can only partially wet the inner corner area; when θ 0 < π 2 À α, the droplet can completely wet the inner corner area. [32]The Navier-Stokes equation can be used to describe the flow behavior of the liquid at the inner corner.
Due to the effect of the van der Waals force, the droplets were wetted along the internal angle bisector toward the internal angle area when placing the water droplet at the hydrophilic inner corner.Then, driven by the separation pressure, the droplets spread on the substrate.At 2α < 135°, the precursor chain moved forward along the inner corner, while the precursor film moved forward on the wedge surface and completely wet the inner corner area.At 2α > 135°, the precursor film could partially wet the inner corner area.
As Figure 4G shows, during liquid droplet spreading, the MCL will always be nailed along the straight line formed by the pattern, and the intersection of the triangle and square elements will form a long straight line (with no inner corner), while the intersection of the hexagon elements will always have an inner angle of 120°(< 135°).

Conclusion
The rapid spread of mucus over the whole-body surface is crucial for rock-climbing fishes in adaptive survival in environments with strong water flows.We identified the microhexagonaltextured structures on the fish's back surface using AFM.According to the tessellation principle, we designed triangular, square, and hexagonal textures.The corresponding groove units were obtained by etching silicon substrates with femtosecond laser processing.After casting and curing Sylgard 184 onto these silicon masters, three texture structure PDMS films were created.The spreading speed and the spreading area of the droplets on three texture surfaces and one unstructured surface showed that the hexagonal texture structure could enhance the spreading speed of the liquid and facilitate droplets of the same volume spreading over a larger area.The spreading process of the MCL at the inner corner revealed that the hexagonal structure was more conducive to liquid spreading on the surface.
Since it occurs at the three-phase interface, the spreading phenomenon of the liquid at the three-phase interface of solid, liquid, and gas was investigated.It should be noted that the underwater spreading of mucus is affected by the viscosity, surface tension, surface energy, and microstructure scale, which should be studied in the future.In addition, the microtextured surface might be capable of maintaining mucus.Since the water droplets evaporate rapidly in the air, this retention phenomenon was not obvious in our study.The maintenance phenomenon of mucus underwater will be a very interesting issue, and remained being studied in future works.

Experimental Section
Animals: All experiments and surgical procedures were approved by the Animal Use and Care Committee at Chengdu Institute of Biology, Chinese Academy of Sciences, which complies with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (AUCC number: CIBDWLL2022032).All efforts were made to minimize the number of animals used and their suffering.We collected five rock-climbing fishes (B.kweichowensis) in the field, ranging from 4 to 6 cm in body length, and raised them in a 60 Â 100 Â 50 cm fish tank.The fish tank was equipped with a filter device, an air pump, and a thermostat.The temperature was maintained at 22 AE 1 °C, and the fish were fed every day with multiple algae wafers.
Characterization of Rock-Climbing Fish Skin: The fish were immersed in 0.5 g L À1 MS-222 (Sigma Chemical, USA) for 15 min.The fish body was divided into small pieces with uniform thickness, washed with phosphate buffer solution (Sigma Chemical, USA) for 10 min, and then fixed with 2.5% glutaraldehyde (EM Grade) (Solarbio Co. Ltd. (Beijing)) for 24 h.To reduce the damage of glutaraldehyde to the enzyme activity of tissue cells, fixation was carried out at 4 °C.All utensils, instruments, and glutaraldehyde fixatives were precooled.After fixation, some of these samples were scanned with an AFM (Dimension Icon, Bruker Corporation, USA), and an analyst in fluid mode was used.
Fabrication of the Texture Surfaces: Through femtosecond laser etching technology, silicon substrate molds with triangular-, square-, and hexagonaltextured grooves were obtained.The processed silicon mold was ultrasonicated in absolute alcohol for 1 h, and then the silicon surface was silanized with dimethylchlorosilane for 2 h.Sylgard 184 (Dow Corning, USA) prepolymer and crosslinker at a ratio of 10:1 were mixed, degassed, and poured onto these silicon molds.After being victimized for 0.5 h, the silicon mold with the mixture was sent to a baking oven for 2 h at a temperature of 80 °C.After natural cooling, the film was removed to obtain triangular-, square-, and hexagonal-textured films.
Characterization of the Texture Surfaces: The sample was placed in a vacuum spraying apparatus, spraying gold till reaching 25 nm thickness, and scanning electron microscopy (SEM; Evo ma10, Zeiss, Inc., Germany) was used for scanning observation.
Hydrophilic Treatment: The PDMS films were placed in the chamber of a plasma cleaner (Diener Femto, Sinti, Inc., America) under vacuum for 5 min, injected with oxygen for 5 min, kept at a pressure gauge reading of 15-20, and then treated with oxygen plasma for 5 min to obtain a super-hydrophilic surface.
Controlling Microdroplets and Spreading Observation: Using the drawing parameters recommended, glass microtubules with an outer diameter of 1 mm and an inner diameter of 0.6 mm were drawn into approximately 10 μm microneedles through a laser drawing instrument (MODEL P-2000, Sutter Instrument Co., USA).The microneedle was connected to the syringe through a hose, and the microinjection pump was used for quantitative injection.Calibrating the flow was conducted before injection.The microneedle was fixed on the 3D microdisplacement platform to control the position of the microneedle.The spreading process of droplets on the textured surface was observed under an optical microscope (TI-2E, Nikon, Inc., Japan).

Figure 2 .
Figure2.Bionic microtextured surface.A) Scanning electron microscope (SEM) photographs of bionic triangular-, square-, and hexagonal-textured surfaces; B) transversal section view of the bionic triangle, square, and hexagon textures with stripe height of approximately 2 and 4-6 μm gap between stripes; C) AFM figures of the bionic triangular-, square-, and hexagonal-textured surfaces, and the morphology information of the connection of the cells: the connections of the triangle and square cells were a straight line, and that of the hexagon cells had an inner angle of 120°.

Figure 3 .
Figure3.The droplet spreading phenomenon: spreading area of 1 μL droplet in the fourth second A) on the nontextured surface, B) on the regular triangular-textured surface, C) on the square-textured surface, and D) on the regular hexagonal-textured surface, respectively; the shape of the droplet during spreading E) on the nontextured surface close to circular, F) on the regular triangular-textured surface constricted to the shape with the triangle edge as the contact line, G) on the square-textured surface limited to the quasi-square shape with the straight edge as the contact line, and H) on the regular hexagonal-textured surface similar to the circle.

Figure 4 .
Figure 4. Schematic diagrams of droplet spreading: surface and interface energy of liquid drop and solid surface A) in an initial state and B) in equilibrium; C) cross-sectional diagram of droplet contact line; D-F) area factor and contact angles of the triangle, square, and hexagon structures, respectively; and G) triangle, square, and hexagon structures and MCLs of liquid drops.