Bioinspired Directional Surfaces for Adhesion, Wetting and Transport

Abstract

In Nature, directional surfaces on insect cuticle, animal fur, bird feathers, and plant leaves are comprised of dual micro-nanoscale features that tune roughness and surface energy. This feature article summarizes experimental and theoretical approaches for the design, synthesis and characterization of new bioinspired surfaces demonstrating unidirectional surface properties. The experimental approaches focus on bottom-up and top-down synthesis methods of unidirectional micro- and nanoscale films to explore and characterize their anomalous features. The theoretical component of the review focuses on computational tools to predict the physicochemical properties of unidirectional surfaces.

Keywords: Biomimetics; Nanostructures; Polymeric Material; Superhydrophobic/philic Surfaces; Surface Modification.

Figure 1

Sources of asymmetry in materials science. The asymmetry of (a) materials structure or geometry, (b) the external field or stress, (c) surface or boundary condition, (d) the physical property relating stress to strain, and (e) strain or deformation.

Figure 2

Directional textured surfaces in nature. (a) Beetle (Hemisphaerota cyanea) tarsus consists of pads arranged in rows (left) and stuck together in clusters (middle, right). Scale bars, left to right, 200 µm, 40 µm, 10 µm. Source: Eisner and Aneshansley,^[17] copyright (2000) National Academy of Sciences, U.S.A. (b) Tokay gecko (Gekko gecko) foot setae (left) and the finest terminal branches of a seta, called spatula (right). Scale bars 50 µm (left) and 1 µm (right). Source: Autumn et al.,^[66] reprinted with permission from Macmillan Publishers: Nature, copyright 2000. (c) Wet-rebuilt silk of the cribellate spider (Uloborus walckenaerius), showing overall structure (top) with ESEM zooms of a spindle knot (bottom left) and joint (bottom right). Scale bars 50 µm (top) and 2 µm (bottom). Source: Zheng et al.,^[23] reprinted with permission from Macmillan Publishers: Nature, copyright 2010. (d) The non-wetting leg of the water strider (Gerris remigis). Individual hairs are deflected by capillary forces as the drop advances against the grain (top). SEM images of the oriented microsetae (bottom left) and the nanoscale grooved structures on a seta (bottom right). Scale bars 100 µm (top), 40 µm (bottom left), 400 nm (bottom right). Top image reprinted from Prakash et al.,^[24] copyright (2011), with permission from Elsevier. Bottom image from Gao and Jiang,^[25] reprinted with permission from Macmillan Publishers: Nature, copyright 2004. (e) Overlapping microscales (top) on the wings of the butterfly (Morpho aega) are comprised of aligned nanostripes. Scale bars 100 mm (top) and 400 nm (bottom). Source: Zheng et al.,^[28] reproduced by permission of The Royal Society of Chemistry. (f) Peristome surface of the Nepenthes pitcher plant has first and second order radial ridges. Scale bars 100 µm. Source: Bohn and Federle,^[29] copyright (2004) National Academy of Sciences, U.S.A. (g) Micro-barbs on the surface of the grass species Hordeum murinum. Scale bar 50 µm. Source: Kulic et al.,^[30] reprinted with permission of The Royal Society, copyright 2009.

Figure 3

Engineered textured directional surfaces with asymmetric or periodic structures. (a) Micropost array for directional drop transport. Zoomed image shows details of asymmetric positioning of posts. Scale bars 200 µm (left) and 100 µm (right). Reprinted with permission of Chung et al.^[32] Copyright 2010, SPIE. (b) Wax microratchet surfaces for directional drop transport. Scale bar 1 mm. Reprinted with permission from Sandre et al.,^[34] copyright (1999) by the American Physical Society. (c) Milliratchet surface for directional drop transport powered by the Leidenfrost effect. Scale bar 1 mm. Source: Lagubeau et al.,^[36] reprinted with permission from Macmillan Publishers: Nature Physics, copyright 2011. (d) Shaped micropillar array for directional droplet spreading. Scale bars 200 µm (left) and 50 µm (right). Source: Jokinen et al.,^[37] copyright (2009) John Wiley & Sons. (e) Nanofilm with tilted nanorods acting like nanoratchets for directional drop transport and adhesion. Scale bars 5 µm (left), 1 µm (middle, right). Source: Malvadkar et al.,^[39] reprinted with permission from Macmillan Publishers: Nature Materials, copyright 2010. (f) Ion track textured surface with directional contact angle hysteresis. Scale bar 50 µm. Reprinted with permission from Spohr et al.^[40] Copyright (2010) American Chemical Society. (g,h) Tilted nanohairs for directional wetting and drop spreading. Scale bars (g) 1 µm and (h) 10 µm. Sources: (g) Kim and Suh^[41] – reproduced by permission of The Royal Society of Chemistry. (h) Chu et al.,^[42] reprinted with permission from Macmillan Publishers: Nature Materials, copyright 2010. (i) Microratchet surface for directional dry adhesion. Scale bars 500 µm (left) and 20 µm (right). Source: Parness et al.,^[43] reprinted with permission of The Royal Society, copyright 2009. (j) Tilted nanorods for directional dry adhesion. Scale bar 10 µm. Reprinted with permission from Lee et al.^[44] Copyright 2008, American Institute of Physics. (k) Hierarchical texture (micro/nano) for directional dry adhesion. Scale bars 10 µm (left), 1 µm (right). Source: Jeong et al.,^[45] copyright (2009) National Academy of Sciences, U.S.A. (l) Microchannel with ratchet-shaped walls for directional particle transport. Scale bar 2 µm. Reprinted with permission from Kettner et al.^[47] Copyright (2009) by the American Physical Society.

Figure 4

Applications of directional surfaces. (a) Snapshots of a droplet spreading in one direction on a directional surface. Schematic indicates direction of nanorod tilt. Scale bars 1 mm. Source: Chu et al.,^[42] reprinted with permission from Macmillan Publishers: Nature Materials, copyright 2010. (b) Drops and drops containing cargo being transported on directional surfaces forced with mechanical vibration. The forcing frequency was chosen close to the resonant frequency of the largest drop, and amplitude low enough so that smaller drops did not move. When two drops coalesced, their size increased and resonant frequency decreased. Thus, the forcing frequency was also reduced to keep the largest drop moving. Scale bar 5 mm. Reprinted with permission from Sekeroglu et al.^[54] Copyright 2011, American Institute of Physics. (c) Drops impacting a directional surface with the grain (top) bounce further than those impacting against the grain (bottom). Dashed vertical lines demark the extent of the trajectory in the top image; schematics indicate the grain, i.e. nanorod direction. Scale bar 1 cm. Source: new unpublished work by the authors (see Experimental for details). (d) Soft gel transport on a PDMS surface with angled cuts aligned in the direction of motion. Snapshots shown 1 s apart. Scale bar 1 cm. Source: Mahadevan et al.,^[55] copyright (2004) National Academy of Sciences, U.S.A. (e) Directional control of gas release. Gravity and buoyancy force point in (above) and against (below) the direction of nanorod tilt, as indicated by the schematic. Scale bar 1 mm. Source: new unpublished work by the authors (see Experimental for details). (f) Directional folding. Polymer sheets were folded by evaporating droplets. Folding was symmetric and asymmetric for sheets with isotropic and directional surfaces, respectively. Scale bar 4 mm. Source: new unpublished work by the authors (see Experimental for details). (g) Directional cell adhesion on a directional surface. Zoomed image indicates cell filopodia penetrating between the nanorods. Scale bars 10 µm (top) and 1 µm (bottom). Reprinted with permission from Christophis et al.^[60] Copyright 2011, American Vacuum Society. (h) Channels with ratchet surfaces for cell transport. Scale bar 50 µm. Source: Mahmud et al.,^[61] reprinted with permission from Macmillan Publishers: Nature Physics, copyright 2009.

Figure 5

Modeling wet adhesion on textured surfaces. (a) Initial and final Surface Evolver simulation profiles for a static drop on a pillared surface. Reprinted from Chen et al.,^[94] copyright (2005), with permission from Elsevier. (b) Surface Evolver simulation of a portion of a droplet in a Wenzel state on a pillared surface. Reprinted with permission from Dorrer and Rühe.^[97] Copyright (2008), American Chemical Society. (c) Surface Evolver simulation of a portion of a droplet in a Cassie state on a pillared surface. Reprinted with permission from Dorrer and Rühe.^[95] Copyright (2007), American Chemical Society. (d) Surface Evolver simulation of a droplet advancing or receding on a checkerboard chemically patterned surface. Reprinted with permission from Kwon et al.^[93] Copyright (2010), American Chemical Society. (e) Lattice Boltzmann simulation of directional spreading through an array of triangular pillars. Reprinted with permission of Blow et al.^[106] Copyright 2009, Institute of Physics. (f,g) Lattice-Boltzmann simulations of drops sliding on pillared and grooved surfaces. Source: Hyväluoma et al.,^[107] with kind permission of The European Physical Journal (EPJ). (h) Lattice Boltzmann simulation of a portion of a drop in a Cassie state receding on a pillared surface. Reprinted with permission from Mognetti and Yeomans.^[111] Copyright (2010) American Chemical Society.