The biological surfaces in nature can transport liquids in a given direction without any external energy or additional setup. Inspired by the natural surface, the liquid can be successfully transported by artificial surfaces. In recent years, liquid directional transportation has received extensive attention and in-depth research, and it is expected to have prospects in the fields of directional water collection, macroscopic liquid transportation, oil-water separation, microfluidic systems, etc. This review systematically introduces the principle of liquid transport, processing methods and potential application of biomimetic surfaces. Additionally, the limitations, challenges and future opportunities of liquid transportation are also scoped in this review.
Contents
1 Introduction
2 The principles of directional liquid transport
2.1 Theory of roughness gradient surface
2.2 Theory of chemical gradient surface
2.3 Theory of hierarchical structure
3 Biological surface for directional transport of liquids
Since life appeared on the earth, organisms have undergone at least 3.5 billion years of evolution. Under the action of natural selection, organisms have evolved high-performance water-collecting surfaces that can collect water from humid air, transport water against gravity, and collect micro-droplets without relying on additional energy[1⇓⇓⇓~5]. For example, the "mouth edge" surface of pitcher plants can drive droplets to "flow up", and the skin of Texas horned lizards can absorb water from wet sand and dew. The phenomenon of directional transport of these droplets is mainly due to the unique micro-nano structure and wettability gradient of the biological surface. Based on the study of natural animal and plant surfaces, researchers have found that the surfaces that can transport water directionally in nature usually have physical roughness gradients, chemical roughness gradients, and micro-scale structures of micron/nanometer.These structures lead to the formation of the surface energy gradient of natural organisms and the Laplace pressure difference on the droplet, and the combined effect makes the force imbalance of the droplet and drives the directional transport of the liquid.
Inspired by the biological surface with high performance directional water collection, researchers have synthesized biomimetic functional material surfaces with physical roughness gradient and chemical roughness gradient structures by traditional techniques such as vapor diffusion, liquid diffusion, surface patterning, laser ablation, plasma treatment and mechanical stretching[6,7][8,9,10][11][12][13][14]. The surface of biomimetic functional materials has the advantages of precision, speed, green, environmental protection, no need to input energy and external equipment, so biomimetic functional materials that can transport liquid directionally have broad application prospects in industry, medical treatment and agriculture[11]. In recent years, the commercialization of lithography and 3D printing has greatly promoted the bionic simulation of microscale/nanoscale structures[15]. Based on this, this paper mainly summarizes the biomimetic functional material surface in recent years, and systematically discusses its application prospects in pipeline liquid transportation, planar directional water collection, oil-water separation, microfluidic systems and other fields[16][17][18,19][20].
2 Principle of directional liquid transport
In the absence of external energy, the directional motion of the droplet is very difficult, because the contact angle hysteresis effect and the static friction force will become the energy barrier to hinder the motion of the droplet when the droplet contacts with the material surface[21,22]. However, organisms such as spider silk, cactus, pitcher plant and desert beetle in nature can realize the directional collection of water droplets through the physical roughness/chemical composition gradient or microscale/nanoscale structure on their surfaces, which provides a new perspective for solving this problem.
The main reason for the directional movement of droplets on the biological surface is that there is a physical roughness/chemical composition gradient or micro-scale/nano-scale structure on the biological surface, which makes the surface tension on both sides of the droplet unbalanced. When the unbalanced surface tension is greater than the resistance of the droplet movement, the droplet will move directionally[23,24,25]. It is well known that the surface tension and contact angle of a droplet on a smooth and chemically homogeneous solid surface (Fig. Young's equation) can be described by Young's equation:[26]
Where σLO, σSO, and σSL are the surface tensions of the liquid-gas, solid-gas, and liquid-solid interfaces, respectively, and θ is the contact angle between the droplet and the surface.
When a droplet is placed on the surface, it will convert the surface energy into kinetic energy due to the difference of surface energy between the two sides of the droplet, so that the droplet can move spontaneously along the specified path without additional energy[27]. In order to understand the mechanism of droplet motion, the energy conversion process in this motion can be expressed as:
Where represents the kinetic energy of the droplet, represents the work done to prevent the droplet from moving, such as friction loss work, gravity loss work, and represents the surface energy difference between the initial and final States of the droplet.
It can be seen that the directional transport of droplets on the surface requires increasing the energy of directional motion, that is, increasing the , which can be achieved by two ways: one is to increase the surface energy gradient ().From the point of view of increasing the chemical composition gradient/physical roughness gradient, a surface with a larger surface energy gradient is constructed, and the droplet is driven from a low surface energy position to a high surface energy position; The second is to reduce the resistance energy in a specific direction () and increase the resistance energy in other directions (), thereby constructing a surface with an asymmetric topological microstructure. On this special surface, the energy barrier for the droplet to flow in a specific direction is relatively low, and the energy barrier for the droplet to flow in other directions is relatively high, so the droplet will flow in a specific direction.
2.1 Wetting theory of physically rough graded surfaces.
The effect of physical roughness on wettability was resolved by Wenzel in 1936. Increasing the surface roughness of a substance amplifies the wettability of a solid[28]. When the smooth surface is hydrophilic, the roughened solid surface will be more hydrophilic; When a smooth surface is hydrophobic, the surface of the roughened solid will be more hydrophobic. This phenomenon can be expressed by the following formula:
Where θw is the apparent contact angle, which is the contact angle of a droplet on a rough surface in actual measurements, and θy is the intrinsic contact angle, which is the contact angle of a droplet on an absolutely smooth surface. ϕ is the roughness factor, defined as the ratio of the actual surface area to the geometrically projected area. When ϕ = 1, the base is a smooth plane; When ϕ > 1, the substrate is in the Wenzel state. While in the Wenzel state, when θy>90°, the θw>θy can be obtained, and when θy<90°, the θw<θy holds in the above formula, so the roughness can amplify the hydrophilicity and hydrophobicity of the material[29,30].
However, the physical roughness gradient will produce a surface free energy gradient, as shown in the figure (Fig. 1, a). Assuming that the substrate is a hydrophilic substance, the area with large roughness is more hydrophilic, and the water droplet has a tendency to move spontaneously to the hydrophilic end, the roughness of the substrate surface will lead to different contact angles on both sides of the droplet, and the surface energy difference ) can be expressed as:[31]
Where σ is the surface tension of water, αA and αB are the advancing and receding angles of the droplet on the physically rough gradient substrate, respectively, and is the integration variable along the direction of the physically rough gradient.
On the hydrophilic surface with roughness gradient, the roughness will increase the hydrophilicity of the surface, and the water droplet has a tendency to move spontaneously to the end with larger roughness. On a hydrophobic surface with a roughness gradient, the roughness increases the hydrophobicity of the surface, and the water droplet has a tendency to move spontaneously to the end with smaller roughness.
2.2 Wetting theory of chemically graded surfaces
The Cassie-Baxter model was proposed in 1944 to extend Young's equation (Young's equation) to smooth and chemically heterogeneous surfaces[32]. The surface is assumed to consist of two species (Fig. 1, B) with their own contact angles θ1 and θ2. Assuming a region much smaller than the area of the water droplet, the proportion of the area of each species over the region is denoted by f1, f2 (f1+f2=1). Then in this region, the contact angle of the droplet on the surface of the material is related to the area ratio of the two substances and the intrinsic contact angle, which can be defined as:
Where θ is the apparent contact angle, θ1, θ2 are the intrinsic contact angles of substance 1 and substance 2, respectively, and the cosine value of the apparent contact angle can be calculated from the average of the cosines of the characteristic angles of each component.
When there is a chemical composition gradient on the surface of the material, the contact angle of the droplet on the surface of the material will change with the change of the chemical composition of the surface, which is mainly reflected in the change of the surface energy . For example, when the droplet moves from point A in the hydrophilic region to point B in the more hydrophilic region on the plane, the distance traveled is , and the variable of the surface energy can be expressed as:[22]
In the above equation, σSO and σSL are the surface tensions of solid-gas and liquid-solid interfaces, respectively, σ is the surface tension of water, βA and βB are the advancing and receding angles of water on the chemical composition gradient substrate, respectively, and is the integration variable along the chemical composition gradient direction. The larger the is, the greater the surface energy gradient is, and the greater the tendency of droplets to move to the hydrophilic end is.
On the surface with chemical composition gradient, the droplet will be more hydrophilic in the region with a large proportion of hydrophilic surface, while it will be less hydrophilic in the region with a large proportion of hydrophobic surface, so the droplet has a tendency to move spontaneously to the direction of the hydrophilic surface region.
2.3 Wetting phenomenon on the surface of microscale structure
In recent years, researchers have found that surfaces with microscale structures can also carry out small-scale liquid precision transport without surface energy gradient, and the principle of this liquid transport is based on the destruction of the symmetrical wettability of the surface[33⇓⇓⇓~37]. The asymmetric surface topography of the substrate (Fig. 1,c1) makes the liquid on the surface have less resistance to flow along a specific direction and more resistance to flow in other directions, while the three-dimensional topography of the substrate surface (Fig. 1,c2) generates Laplace pressure on the droplet and drives the droplet to flow[38,39]. For example, the conical burr of a cactus and its surface texture cause a water droplet to move along the tip of the burr toward the root; The spider will use the Laplace pressure generated by its fiber filaments to push the droplets to move directionally. Zheng et al. First revealed the traction phenomenon of Laplace pressure on droplets on spider silk, and gave the reason for the formation of Laplace pressure difference-the periodic conical structure of spider silk[31].
Another method is to introduce a micron/nanoscale sharp Edge to construct an energy barrier. The sharp edge at the surface microscale structure will pin the three-phase contact line of water-solid-air, that is, the apparent contact angle of the droplet at the sharp edge is much larger than the intrinsic contact angle, forming an energy barrier to prevent the droplet from flowing to the sharp edge[15]. If the unidirectional Laplace pressure on the liquid and the three-dimensional structure of the energy barrier are combined, the surface with the function of directional transportation of liquid can be constructed, and the directional movement behavior of water droplets can be regulated.
In a word, the chemical/physical gradient surface with continuous liquid transport function has been concerned by many researchers for decades, but the self-driven phenomenon of droplets on the surface with chemical/physical gradient is mainly caused by the surface energy gradient. If the liquid can be self-driven on the gradient surface, the surface of the material must satisfy three conditions:
(1) The droplet has low viscosity resistance;
(2) the droplet has a low contact angle hysteresis;
(3) The surface energy gradient of the substrate is large.
Only when the above conditions are satisfied, the surface energy of the gradient material can be converted into mechanical energy. If the droplet has high viscosity resistance and high contact angle hysteresis, the energy barrier () required for driving is too large, and the droplet is difficult to be driven by the surface energy gradient. If the surface energy gradient ) of the substrate is too small, the driving force for the formation of droplets is not enough to drive the droplets, that is, the phenomenon of directed transport of droplets driven by the surface needs to satisfy . At the same time, this kind of surface also faces the disadvantages of limited driving distance and slow driving speed.
Compared with the traditional gradient surface, the biomimetic surface with micro-scale structure has the advantages of faster continuous liquid transmission ability, energy barriers in multiple directions and asymmetry in micro-scale structure.The Laplace pressure generated by the interaction between the droplet and the surface of the biomimetic material can control the flow direction and transport rate of the liquid. These advantages make the biomimetic surface of the micro-scale structure have broad application prospects and good development space.
3 Biological surface for directional transport of liquid
Many animals and plants in nature have formed their own unique and practical skills of water collection and directional transportation in natural selection to adapt to various harsh living environments. For example, the micron-scale grooves on the conical spikes of cactus, the periodic structure of spider silk, the uneven wax coating on the back of desert beetles, and the micro-scale texture on the "mouth margin" surface of pitcher plants can induce directional and continuous water transport[40][31][41][42]. Texas horned lizards, which similarly live in arid regions, have special skin microscale textures that constitute "liquid diodes," which allow them to directionally collect and deliver water from wet sand and dew without the need for external energy delivery[43]. These naturally evolved animals and plants are still a treasure house for biomimetic material scientists to learn from and explore in terms of water collection and directional water transport.
3.1 Cactus
Cactus can live in extremely arid desert environment, especially in the environment of strong sunshine, low humidity, less precipitation and strong evaporation, and has strong drought tolerance. The key to the cactus's survival is the extremely efficient water mist collection surface on its needle-like burrs[44].
The needle-like burr of cactus has a cone-like multilevel structure (Fig. 2, a ~ C). Scanning electron microscopy (SEM) observation showed that each needle-like burr consisted of three parts (Fig. 2, d, e): an oriented barb at the tip, a groove containing a gradual gradient in the middle, and a ribbon-like burr at the bottom. The conical needle-like burr can directionally move the droplet from the burr tip to the bottom without external energy input[45⇓~47]. The directional transport of droplets mainly occurs in the middle of the gradient groove. During the directed motion of the droplet, the Laplace pressure difference and the surface energy gradient determine the directed motion of the droplet on the cactus burr[48]. The Laplace pressure difference () on the droplet surface is the directional driving force for the droplet motion, and the larger the driving force is, the faster the droplet moves from the tip to the root. Ju et al. Analyzed the behavior of cactus to collect water mist, and pointed out that the radius difference of cactus needle burr produces Laplace pressure difference ():[40]
Where is the local radius of the needle burr, and are the needle burr radii at the two sides of the water droplet, σ is the surface tension of water, is the radius of the water droplet, α is the semi-apex angle of the needle bristle, and dx is the integral variable of needle bristle radius. The difference in the radius of the needle burr (R2-R1) on the left and right sides of the droplet will induce a Laplace pressure difference that drives the droplet directionally (Fig. 2, J).
The existence of surface energy gradient on cactus needle-like burrs is still controversial. Ju believes that there is a surface energy gradient on cactus needle-like burrs: the surface of cactus needle burrs is covered with vegetable wax (hydrophobic substance), and the increase of roughness leads to the increase of surface hydrophobicity.The tip (rough area) of the needle-like burr with dense grooves is the hydrophobic end, and the droplet will spontaneously flow to the hydrophilic end of the bottom (smooth area) of the needle-like burr with sparse grooves, and the surface energy difference between the tip and the bottom is , which makes the droplet move spontaneously:[40][49]
Where θ2 and θ1 are the advancing and receding contact angles of a water droplet on a needle burr, respectively, σ is the surface tension of water, and dx is the length-integrated variable along the needle burr (from the tip region to the base region).
Different from the above inference, through the observation of the water transport process of the cactus needle-like burr from the top to the root, the advancing contact angle of the water droplet is less than 90 °, so the surface of the cactus needle-like burr should be hydrophilic[50]. Luo believes that the droplet on the cactus needle burr is mainly driven by the Laplace pressure difference of the conical structure induced droplet (Fig. 2, I), the size of the tip angle (α) of the needle burr is negatively correlated with the Laplace pressure difference ), and the roughness factor ϕ is positively correlated with the Laplacian pressure difference ):[51][52]
Where ϕ is the roughness factor, which is defined as the ratio of the actual surface area to the geometrically projected area, σ is the surface tension of water, and θ2 and θ1 are the apparent contact angles on both sides of a water droplet when the needle-like burr is at rest. The represents the distance between o and a, and the represents the distance between a and B.
3.2 Spider silk
Spider silk is a biological fiber composed of moisture-sensitive hydrophilic flagellated proteins. Researchers have observed that the surface of spider silk fibers is an orderly arrangement of nano-scale micro-morphology[53][54,55]. When the dry spider silk is exposed to water mist, its morphology will change greatly, and the spider silk will absorb water to "wet rebuild" and contract to form periodic Spindle-shaped biological fibers (Fig. 3, a). Spindle-knots and joints are arranged periodically and alternately on the spider silk[56⇓~58]. By scanning electron microscopy (SEM), in the periodic spindle-shaped structure, the nanofibers on the spindle knot are randomly intertwined with each other (Fig. 3, C, d), so the surface of the spindle knot is rough, while the joint is smooth composed of regular nanofibers parallel to the spider silk (Fig. 3, e, f).
Zheng et al. Attributed the ability of spider silk to transport liquid directionally to two reasons — surface energy gradient and Laplace pressure difference [31]. After wet reconstruction (Fig. 3, G), the radius from the joint to the spindle knot of the spider silk gradually increases, so the joint and the spindle knot can be seen as an incomplete cone[59,60]. The curvature gradient of the cone will cause the water droplet on its surface to form a Laplace pressure difference and drive the droplet to move to the spindle knot (larger radius)[51]. The Laplace pressure difference of the water droplet on the spider silk can be expressed by the following equation:
Where is the local radius of the spider silk, is the joint-side radius of the spider silk at the droplet boundary, is the spindle-knot-side radius of spider silk at the droplet boundary, is the surface tension of water, is the radius of the water droplet, β is the semi-apex angle at the spider silk spindle knot, while is the integral variable of the spider silk radius.
At the same time, spider silk is composed of flagellin (hydrophilic substance), and the surface of the spindle knot is rougher than that of the linker, which leads to the formation of a surface energy gradient between the spindle knot and the linker. The high roughness on the spindle junction leads to the enhancement of its surface hydrophilicity. Relatively speaking, the joint region with lower roughness is the hydrophobic end, and the droplet is driven to the hydrophilic spindle junction (rough region) by the surface tension difference. The surface energy difference resulting from the difference in the physical roughness of the upper surface of spider silk can be expressed as:[31]
Where θA and θR are the advancing and receding contact angles of a water droplet on spider silk, respectively, σ is the surface tension of water, while dx is the length integral variable in the direction along the principal axis of spider silk.
3.3 Desert beetle (Namib Desert beetle)
The Namibian beetle is able to collect moisture from moist winds with its back. Parker et al. Pioneered the idea that the Namibian beetle has a bumpy dorsal surface consisting of alternating wax-coated areas (hydrophobic material) and wax-free areas (hydrophilic material).The repeated alternating distribution of these two regions forms the bumpy surface topography of the Namibian desert beetle back (Fig. 4, a), while inducing the convergence of droplets to the hydrophilic region[41]. Uneven distribution of wax coating on beetle back induces the formation of surface tension difference , which can be expressed by the following equation:
Where σ is the surface tension of water, and θ4 and θ3 are the contact angles of water on the wax-coated and wax-free regions, respectively.
However, further studies by Dacke et al. Found that there were no hydrophilic regions on the elytra, prothorax, or head of the Namibian beetle[61]. They used Sudan III to stain the back of the beetle (Fig. 4, B). This method can make the wax area (hydrophobic material) shiny. The experiment showed that the back of the beetle had a shiny hydrophobic wax coating, which indicated that the back had uniform hydrophobicity. Therefore, the chemical pattern may not be the main factor affecting the collection of fog water by Namibian beetles.
Another possible explanation is that the high infrared emissivity of the back of the Namibian beetle efficiently radiates the heat of the substrate in the form of radiation, thus reducing the substrate temperature, and the low temperature surface of the Namibian beetle ensures the continuous condensation of droplets[62]. In the absence of local chemical patterns and local chemical gradients, the vapor diffusion flux of water droplets at the bulge can be increased only by relying on the surface structure of the millimeter-scale specific geometry of the bulge on the back of the beetle. Park et al. Used COMSOL software to numerically simulate the intensity distribution of vapor diffusion flux around the bulge on the back of the beetle, and the simulation results showed that the millimeter-scale geometry of the bulge on the back of the beetle in Namibia could increase the vapor diffusion flux around the bulge (Fig. 4, C, d), and the high vapor diffusion flux promoted the rapid nucleation and growth of water droplets at the bulge[1].
3.4 Nepenthes
The surface of the pitcher plant can be completely wetted by water, and the smooth water film will cause insects to slide into the mouth of the pitcher plant, thus "capturing" and decomposing the insects[63]. Through the analysis of the mechanism of water film formation, the water on the surface of Nepenthes can overcome its own gravity and transport upward[64]. In this process, the microchannel composed of periodic arched microcavities can transport water (Fig. 5, B, C). The microcavity structure directionally enhances the capillary rise phenomenon in the transport direction, and prevents the bypass flow by pinning the liquid, so that the liquid can flow unidirectionally at a high speed on its surface without any surface energy gradient.
Chen et al. First found that the surface at the "mouth edge" of Nepenthes (Fig. 5, a) has the function of directional water transport. In the absence of chemical composition gradient/physical roughness gradient, the micron-scale/nano-scale multi-layer structure at the "mouth edge" surface can form a "liquid diode", which has a blocking effect on the inward fluid and no blocking effect on the outward fluid[65]. Zhang et al. Proposed a possible explanation from the difference between the inward and outward motion of the droplet[66]. Due to the sharp edges of the microcavity, the fluid diffusing inwards tends to be pinned at the edges[67]; Similarly, due to the curvature inside the microcavity, the microcavity can be seen as a wedge-shaped capillary, and the fluid diffusing outward will preferentially fill the microcavity under capillary action, and the fluid will discharge air and converge inside the microchamber, and then cause the liquid level to climb after filling part of the microchamber (Fig. 5, d)[68]. Thus, the "liquid diode" surface on the pitcher plant enables water to be transported upward against gravity.
Under the action of natural selection, organisms have evolved a variety of ways to collect and control the directional transport of water in the environment independent of external energy input. According to the different driving modes of biological surface to water, it can be divided into three characteristic structures: physical roughness gradient, chemical composition gradient, and microscale/nanoscale structure. The three characteristic structures realize the directional control of the motion of the water droplet by regulating the surface energy difference, the Laplace pressure of the water droplet, or both the surface energy and the Laplace pressure of the water drop. The surface with physical roughness gradient and chemical composition gradient can produce surface energy gradient to form the driving force for the droplet, and then drive the droplet to flow. The surface of the micro-scale structure mainly relies on the local three-dimensional structure (such as wedge-shaped capillaries) created by the asymmetric surface to form a Laplace pressure difference to induce the liquid to flow in a specified direction.At the same time, its surface defects (such as sharp edges) pin the liquid flowing in other directions, so it can directionally regulate the movement of droplets on its surface. These bio-regulated directional transport modes of water droplets provide important inspiration for the design of biomimetic functional material surfaces.
4 Types of surfaces of biomimetic functional materials
In the past two decades, with the continuous breakthrough of advanced manufacturing technology, researchers have continuously imitated the surface morphology and structure of animals and plants in nature, and produced a variety of materials with directional liquid transport functions[10,69⇓ ~71]. Traditional methods mainly produce surface energy gradients that can drive liquids by making chemical/physical gradients. In recent years, the promotion of 3D printing technology and laser technology has greatly promoted the development of materials with micro-scale structured surfaces, as shown in Table 1.
4.1 Chemical composition gradient surface
Chemical composition gradient means that the ratio of one or more chemical species on the substrate changes gradually along one direction, and the surface energy also changes with position. On this substrate, the surface energy of the droplet can be converted into mechanical energy, and the droplet moves directionally along the direction of the chemical gradient change. Chemical composition gradient surface preparation methods usually include vapor diffusion method, liquid diffusion method, patterning method and plasma treatment method. These methods mainly modify the surface of the substrate (Fig. 6, a ~ C) to prepare chemical composition gradient surface with gradual change of chemical composition/pattern[6,7][8,9][11][13]. If the contact angle on the high surface energy side is smaller than that on the low surface energy side, the droplet will be pulled to the high surface energy side by the unbalanced surface tension.
It is a common method to construct chemical composition gradient by gas volatilization. For example, Chaudhury et al exposed the polished silicon wafer to decyltrichlorosilane (Cl3Si(CH2)9CH3) vapor, and the vapor concentration decreased along the length of the wafer when the silane evaporated and diffused in the gas phase, and the silicon wafer near the silane solution became hydrophobic after modification, while the silicon wafer far away from the silane solution remained hydrophilic[6]. Using this method, a surface with a surface energy gradient of more than 10 mm and a low contact angle hysteresis can be prepared. The chemically graded silicon wafer enables water droplets to climb spontaneously against gravity (Fig. 7, a). Similarly, Daniel et al. Used the vapor diffusion method to prepare a circular silicon wafer with a hydrophobic middle and a hydrophilic outer ring (Fig. 7, B), which can condense water vapor on its surface and spontaneously move to the more wettable outer ring side surface[7].
In addition, patterning with different chemical components can also be used to construct chemical gradients. Zhang et al. Patterned mica (hydrophilic substance) onto wax/low density polyethylene (hydrophobic substance), as shown in the figure (Fig. 7, C), the tip of the triangle is the hydrophobic end (the area occupied by mica is small), while the base of the triangle is the hydrophilic end (the area occupied by mica is large)[72]. The triangular mica pattern on the wax causes the substrate to form a surface energy gradient in the direction of the white arrow, which also causes the droplet to move spontaneously uphill on the substrate surface.
4.2 Physical rough gradient type surface
Physical roughness gradient means that the roughness of the material surface changes gradually along one direction when the substrate is chemically homogeneous. Because the roughness is positively correlated with the actual surface area, changing the roughness of the substrate makes the contact angle, surface energy , and energy barrier () change with position. On this substrate, the surface energy of the droplet can be converted into the mechanical energy that drives the droplet to move, and the conversion of these two energies can cause the droplet to move directionally along the direction of the change in physical roughness.
The fabrication methods of physically rough gradient surface include laser ablation, mechanical stretching and hot embossing. These methods mainly prepare physically rough gradient by changing the width and shape of the surface groove (Fig. 6, d, e)[12,73][14][33]. When the droplet is in the Wenzel state and the depth of the surface groove is constant, the contact area between the droplet and the dense part of the groove is large, and the contact area between the droplet and the sparse part is small. If the substrate material is a hydrophilic substance, the surface on the side of the groove will be more hydrophilic, and the contact angle of the water droplet will be smaller, and vice versa. For example, a groove with a certain width is made on the surface of a non-hard material by a template method, and a groove with a gradual gradient can be constructed by asymmetric mechanical stretching of the whole substrate. Zhang et al. Prepared a flexible gel with a groove width of 20 – 80 μm by free radical copolymerization of butyl methacrylate (BMA) and lauryl methacrylate (LMA) on the template of microgrooves, which could form grooves with different width gradients after asymmetric stretching (Fig. 8, a), and the difference of energy barriers () for droplets moving in different directions on this surface made the droplets move unidirectionally[14].
When the substrate is a hard material, the micro-grooves formed by laser ablation and hot embossing are often used to artificially construct rough gradients. Neuhaus et al. prepared surfaces with grooved or square microstructures by hot embossing an ethylene-tetrafluoroethylene copolymer (ETFE) substrate (Fig. 8, B), and the wettability of the surface could be directionally changed by reacting different compounds with the substrate surface[33]. By combining different chemical compositions and different groove sizes, the characteristics of this artificial microstructure can determine the shape of the droplet. Sun et al. Used an excimer laser to generate a surface roughness gradient on a polished silicon wafer, and prepared a gradient surface from superhydrophobic to hydrophobic by changing the width of the groove, so that the water droplets could spontaneously move along the designed route (Fig. 8, C)[73].
It is worth mentioning that the main method of artificial control of roughness is to control the width and shape of grooves, and the density of grooves is positively correlated with roughness[27]. However, when the groove density reaches a certain threshold, the air layer at the bottom of the droplet will enter the groove, so the Cassie-Baxter model should be used to describe the wetting behavior of the droplet. Because the method of artificially forming this kind of groove surface does not involve the chemical composition gradient, the phenomenon of air layer entering the groove is mainly caused by the physical parameters of the groove, so this kind of gradient surface is temporarily classified as "physically rough gradient surface" in this paper.
4.3 Microscale structure
Microscale structure refers to the introduction of energy barriers in multiple directions that can effectively hinder liquid flow based on the unique asymmetric topological microstructure of the surface in the absence of surface energy gradients. The apparent contact angle of the droplet at the energy barrier is much larger than the intrinsic contact angle (Fig. 9, d ~ f), and the liquid can only be transported along a specific direction, while it is pinned along other directions[74⇓⇓~77]. The micro-scale structured surface can be shaped by 3D printing technology to create a micro-Edge and local wedge-shaped bionic structure on the surface of the material (Fig. 6, f)[16].
Li et al. Used 3D printing technology to create a pitcher-like surface, which has a bionic structure of ratchets, grooves and arched channels, and can be used in artificial water collectors and oil collectors. Such surfaces can significantly improve the transport rate of liquids[15,78]. The surface of the artificial pitcher plant has a multi-curvature structure, and the process of continuously collecting and transporting water can be divided into three steps: nucleation of water droplets at ratchets, increase of water collection at grooves, and overflow of arch-shaped channels. The water will form a water film at the micro-scale structure of the surface, which greatly reduces the self-driven transport resistance of the surface to the liquid. As shown in Fig. 9, a ~ C, the average liquid transport velocity of this kind of artificial collector in the wet state of the surface is 300 times that in the dry state of the surface. Feng et al. prepared the Araucaria surface by 3D printing technology. This kind of surface has a unique bionic three-dimensional morphology. Affected by the solid-liquid contact angle, the surface has different transport mechanisms for high/low surface tension liquids, thus controlling the diffusion of high/low surface tension mixed liquids in different directions (Fig. 9, G, H)[74].
In general, the chemical/physical gradient surface has a relatively perfect preparation system, and the chemical/physical gradient surface with the function of directional transport of droplets can be synthesized by traditional technologies such as vapor diffusion, liquid diffusion, surface patterning, laser ablation, plasma treatment, and mechanical stretching[6,7][8,9,10][11][12][13][14]. However, the action principle of gradient surface transporting droplets leads to its inherent defects, and the gradient surface has the disadvantages of limited chemical/physical gradient distance and small scale of transporting liquid.
In recent years, the commercialization of lithography and 3D printing has greatly promoted the bionic simulation of micro-scale structures on biological surfaces[15,25]. Relatively speaking, the biomimetic surface that relies on microscale structure to induce directional liquid transport has a longer distance and increases the scale of liquid transport to a certain extent.
5 Application of Biomimetic Functional Material Surface
By imitating the surface topography of animals and plants in nature, researchers have created materials that can directionally transport liquid functions. Based on its unique surface energy distribution and morphology, the surface of biomimetic functional materials has the advantages of accuracy, speed, green, environmental protection, no need to input energy and external equipment[11]. Therefore, biomimetic functional materials that can transport liquid directionally have broad application prospects in the fields of directional water collection, liquid transportation, microfluidic systems, oil-water separation and so on[17][16][20][18].
5.1 Directional catchment
Collecting water from fog is an effective way to solve the global freshwater crisis. There are many organisms that can get water from fog, such as green tree frog, Arabian sand lizard (Phrynocephalus arabicus) and Namibian beetle[79]. Inspired by these natural structures, surface energy patterning on the condensation surface can directionally collect the condensed liquid, and the surface with different surface energy gradients can make the droplets move directionally.
The droplet will move from the region with lower surface energy to the region with higher surface energy on the surface with surface energy gradient, and the droplet in Cassie-Baxter state and Wenzel state can also be directionally regulated due to the different pinning force of the contact line, so the difficulty of the detachment of the condensed droplet on the surface and the condensation rate can also be directionally regulated. Based on this, Chatterjee et al. Constructed hydrophilic island patterns and tree patterns on the hydrophobic condensation surface (Fig. 10, a ~ d). The hydrophilic area is covered by larger droplets, while the hydrophobic area is covered by relatively smaller droplets, so the droplets will be absorbed from the hydrophobic area to the hydrophilic area[17]. Zhang et al. Used inkjet printing technology to directly produce superhydrophilic micropattern droplets (Fig. 10, e, f) on the superhydrophobic surface. After condensation on the superhydrophobic surface, they will be driven by the surface energy gradient to preferentially move to the polydopamine-modified superhydrophilic region, and then merge into larger droplets[80]. Seo et al. Used photolithography to fabricate a surface with a microstructure of columnar/grid structures on a silicon substrate (fig. 10, G, H), and controlled the condensation characteristics of moisture on the surface by adjusting the height, width, and spacing of the columnar bodies and the grid bodies.The researchers found that the geometric characteristics of the surface can affect the nucleation site and condensation rate during the droplet condensation process, and the water droplets on the Cassie surface are easier to fall off than those on the Wenzel surface, so the Cassie surface has better water collection performance[81].
5.2 Liquid transfer
In the petroleum, textile, and pharmaceutical industries, directional long-distance transport of liquids and self-driven transport without external energy input are of great significance, but the existing methods to improve the transport distance are still limited[82][83]. In order to solve the problem of long-distance, fast and high-flux liquid transport, the surface of the substrate is modified to form a local Laplace pressure gradient, and surface defects are introduced in a specific direction to pin the fluid in an unnecessary flow direction, thereby inducing the self-driven transport phenomenon of the liquid[15].
Li constructs a pitcher-like microscale structure inside the capillary tube (Fig. 11). In the pitcher-like capillary tube, the capillary action and the Laplace pressure difference formed by the local microscale structure will cooperate to push the water level up[16]. At the same time, the water will form a stable water film on the micro-scale structure of the capillary inner wall, which reduces the viscous resistance of the liquid rising. Compared with the capillary without any micro-scale structure, the rising speed of the liquid surface of the imitated "pitcher plant" capillary in 1 s is 17. 9 times of that of the smooth capillary in 1 s. Compared with the smooth capillary tube, the rising height of the liquid level is also increased by 2.1 times.
5.3 Microfluidic
Microfluidics mainly refers to the use of microchannels to manipulate small fluids, which is widely used in biology, chemistry, medicine and other fields, in which the hydrophilicity of the substrate of the microfluidic system has a great impact on the flow of microfluids[84,85]. Based on the influence of surface tension, the hydrophilic-hydrophobic patterns and microscale structures on the substrate material can construct "virtual walls" to restrict the flow of liquid in unnecessary directions and "microvalves" to select the channels through which microfluids flow, and the surface energy gradient can play an important role in promoting the movement of microfluids together with external energy[10,69,86⇓ ~88].
How to control and drive the movement of fluid in microchannels is a technical challenge for microfluidic systems, which usually requires microvalves, microchannels, and external actuators, while traditional mechanical devices increase the cost and size of microfluidic systems[89]. In order to overcome this shortcoming, non-mechanical devices have been developed to manipulate microfluids by changing the surface properties, which can achieve directional control of the flow behavior of microfluids only by modifying the wettability of the substrate without external mechanical devices. In recent years, researchers have successfully realized the manipulation of microfluids using substrates by pre-designing the hydrophilic and hydrophobic patterns and micro-scale structures of the substrates using surfaces with different wettability. Fang proposed a finite element analysis method to formulate the study of non-uniform rough surface, and designed the surface energy gradient distribution of the microfluidic channel for transporting droplets (Fig. 12, a), which provided theoretical design ideas and guidance for the control of microfluidics by hydrophilic-hydrophobic patterns and microscale structures on the substrate material[90]. Liimatainen et al. Used the principle of micro-scale sharp edge pinning liquid (Fig. 12, B) to create an energy barrier to block the movement of microfluids at the wall surface of the "virtual wall", thus actually constructing a microfluidic channel to manipulate the flow of microfluids in the "virtual wall"[20]. Through the surface energy difference of chemical heterogeneity (Fig. 12, C), Wang et al. developed a chemical composition striped surface with microvalve effect in microchannels, which can set different "energy barriers" in different microchannels[11]. Under the condition of the same flow rate, the microfluid will preferentially flow to the channel with low "energy barrier", thus realizing the anisotropic flow of water in the microchannel.
5.4 Oil-water separation
Environmental protection and economic development have increased the demand for effective separation materials for oil-water mixtures, and the separation of tiny oil droplets from water has a great role in pollution prevention and oil recovery. However, the common separation method is the phase separation of oil-water mixture, and the micron-sized droplets which are not affected by phase separation are difficult to be treated on a large scale by conventional phase separation methods. In recent years, researchers have designed conical arrays imitating cactus spikes and micro-groove arrays imitating pitcher plants on the surface of materials. This bionic surface uses the structure of natural biological surface to transport liquid unidirectionally, which can transport droplets touching the bionic surface directionally, thus successfully collecting tiny droplets in oil-water mixture directionally[82][18].
Li et al. Made a reverse template by 3D printing, and then cast polydimethylsiloxane/polyvinyl alcohol (PDMS/PVA) on the template, thus successfully making a pitcher-like surface, which can separate micron-sized oil-water mixtures[91]. They dropped the oil-water mixture one by one onto the surface of the arched pitcher plant, and then obtained pure water and oil droplets at both ends of the arch. Zhou et al. Put a gel material with a pitcher-like surface into an oil-water mixture, and placed its surface horizontally in an oil or water environment, and then used a micro-pump to dispense water or oil droplets onto the surface[18]. Under weak interactions (such as hydrogen bonds, van der Waals forces and hydrophobic interactions), the gel will entrap the liquid and form a liquid film on its surface, which reduces the one-way transport resistance of the liquid in the liquid-liquid-solid three-phase.At the same time, the surface microscale structure drives the droplets directionally (Fig. 13), and these factors work together to achieve the directional collection of oil droplets in water and water droplets in oil.
By imitating the unique surface energy distribution and morphology of natural organisms, researchers have prepared biomimetic functional materials in the laboratory that can be used in various fields. Biomimetic functional materials also have great potential for industrial, medical and agricultural applications due to their characteristics of precision, speed, green, environmental protection, no input of energy and external equipment[11].
6 Conclusion and prospect
In this paper, the biological structure characteristics, functional principles, artificial manufacturing methods and potential application fields of biomimetic functional material surfaces driven by liquid directional movement are summarized. The surface of biomimetic functional materials has unique physical/chemical composition gradient and micro-scale structure. Based on this unique surface distribution and morphology, the surface of biomimetic functional materials can control the wetting behavior of droplets and the transport behavior of fluids, which shows good application prospects in the fields of directional water collection, droplet transport, microfluidics, oil-water separation and so on. With the rapid development of the surface fabrication of biomimetic functional materials, the bottleneck factors limiting the practical application of these materials have gradually emerged. How to move biomimetic functional materials from laboratory preparation to complex practical application will become the direction of future development.
6.1 Long Distance Liquid Transportation
The directional long-distance transportation of liquid is of great significance in the chemical and food industries. The traditional long-distance liquid transportation method is to drive the liquid by means of mechanical pumps, siphon effect and other means. The combination of biomimetic functional material surface and long-distance liquid transportation method will bring significant economic benefits in drag reduction and energy saving. As a result, such surfaces can enhance the effectiveness of traditional methods of long distance liquid transport. However, the liquid transport distance is severely limited by the geometric shape and the driving force of Laplace pressure of the liquid transport surface prepared from the biomimetic point of view. How to regulate and control the surface topography structure in the liquid conveying channel and form a reasonable transition, effectively improve the transportation distance of the liquid and reduce the energy loss in the transportation process of the liquid in the channel by using the surface energy gradient and the micro-scale structure on the surface,The combination of traditional liquid transportation means and biomimetic functional material surface will effectively improve the efficiency of long-distance liquid transportation and achieve energy saving and emission reduction[16].
6.2 Natural environment catchment
Natural environment water harvesting has many uses in military, agriculture, tourism and other fields. Most of the existing catchment surfaces induce the directional aggregation of droplets by controlling the hydrophilic and hydrophobic patterns, and at the same time, they need to meet certain temperature difference or humidity conditions in the process of catchment, which usually has a significant catchment effect in the experimental environment[17]. Although researchers have tried and carried out some simulation tests on artificial catchment surfaces in natural environments, there are relatively few studies on different natural environments such as islands and deserts. However, it is still a difficult problem whether the water-harvesting materials prepared in the experiment can be applied to different natural environments and maintain a high water-harvesting efficiency. Therefore, there is still a broad research space for catchment materials in the actual environment of islands and deserts with different isothermal/humidity.
6.3 Application of anticorrosive coating
Infrastructure and industrial facilities serving in the marine environment are often damaged and scrapped due to corrosion problems, which seriously affects the service safety, life and reliability of ships, offshore engineering and ocean-going facilities. Especially in the marine atmospheric region, the high humidity and salinity in the marine environment will accumulate into a thin liquid film on the metal surface, forming a good conductive electrolyte solution and promoting the occurrence of electrochemical corrosion. Coating materials can effectively prevent the diffusion of external corrosive medium to the surface of metal substrate, which is an important means of corrosion control for ship and marine structures. However, if the coating is in contact with water or periodic alternation of drying and wetting during long-term service, it is very easy to cause aging, damage and even corrosion of the metal. Therefore, how to construct a bionic liquid directional transport structure on the surface of the coating, conduct the water in the thin liquid film on the surface of the coating in time, and reduce the contact time and contact area between the coating and the thin liquid film will effectively increase the service life of the coating and enhance the service time of marine engineering equipment, which has an important development prospect in the corrosion protection of marine equipment.
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