Fig. 1 Three water transport mechanisms, physical gradient, chemical gradient and hierarchical structure. (a) Physical gradient: a surface energy gradient is generated by physical surface roughness, by which a droplet is promoted to move forward to a coarser and hydrophilic region. (b) Chemical gradient: the asymmetry surface tension generated by chemical gradient drives the droplet to high surface energy region[23]; (c1) Hierarchical structure: the hierarchically maro/nano edge on the substrate generates an energy barrier that inhibits the movement of water to the direction of high energy barrier region, the water would move in a given direction. (c2) The difference in the radius on both sides of the cone will induce the droplet to move

Fig. 2 Mechanism of directional water transport in cactus. (a) Optical image of a plant of Opuntia microdasys stem covered with well-distributed clusters of spines and trichomes. (b,c) Magnified optical images of a single cluster with spines growing from the trichomes. (d,e) SEM image of a single spine. (f,g) Magnified images of regions near the base and tip of the cactus spine, respectively. The microgrooves near the base are wider and sparser than those near the tip. (h) Magnified image of a single barb[40]. (i) Cross-sectional schematics of liquid drops[51]. (j) In addition to the conical shape, the surface of the spine was covered with multi-level grooves[40,51].Scale bars, a: 5 cm, b,c: 500 μm, d: 100 μm, e~g: 20 μm, and h: 2 μm

Fig. 3 Mechanism of directional water transport in spider silk. (a) Macroscopic photos of spider silk. (b) Environmental SEM images of periodic spindle-knots linking with slender joints. (c,d) Low-magnification and zoomed images show that the spindle-knot is randomly interweaved by nanofibrils. (e,f) Low-magnification and high-magnification images of the joint, which is composed of nanofibrils aligned relatively parallel to the silk axis. (g). Surface structural anisotropy generates a surface energy gradient so that spindle-knots possess higher apparent surface energy than joints[31]

Fig. 4 Mechanism of directional water transport in Namib desert beetle. (a) Hydrophobic dorsal surface of Physasterna cribripes. An example of a Physasterna cribripes treated with Sudan Ⅲ staining in order to examine if the beetles have hydrophilic zones that could facilitate the collection of water from fog. The Sudan Ⅲ staining makes wax covered i.e. hydrophobic areas shiny. The magnification shows the shining hydrophobic peaks of the bumps on the elytra. (b) Scanning Electron Microscope images of the apex of the elytra. Scale bar = 1 mm[61].(c,d) Numerically calculated intensity profile of diffusion flux (COMSOL-Multiphysics)[1]

Fig. 5 Mechanism of directional water transport in Nepenthes. (a) Optical images of a pitcher of Nepenthes. (b) High-speed digital images showing how the starting boundary of a water droplet (dashed lines) changes after the droplet is deposited on the peristome surface. (c) High-speed digital images of the continuous water-transport process between overlapping duck-billed microcavities. (d) Three-dimensional illustration of the water-transport process. The lower water layer, layer Ⅲ, fills up a single microcavity (yellow arrows) and overflows to generate the upper water layer, layer Ⅱ. Before the second microcavity is completely filled, the upper water layer (brown arrows) becomes the top water layer, layer Ⅰ (green arrows), filling the third microcavity. Transport in the reverse direction does not occur because the water is pinned in place, with the water boundary coinciding with the sharp edge[42]

Fig. 6 Various methods of creating chemical gradients, physical gradients and hierarchical structure. (a) Liquid diffusion of organosilanes. (b) Vapor diffusion of organosilanes. (c) Producing a gradient with radio frequency plasma discharge. (d) The scan head delivers the laser beam on the sample surface to produce microstructures[12].(e) Asymmetrically stretching the micro-grooved organogel surfaces. (f) Fabrication of inner microstructured capillary tubes through replication[16]

Fig. 7 Artificial surfaces of chemical gradient for directional water transport. (a) The surface of a silicon wafer reacts with vapors of a volatile alkylchlorosilane by using a diffusion-controlled process[6]. The type of drop motion is a consequence of chemical gradient. (b) The surface of a silicon wafer has a radially outward gradient of chemical composition that was prepared by diffusion-controlled silanization[7]. (c) Patterning the shape-gradient hydrophilic material to the hydrophobic matrix. The surface has a spatial gradient in the surface free energy, the unbalanced Young’s force drives water uphill[72]

Fig. 8 Artificial surfaces of physical roughness for directional water transport. (a) The unidirectional sliding of water droplets is controlled by asymmetrically stretching the surface of the microgrooved organogel[14]. (b) Microstructures composed of grooves or squares are produced via hot embossing of poly(ethylene-alt-tetrafluoroethylene) (ETFE) substrates. Modify the structure substrate with a polymer brush to change its surface function and wettability[33]. (c) Using an excimer laser to generate surface roughness gradient on a polished silicon wafer, a surface wettability gradient from superhydrophobicity to hydrophobicity can be generated[73]

Fig. 9 The bionic surface with hierarchical structures. (a)Schematic diagram of the artificial peristome harvestor. (b) Time-sequence images of the water condensation and transport process on the artificial peristome harvestor. (c)The water droplet with a diameter d1 condensate at the cone side transports along the wet surface to the container side[78].(d) From S1 to S2, upper liquid exceeds and overflows around the sharp overhang as well as fills up the upper microcavity. The liquid-solid contact line pins in place and follows the outline of the microcavities. (e,f)Side view e and cross-section view f of the advancing and receding contact angles of the liquid on the surface, respectively[15]. (g,h) Transport of water-ethanol mixtures[74]. The liquid of low surface energy displays forward transport, whereas liquid of high surface energy displays backward transport

Fig. 10 The artificial surface which can collect water. (a~d) Patterns of distinct hydrophobic and hydrophilic regions on the 25.4 mm condensation surface[17].(e, f) Water collection processes on micropatterned surfaces, these droplets preferentially moved toward the polydopamine-modified superhydrophilic regions and subsequently coalesced into bigger droplets in these regions. Areas 1 and 2 are super-hydrophilic regions composed of polydopamine[80].(g,h) Time-lapse images of the water condensation. The condensed surface is in the Cassie-Baxter state, and the distance of the micro-pillars on the condensation surface is 60 microns, the height is 60 microns, and the radius is 20 microns[81]

Fig. 11 Capillary uplift dynamics and quantification. (a)Image of water capillary rise inside tubes. (b) Schematic of water capillary rise. (c,d)Variations in the precursor and bulk water elevation heights. (e,f) Comparison of experimental elevation heights and elevating velocities (colored open triangles), H and v, respectively, and theoretical elevation heights and elevating velocities (colored lines) as a function of time, t. (f, Inset) Enlarged diagram within the time range from 0.3 to 2.0 s[16]

Fig. 12 Designed surface controls microfulid and confines the liquid flow along the undesired directions. (a)Schematic illustration of the view of the surface which is designed for microfluidic channels and the surface can guide the droplets[90]. (b)Microstructures with sharpen edge is designed in the surface. The liquid spread without external actuation after initial dispensing[20]. (c)Water is controlled in microfluidic systems, the patterned surface acts as microvalve for the microfluidic system[11]

Fig. 13 Artificial surface can transport water directionally under oil and transport oil directionally under water. (a,c) Schematic of the experimental setup. (b)Time-lapse images of the water transporting process from the top view.(d) Time-lapse images of the oil transporting process from the top view. (e) Time sequences of the unidirectional transporting distances of the water in oil and the oil in water.(f) 3D presentations of sample structures using 3D X-ray microscopy[18]