Figure 1 Wettability of textured surfaces: (a) Rapid spreading of water droplets on a superhydrophilic surface. Reprinted from reference [11], Copyright 2006, with permission from American Chemical Society. (b) Water droplets on a superhydrophobic duck feather. Reprinted from reference [19]. (c) A schematic illustration of a Wenzel state. In this state, the contacting liquid droplet (represented by blue color) completely fills any gap between protrusions present on the surface (represented by red color). (d) A schematic illustration of a Cassie-Baxter state. In this case the liquid droplet is supported partially on the solid substrate and partially on vapor, forming a solid-liquid-vapor composite interface.

Figure 2 Drag reduction using textured hydrophobic surfaces: (a) A schematic illustrating a conventional, no-slip interaction between a liquid flow and a solid surface. (b) A liquid (represented by purple color) flow on a superhydrophobic surface, possessing a non-zero wall velocity Vw at the liquid-vapor interface (white color) and conventional no-slip boundary condition on solid-liquid interfaces (blue solid line). Image is reproduced and modified from reference [44] - Published by The Royal Society of Chemistry. (c), (d) The effects of gas fraction (1 - ϕs) (i.e., the areal fraction of liquid-air interface) and pitch on the slip length. Images c and d are reproduced from reference [37], Copyright 2008, with permission from American Physical Society.

Figure 3 Surfaces that exhibit significant reduction of drag: (a) The irreversible transition from the Cassie-Baxter to Wenzel regime occurs when the meniscus of water forms Young's contact angle on the vertical wall of surface texture. (b) Superhydrophobic surfaces with dual-scale texture (left) or re-entrant texture (right) may display higher resistance against the penetration of water as the apparent contact angle on the main asperities can reach as high as 180°. Images in (a) and (b) are reprinted from reference [42], Copyright 2009, with permission from American Chemical Society. (c) The collapse of air pockets due to pressure perturbation (left), which can be prevented by introducing a feedback channel (right). Image is reprinted from reference [49], Copyright 2010, with permission from American Chemical Society. (d) The flow of water through a superhydrophobic (left) and clean, hydrophilic copper tube (right). Water enters from the top through a vertical pipe at the center, splits at the T junction toward the two horizontal pipes. Two graduated cylinders at the bottom collect the discharged water. After 90 seconds, the left cylinder becomes almost half full while the right one remains to be almost empty, indicating that water preferentially flows along the superhydrophobic pipe. Image is reprinted and modified from reference [50], Copyright 2009, with permission from American Chemical Society.

Figure 4 Boiling curve and the effect of surface wettability: (a) An illustration of a typical boiling curve on a smooth surface, showing heat flux as a function of wall superheat. Image is reprinted from reference [53], Copyright 2010, with permission from American Chemical Society. (b) A comparison of HTCs (heat transfer coefficient) on a hydrophobic (blue circle) and a hydrophilic (hollow square) surface. The merit of hydrophobic surface is obvious at low heat flux regime, but the merit quickly disappears as the wall superheat increases beyond 25K. Approximate CHFs (critical heat flux) are 200 kW/m2 and 800 kW/m2 for hydrophobic and hydrophilic surfaces respectively. Reprinted from reference [54], Copyright 2011, with permission from Elsevier.

Figure 5 Surfaces that affect the boiling characteristics: (a) The first surface (a.a) is patterned with continuous hydrophilic matrix (black) with discrete hydrophobic domains (gray), while the second surface (a.b) is patterned with the opposite polarity. The third panel (a.c) shows the nucleation of bubbles on the patterned surface. Image is reprinted from reference [61], Copyright 2010, with permission from AIP Publishing LLC. (b) The mutual interaction among bubbles nucleating from nearby sites. The figure shows two exemplary patterns - vertical coalescence (top) and declining coalescence (bottom), depending on the spacing between nucleation sites. Image is reproduced from reference [62], Copyright 2003, with permission from Elsevier.

Figure 6 Overview of arid regions and natural examples of fog harvesting surfaces: (a) Hyperarid and arid lands, which are deserts, have annually no rainfall and less than 250 mm of rainfall, respectively. In some of hyperarid and arid regions near ocean, abundant fog is one of the most promising sources of water. Reprinted with permission from reference [64]. Source: https://pubs.usgs.gov/gip/deserts/what/world.html. (b) The image of the Namib desert beetle and (c) SEM image of its back with hydrophilic bumps surrounded by hydrophobic channels. Reprinted with permission from reference [3], Copyright 2001, Nature Publishing Group. (d-f) Three images of cactus spine structures at different magnifications. Reprinted with permission from reference [73]. Copyright 2012, Nature Publishing Group. (g,h) The droplets formed on spiderweb in early mornings and result from the repeated spindle-knot and joint structures with different wetting characteristics. ((g) Image Courtesy of William Lee (h) Reprinted with permission from reference [74]. Copyright 2010, Nature Publishing Group.)

Figure 7 Artificial fog harvesting surfaces: (a,b) Water droplets formed by spraying mist on poly(acrylic acid) (PAA) patterned superhydrophobic surfaces that mimic the fog-collecting capabilities of the Namib desert beetle. Reprinted with permission from reference [75]. Copyright 2006, American Chemical Society. (c,d) Illustration of water collection state by hydrogen bonding between molecules of poly(N-isopropylacrylamide) (PNIPAAm) at lower temperature than low critical solution temperature (LCST). Reprinted with permission from reference [77]. Copyright 2013, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (e,f) Illustration of PNIPAAm intermolecular bonds at higher temperature than LCST, resulting in water release (superhydrophobic) state. Reprinted with permission from reference [77]. Copyright 2013, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (g) Image of a large fog collector made of Raschel mesh. (Image Courtesy of Pilar Cereceda) (h) Illustration of fog harvesting mechanism on permeable network of fibers. A portion of small fog droplets coming to the woven mesh structure are captured and deposited droplets are drained downward when gravitational force acting on the deposited droplets exceeds the pinning force that is attributed to water contact angle hysteresis of the fiber surface material. Reprinted with permission from reference [83]. Copyright 2013, American Chemical Society. (i,j) Two distinct factors that reduce the fog collection efficiency – (i) the re-entrainment of collected droplets by air drag force and (j) blockage of the mesh by deposited droplets. These factors are attributed to the wetting characteristics of mesh surface material and can be tuned by the application of surface coating to achieve a higher fog collection efficiency. Reprinted with permission from reference [83]. Copyright 2013, American Chemical Society.