Interfacial Materials with Special Wettability Tak-Sing Wong, Taolei Sun, Lin Feng, and Joanna Aizenberg
Abstract Various life forms in nature display a high level of adaptability to their environments through the use of sophisticated material interfaces. This is exemplified by numerous natural examples, such as the self-cleaning of lotus leaves, the water-walking abilities of water striders and spiders, the ultra-slipperiness of pitcher plants, the directional liquid adhesion of butterfly wings, and the water collection capabilities of beetles, spider webs and cacti. The versatile interactions of these natural surfaces with fluids, or special wettability, are enabled by their unique micro/nanoscale surface structures and intrinsic material properties. Many of these biological designs and principles have inspired new classes of functional interfacial materials, which have remarkable potential to solve some of the engineering challenges for industrial and biomedical applications. In this article, we provide a snapshot of the recent state-of-the-art development of biologically inspired materials with extreme fluid repellency and their potential applications in high/low temperature environments, as well as discuss some promising future directions in the field.
Keywords: Biomimetics, Bioinspiration, Special Wettability, Superhydrophobicity, Superoleophobicity, Superomniphobicity
Introduction Wetting – the interaction of fluids with solid surfaces – impacts many areas of
science and technology in the modern era.(1-3) In particular, creating a robust synthetic surface that I) repels various liquids, II) allows for directional/switchable fluid manipulation, and/or III) operates under various environmental conditions would have broad technological implications for areas related to water, energy, and health but has proved extremely challenging.(4) In nature, many biological surfaces are

engineered to have special interfacial interactions with fluids – or special wettability – in order to survive in their innate environments.(5-23) For example, lotus leaves rely on hierarchical micro/nanoscale textures to trap a thin layer of air (Figure 1a), which then acts as a cushion against liquids and helps to keep the surface clean by carrying away dirt – the lotus effect (6); springtails, arthropods that live in the soil, have evolved overhanging nanostructured skin patterns (Figure 1b) that help prevent them from soiling (20); Nepenthes pitcher plants capture insects with their highly slippery, liquid infused micro-textured peristome (Figure 1c) without the use of any active prey-capturing mechanisms.(10, 24) Central to many of these functional biological surfaces is the presence of unique micro- and nanostructured architectures that allow them to exhibit special wettability. To this end, mimicking these biological surfaces biomimetics - and learning from these biological concepts - bioinspiration - have led to important advances in the manufacturing and design of synthetic interfacial materials in recent years.
Biomimetic and Bioinspired Materials Maturation of high resolution microscopy techniques, together with rapid
advancement in micro- and nanomanufacturing, have enabled scientists and engineers to not only uncover the secrets of functional natural interfacial materials, but also to manufacture these functional surfaces using a broad spectrum of synthetic materials. With these collective advances, the field of biomimetics and bioinspiration, particularly the development of interfacial materials, has progressed tremendously during the last decade.(25-27) In the first article of this issue, Jiang and Wang et al. provide a comprehensive overview of the recent development of bioinspired materials with special wettability, ranging from the superior water-walking ability of water striders, the directional adhesion of butterfly wings, the antifogging functionality of mosquito eyes, the water collection of the cactus and spider silk, to the underwater self-cleaning ability of fish scales.

Among these many biomimetic studies, the lotus effect has been the most widely studied and investigated, and has accounted for >1000 journal papers published in the last decade alone (Figure 2). This reflects the remarkable interest and need to create highly liquid-repellent materials. Since these bioinspired materials utilize structured surfaces to achieve their fluid repellency, it is instructive to look at some of the fundamental theories and terminologies for wetting on structured surfaces.

Wetting on Structured Surfaces When a liquid droplet is deposited on a smooth solid surface in air, three
distinctive interfacial boundaries arise that intersect at a well-defined contact angle, θ (Figure 3a). Competition between the adhesion forces of the liquid, vapor and solid molecules (or atoms) results in a force equilibrium at the triple line (the line where all three phases meet),(28) which can be described by the Young’s equation

g LV cosq = g SV - g SL ,

(1)

where γLV, γSV, and γSL are the surface tensions for liquid-vapor, solid-vapor, and solid-liquid interfaces, respectively, and θ is the intrinsic contact angle at the triple line with the solid surface. By convention, if ������ ≥ 90%, then the solid is said to “hate” the fluid droplet (hydrophobic for the case of water). Likewise, if ������ < 90%, then the

solid is said to “like” the fluid droplet (hydrophilic for the case of water). However, real surfaces are rarely smooth. The contact angles of liquid droplets
observed (or apparent contact angles, θ*) on these real surfaces typically deviate significantly from those described by the Young’s equation. Wetting of liquid droplets on structured surfaces can be roughly described by two distinct modes. In the first wetting mode, the liquid closely follows the topography of the surface forming a continuous liquid-solid interface (Figure 3b). The apparent contact angle can be described by the Wenzel equation developed in 1936

cos ������∗ = ������ cos ������,

(2)

where R is the roughness factor, defined as the ratio between the actual surface area

and the projected surface area of the solid.(29) The Wenzel equation indicates that

roughness can amplify the wettability of a solid. For example, if the solid is

intrinsically hydrophobic, roughness will further enhance the surface hydrophobicity

(i.e., ������∗ > ������ for R > 1).

In the second mode of wetting, the liquid does not follow the topography of

the solid surface; instead the liquid is suspended on a mixed interface composed of

surface protrusions with air pockets trapped between them (Figure 3c). The apparent

contact angle in this mode was first described by the Cassie-Baxter equation in

1944,(30) and was further extended by Cassie to heterogeneous surfaces in 1948,(31)

cos ������∗ = ������/ cos ������/ + ������1 cos ������1,

(3)

where A1 and A2 are area fractions (i.e., A1 + A2 = 1), and θ1 and θ2 are the intrinsic

contact angles of materials 1 and 2, respectively. The Cassie equation indicates that to

achieve a perfect non-wetting situation (i.e., ������∗~	180%), one can maximize the area

fraction of the air pockets trapped beneath the liquid droplet. The concept put forth by

Cassie and Baxter explained the large contact angles observed in many of the plant

and animal surfaces, such as lotus leaves.(32) In addition to the surface energy model

proposed by Cassie and Baxter, recent experimental and theoretical studies have

highlighted the importance of the topography length scale of the surface roughness

(i.e., line energy) to the role of surface wettability.(33-37)

Achieving a high apparent contact angle can reduce the normal adhesion of a

liquid droplet with the solid surface due to reduction of the liquid-solid contact area.

However, contact angle alone does not quantify the resistance to liquid motion in the

direction tangential to the surface.(33, 38-40) In particular, liquids sitting on rough

surfaces exhibit a variety of contact angles bounded by two extreme values. The upper

limit is known as the advancing contact angle, ������6, whereas the lower limit is referred to as the receding contact angle, ������7. The difference between these values is known as contact angle hysteresis, Δθ, whose physical origin is attributed to pinning of the

liquid contact line (CL) on the nanoscopic surface roughness.(41-44) The presence of

the contact angle hysteresis gives rise to a surface retention force, FR, that resists the

motion of a liquid droplet of a characteristic length, L, i.e., (39)

������9 = ������;<������(cos ������9 − cos ������@).

(4)

Therefore, minimizing the hysteresis is the key to minimizing resistance to motion,

resulting in high mobility of the droplets and therefore in significantly improved

liquid-repellency of the surface.

By convention, we describe a material as superhydrophobic if it displays an

apparent contact angle for water of ≥150o with contact angle hysteresis < ~5o – 10o. If

the material displays similar values with oils, we describe the surface as

superoleophobic. If the material meets these criteria for both water and oils, we term

the material as superomniphobic or superamphiphobic (Table 1).

Extreme Fluid Repellency Lotus leaves have exceptional ability to repel water but not oils; therefore
these natural materials are only superhydrophobic. After more than a decade of research and development, we now have many different ways to create synthetic superhydrophobic surfaces,(45-48) but creating materials that are both superhydrophobic and superoleophobic (i.e. superomniphobic) based on the lotus-leaf model has proved more difficult. A fundamental reason is that oils have intrinsically low surface tension, which makes them prone to wet the micro/nanoscopic surface textures more readily than liquids of higher surface tension, thereby displacing the air pockets trapped in between the surface textures and leading to significant liquid pinning.
Despite the challenges, recent efforts have shown that by carefully engineering the surface textures with overhanging features, it is possible to create superomniphobic materials that can repel both water and oils.(49-52) The novelty behind these surfaces is the creation of a local re-entrant curvature such that droplet pinning at the edges of the micro/nanoscopic overhanging structures prevents further penetration. This development has further advanced the capabilities of lotus-leaf inspired surfaces to repel not only water, but also a much broader range of fluids.(53)

In the second article of this issue, Tuteja and Choi et al. discuss the recent advances of superomniphobic surfaces and their durability issues. It is interesting to note that springtails also possess similar overhanging nanoscale textured patterns to protect themselves from soiling (Figure 1b).(20) These natural surfaces were shown to resist wetting of many organic liquids and at elevated pressures, and demonstrate a number of similarities to their artificial counterparts.(49-51, 53)
Anisotropic Fluid Repellency In addition to lotus leaves, which display a high level of omni-directional
water repellency, a number of biological surfaces are able to shed water only in a specific direction – known as anisotropic wetting. For example, the wings of butterflies can shed water droplets easily along the radial outward direction away from their wings, but not in the opposite direction.(16) The legs of water striders are covered with tiny oriented hairs with fine nanogrooves that allow them to propel the strider efficiently on water surface.(11, 54) Another example can be found on rice leaves that consist of one-dimensional arrays of oriented micro/nanotextures that enable the transport of water droplets in a particular direction.(9) Central to these biological surfaces are the orientations and arrangements of the surface textures that provide precise control over the direction of droplet motion. Inspirations from these natural anisotropic surfaces have led to artificial surfaces that display similar anisotropic wetting behaviors.(55-57) In the third article of this issue, Hancock and Demirel summarize recent experimental and theoretical progress in the design, synthesis, and characterization of engineered surfaces that demonstrate anisotropic wetting properties, as well as some of their potential applications.
Towards Industrial Applications in Extreme Environments In addition to the fundamental research on these synthetic bioinspired
materials, important advances have been made in understanding how these materials could be utilized in various industrial applications under different environmental conditions, particularly in industrial processes that involve phase changes such as

condensation (58-62) and icing (63-70). On one hand, vapor condensation is commonly encountered in power generation, thermal management, and desalination plants; on the other hand, ice formation and accretion present serious economic and safety issues for essential infrastructures such as aircraft, power lines, wind turbines, and commercial and residential refrigerators and freezers. Passive coatings that can effectively remove condensed vapor and/or reduce ice adhesion are thus critically needed. In the fourth article of the issue, recent developments in the use of superhydrophobic surfaces for condensation control are discussed by Miljkovic and Wang from an academic research perspective. In the last article of the issue, Alizadeh et al. discuss how some of these bioinspired materials can contribute to the effective removal of condensed vapor and ice from an industrial viewpoint.
Outlook One of the ultimate goals in the field of bioinspired interfacial materials is to
create a robust, scalable, and low-cost surface that can repel any fluids, self-heal upon damage, allow for smart/switchable control of wettability, and operate under a wide range of environmental conditions, such as extreme temperatures, high pressures, and harsh chemicals. As discussed here, cutting-edge development of synthetic liquidrepellent surfaces has primarily been modeled after the lotus-effect, with many important advances made over the last decade (Figure 4). Some of these lotus leafinspired surfaces have been designed to repel both aqueous and organic liquids,(4953), others can be manufactured from low-cost (such as plastics)(71) or mechanically robust (such as ceramic) materials,(72), yet another set of studies demonstrated switchable wettability,(12, 73-76) partially self-healing capability,(77-79) or the ability to operate under moderate pressure (up to ~7 atm).(80) However, these impressive properties, where present, have been demonstrated separately on different materials, rather than integrated into a single material. Thus many of these surfaces face severe limitations to their practical applications: they show limited oleophobicity

with high contact angle hysteresis; fail under high pressure (81) and upon any physical damage; and/or cannot completely self-heal.
Very recently, a conceptually different approach to creating liquid-repellent materials – inspired by the slippery Nepenthes pitcher plants – was developed that may potentially address many of the challenges found in the lotus leaf-inspired surfaces (Table 2). The new bioinspired material consists of a continuous film of lubricating liquid locked in place by a micro/nanostructured substrate (Figure 3d), and is termed as Slippery Liquid-Infused Porous Surfaces (SLIPS),(82) or slippery presuffused surfaces (83) or lubricant-impregnated surfaces (84, 85). The liquid-infused structured surface outperforms its natural counterparts and state-of-the-art synthetic surfaces in its ability to repel various simple and complex liquids (water, crude oil, and blood); maintain low contact angle hysteresis (<2.5o); restore liquid-repellency after physical damage rapidly (within 0.1-1 s); function at high pressures (up to ~676 atm); resist bacterial bio-fouling (86) and ice adhesion (87, 88); enhance condensation (84); and switch wettability in response to mechanical stimuli (89) (see Table 2). Since these properties can all be incorporated into a single material, the slippery surfaces can potentially be used in a wide variety of applications and environments (90), and may provide alternative solutions for designing materials with special wettability that could not be addressed by conventional lotus leaf-inspired surfaces.
Ultimately, the widespread application of any of the aforementioned bioinspired interfacial materials is dictated by their cost, scalability, and robustness, which are important for their practical use on a large scale and accessibility to people with low budgets and around the world. While promising results have been demonstrated for many of these bioinspired materials, continuing research is necessary to bring down the material and fabrication costs, as well as to enhance their longevity and robustness without compromising their functional performances.

Acknowledgments The authors would like to thank Walter Federle and Holger Bohn for
providing the image of the pitcher plant. We thank Alison Grinthal for the help with manuscript preparation. JA and TSW would also like to acknowledge the funding support by the Office of Naval Research MURI grant under award No. N00014-12-10875.

Figures & Figure Captions
Figure 1. The most repellent biological surfaces in nature. (a) A lotus leaf, known for its exceptional water repellency enabled by hierarchical micro/nano-structures (see inset). Scale bar = 10 µm; (b) A springtail, which can resist wetting by organic liquids and at elevated pressures as enabled by overhanging nanostructures (see inset). Scale bar = 500 nm; (c) A pitcher plant, which utilizes a highly slippery, liquid-infused micro-structured peristome to capture prey. Inset shows the microstructures on the peristome. All images are reproduced with permission from the Creative Commons Licenses of (20), (91) Pitcher plant image provided courtesy of W. Federle and H. Bohn.

SPIDER WEB: 104 (2.9%) MOSQUITO: 109 (3.0%)

FISH: 64 (1.8%)

PITCHER PLANT: 59 (1.6%)

CICADA: 172 (4.8%)

LOTUS: 1570 (43.7%)

WATER STRIDER: 609 (17.0%)

LOTUS (1997) WATER STRIDER (2004) DESERT BEETLE (2001) ROSE (2008) CICADA (2004) BUTTERFLY (2006) GECKO (2005) GREEN LACEWING (1996) MOSQUITO (2007) SPIDER WEB (2010) FISH (2008) PITCHER PLANT (2004)

Figure 2. Citations of key papers in biomimicry studies related to interfacial materials with special wettability from the years 2002 to 2012. Citation data obtained from ISI Web of Knowledge provided by Thomson Reuters.

a
Air Liquid
θ Solid

bc

d

θ* Structured Solid

θ* Structured Solid

Lubricant Structured Solid

Figure 3. Wetting on smooth and structured surfaces. A liquid droplet sitting on (a) a smooth surface with an intrinsic contact angle, θ; (b) a textured surface that is completely wetted by the liquid, known as a Wenzel state droplet; (c) a textured surface with trapped air pockets, known as a Cassie state droplet; (d) a textured surface that is infused with an immiscible lubricating fluid (or slippery liquid-infused surfaces).

Challenges Addressed: Omni-repellency Self-healing/Robustness Switchable/Anisotropy Pressure Stability Cost/Scalability
2004 Switchable Superhydrophobic Surfaces

1997 The Lotus Effect 2003 Superhydrophobic Surfaces on Plastics

2007 – 2008 Superoleophobic Surfaces

2010 Self-healing Superhydrophobic Surfaces

2010 Anisotropic Wetting Surfaces

2011 Liquid-Infused Slippery Surfaces
2012 Optically Transparent Superamphiphobic Surfaces

2011 – 2012 Reversible Wenzel-to-Cassie Switching of Superhydrophobic Surfaces

2013

2013

Switchable

Ceramic-based

Omniphobic Surfaces

Superhydrophobic Surfaces

YEAR

Figure 4. Timeline of key materials innovations and developments in bioinspired liquid repellent surfaces in the past decade (2003 – 2013).(6, 49-52, 55, 56, 71-73, 80, 82, 83, 89, 92) Note that this timeline only covers the materials development, and does not include the key fundamental theoretical/computational/experimental discoveries during the period. Readers are referred to the recent reviews on these topics by Quéré (4), Marmur (93), Nosonovsky and Bhushan (94), and Bormashenko (95).

Table 1. Classification of liquid repellent states
State Superhydrophobic Superoleophobic Omniphobic

Liquids θ* (o) Δθ* (o)

Water ≥ 150o ≤ 5 – 10o

Oils ≥150o ≤ 5 – 10o

Water & Oils < 150o
≤ 5 – 10o

Superomniphobic/ Superamphiphobic
Water & Oils ≥150o
≤ 5 – 10o

Table 2. A comparison matrix between the performance of SLIPS and the best available parameters of the lotus leaf-inspired superhydrophobic surfaces published in the literature

Technology Contact Angle Hysteresis (o)

Dynamic Pressure
(atm)

Static Pressure
(atm)

SelfHealing
(sec)

Ice adhesion
(kPa)

SLIPS
Lotus-leafInspired Surfaces

< 2.5o(82) (water & oils)
~ 10o – 30o (oils)(51)
< 5o (water) (51)

> 0.05(82) (water & oils)
~0.01 (oils)(52) >0.05(water)(96
)

676(82) (water & oils)
7 (water)(80)

~0.15(82) ~180(78)

~15(87)
~Order of 100 or above(65)

References
1. P. G. de Gennes, Wetting - Statics and Dynamics. Reviews of Modern Physics 57, 827 (1985).
2. P.-G. de Gennes, F. Brochard-Wyart, D. Quere, Capillarity and Wetting Phenomena : Drops, Bubbles, Pearls, Waves (Springer, New York, 2004).
3. Y. Pomeau, E. Villermaux, Two hundred years of capillarity research. Physics Today 59, 39 (Mar, 2006).
4. D. Quere, Wetting and roughness. Annual Review of Materials Research 38, 71 (2008).
5. T. Wagner, C. Neinhuis, W. Barthlott, Wettability and contaminability of insect wings as a function of their surface sculptures. Acta Zoologica 77, 213 (Jul, 1996).
6. W. Barthlott, C. Neinhuis, Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1 (May, 1997).
7. C. Neinhuis, W. Barthlott, Characterization and distribution of water-repellent, self-cleaning plant surfaces. Annals of Botany 79, 667 (1997).
8. A. R. Parker, C. R. Lawrence, Water capture by a desert beetle. Nature 414, 33 (Nov 01, 2001).
9. L. Feng et al., Super-hydrophobic surfaces: From natural to artificial. Adv Mater 14, 1857 (Dec 17, 2002).
10. H. F. Bohn, W. Federle, Insect aquaplaning: Nepenthes pitcher plants capture prey with the peristome, a fully wettable water-lubricated anisotropic surface. P Natl Acad Sci USA 101, 14138 (Sep 28, 2004).
11. X. F. Gao, L. Jiang, Water-repellent legs of water striders. Nature 432, 36 (Nov 4, 2004).
12. T. L. Sun, L. Feng, X. F. Gao, L. Jiang, Bioinspired surfaces with special wettability. Accounts of Chemical Research 38, 644 (Aug, 2005).
13. W. R. Hansen, K. Autumn, Evidence for self-cleaning in gecko setae. P Natl Acad Sci USA 102, 385 (Jan 11, 2005).
14. D. L. Hu, J. W. M. Bush, Meniscus-climbing insects. Nature 437, 733 (Sep 29, 2005).
15. X. F. Gao et al., The dry-style antifogging properties of mosquito compound eyes and artificial analogues prepared by soft lithography. Adv Mater 19, 2213 (Sep 3, 2007).
16. Y. Zheng, X. Gao, L. Jiang, Directional adhesion of superhydrophobic butterfly wings. Soft Matter 3, 178 (2007).
17. L. Feng et al., Petal effect: A superhydrophobic state with high adhesive force. Langmuir 24, 4114 (Apr 15, 2008).
18. M. J. Liu, S. T. Wang, Z. X. Wei, Y. L. Song, L. Jiang, Bioinspired Design of a Superoleophobic and Low Adhesive Water/Solid Interface. Adv Mater 21, 665 (Feb 9, 2009).
19. Y. M. Zheng et al., Directional water collection on wetted spider silk. Nature 463, 640 (Feb 4, 2010).

20. R. Helbig, J. Nickerl, C. Neinhuis, C. Werner, Smart Skin Patterns Protect Springtails. Plos One 6, (Sep 30, 2011).
21. N. J. Mlot, C. A. Tovey, D. L. Hu, Fire ants self-assemble into waterproof rafts to survive floods. P Natl Acad Sci USA 108, 7669 (May 10, 2011).
22. C. Duprat, S. Protiere, A. Y. Beebe, H. A. Stone, Wetting of flexible fibre arrays. Nature 482, 510 (Feb 23, 2012).
23. J. Ju, Y. Zheng, T. Zhao, R. Fang, L. Jiang, A multi-structural and multifunctional integrated fog collection system in cactus. Nat Commun 3, 1247 (04 December 2012, 2012).
24. Y. Forterre, J. M. Skotheim, J. Dumais, L. Mahadevan, How the Venus flytrap snaps. Nature 433, 421 (Jan 27, 2005).
25. N. F. Lepora, P. Verschure, T. J. Prescott, The state of the art in biomimetics. Bioinspiration and Biomimetics 8, (2013).
26. J. Genzer, A. Marmur, Biological and synthetic self-cleaning surfaces. MRS Bull 33, 742 (Aug, 2008).
27. B. Bhushan, Biomimetics: lessons from nature - an overview. Philos T R Soc A 367, 1445 (Apr 28, 2009).
28. T. Young, An essay on the cohesion of fluids. Philosophical Transactions of the Royal Society of London 95, 65 (1805).
29. R. N. Wenzel, Resistance of solid surfaces to wetting by water. Industrial and Engineering Chemistry 28, 988 (1936).
30. A. B. D. Cassie, S. Baxter, Wettability of porous surfaces. Transactions of the Faraday Society 40, 0546 (1944).
31. A. B. D. Cassie, Contact Angles. Discussions of the Faraday Society 3, 11 (1948).
32. A. B. D. Cassie, S. Baxter, Large contact angles of plant and animal surfaces. Nature 155, 21 (1945).
33. D. Oner, T. J. McCarthy, Ultrahydrophobic surfaces. Effects of topography length scales on wettability. Langmuir 16, 7777 (Oct 3, 2000).
34. C. W. Extrand, Criteria for ultralyophobic surfaces. Langmuir 20, 5013 (Jun 8, 2004).
35. C. Dorrer, J. Ruhe, Advancing and receding motion of droplets on ultrahydrophobic post surfaces. Langmuir 22, 7652 (Aug 29, 2006).
36. T. S. Wong, C. M. Ho, Dependence of Macroscopic Wetting on Nanoscopic Surface Textures. Langmuir 25, 12851 (Nov 17, 2009).
37. E. Bormashenko, General equation describing wetting of rough surfaces. J Colloid Interf Sci 360, 317 (Aug 1, 2011).
38. R. H. Dettre, R. E. Johnson, Contact angle hysteresis .4. Contact angle measurements on heterogeneous surfaces. Journal of Physical Chemistry 69, 1507 (1965).
39. C. G. Furmidge, Studies at phase Interfaces .1. Sliding of liquid drops on solid surfaces and a theory for spray retention. Journal of Colloid Science 17, 309 (1962).

40. W. Chen et al., Ultrahydrophobic and ultralyophobic surfaces: Some comments and examples. Langmuir 15, 3395 (May 11, 1999).
41. J. W. Gibbs, The scientific papers of J. Willard Gibbs. (Dover Publications, New York, ed. New Dover ed., 1961).
42. J. F. Oliver, C. Huh, S. G. Mason, Resistance to spreading of liquids by sharp edges. J Colloid Interf Sci 59, 568 (1977).
43. T. Ondarcuhu, A. Piednoir, Pinning of a contact line on nanometric steps during the dewetting of a terraced substrate. Nano Lett 5, 1744 (Sep, 2005).
44. T. S. Wong, A. P. H. Huang, C. M. Ho, Wetting Behaviors of Individual Nanostructures. Langmuir 25, 6599 (Jun 16, 2009).
45. D. Quere, Non-sticking drops. Reports on Progress in Physics 68, 2495 (Nov, 2005).
46. X. M. Li, D. Reinhoudt, M. Crego-Calama, What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces. Chem Soc Rev 36, 1529 (2007).
47. P. Roach, N. J. Shirtcliffe, M. I. Newton, Progess in superhydrophobic surface development. Soft Matter 4, 224 (2008).
48. C. Dorrer, J. Ruhe, Some thoughts on superhydrophobic wetting. Soft Matter 5, 51 (2009).
49. A. Tuteja et al., Designing superoleophobic surfaces. Science 318, 1618 (Dec 7, 2007).
50. A. Ahuja et al., Nanonails: A simple geometrical approach to electrically tunable superlyophobic surfaces. Langmuir 24, 9 (Jan 1, 2008).
51. A. Tuteja, W. Choi, J. M. Mabry, G. H. McKinley, R. E. Cohen, Robust omniphobic surfaces. P Natl Acad Sci USA 105, 18200 (Nov 25, 2008).
52. X. Deng, L. Mammen, H. J. Butt, D. Vollmer, Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating. Science 335, 67 (Jan 6, 2012).
53. S. Pan, A. K. Kota, J. M. Mabry, A. Tuteja, Superomniphobic Surfaces for Effective Chemical Shielding. J Am Chem Soc 135, 578 (2013).
54. D. L. Hu, B. Chan, J. W. M. Bush, The hydrodynamics of water strider locomotion. Nature 424, 663 (Aug 7, 2003).
55. K. H. Chu, R. Xiao, E. N. Wang, Uni-directional liquid spreading on asymmetric nanostructured surfaces. Nature Materials 9, 413 (May, 2010).
56. N. A. Malvadkar, M. J. Hancock, K. Sekeroglu, W. J. Dressick, M. C. Demirel, An engineered anisotropic nanofilm with unidirectional wetting properties. Nature Materials 9, 1023 (Dec, 2010).
57. M. J. Hancock, K. Sekeroglu, M. C. Demirel, Bioinspired Directional Surfaces for Adhesion, Wetting, and Transport. Adv Funct Mater 22, 2223 (Jun 6, 2012).
58. C. H. Chen et al., Dropwise condensation on superhydrophobic surfaces with two-tier roughness. Appl Phys Lett 90, (Apr 23, 2007).
59. J. B. Boreyko, C. H. Chen, Self-Propelled Dropwise Condensate on Superhydrophobic Surfaces. Phys Rev Lett 103, (Oct 30, 2009).

60. R. Enright, N. Miljkovic, A. Al-Obeidi, C. V. Thompson, E. N. Wang, Condensation on Superhydrophobic Surfaces: The Role of Local Energy Barriers and Structure Length Scale. Langmuir 28, 14424 (Oct 9, 2012).
61. N. Miljkovic, R. Enright, E. N. Wang, Effect of Droplet Morphology on Growth Dynamics and Heat Transfer during Condensation on Superhydrophobic Nanostructured Surfaces. Acs Nano 6, 1776 (Feb, 2012).
62. N. Miljkovic et al., Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces. Nano Lett 13, 179 (Jan, 2013).
63. L. L. Cao, A. K. Jones, V. K. Sikka, J. Z. Wu, D. Gao, Anti-Icing Superhydrophobic Coatings. Langmuir 25, 12444 (Nov 3, 2009).
64. S. A. Kulinich, M. Farzaneh, Ice adhesion on super-hydrophobic surfaces. Appl Surf Sci 255, 8153 (Jun 30, 2009).
65. A. J. Meuler et al., Relationships between Water Wettability and Ice Adhesion. Acs Appl Mater Inter 2, 3100 (Nov, 2010).
66. L. Mishchenko et al., Design of Ice-free Nanostructured Surfaces Based on Repulsion of Impacting Water Droplets. Acs Nano 4, 7699 (Dec, 2010).
67. K. K. Varanasi, T. Deng, J. D. Smith, M. Hsu, N. Bhate, Frost formation and ice adhesion on superhydrophobic surfaces. Appl Phys Lett 97, (Dec 6, 2010).
68. M. He et al., Super-hydrophobic film retards frost formation. Soft Matter 6, 2396 (2010).
69. S. A. Kulinich, S. Farhadi, K. Nose, X. W. Du, Superhydrophobic Surfaces: Are They Really Ice-Repellent? Langmuir 27, 25 (Jan 4, 2011).
70. V. Bahadur et al., Predictive Model for Ice Formation on Superhydrophobic Surfaces. Langmuir 27, 14143 (Dec 6, 2011).
71. H. Y. Erbil, A. L. Demirel, Y. Avci, O. Mert, Transformation of a simple plastic into a superhydrophobic surface. Science 299, 1377 (Feb 28, 2003).
72. G. Azimi, R. Dhiman, H.-M. Kwon, A. T. Paxson, K. K. Varanasi, Hydrophobicity of rare-earth oxide ceramics. Nature Materials, (2013).
73. T. L. Sun et al., Reversible switching between superhydrophilicity and superhydrophobicity. Angew Chem Int Edit 43, 357 (2004).
74. A. Sidorenko, T. Krupenkin, A. Taylor, P. Fratzl, J. Aizenberg, Reversible switching of hydrogel-actuated nanostructures into complex micropatterns. Science 315, 487 (Jan 26, 2007).
75. A. Sidorenko, T. Krupenkin, J. Aizenberg, Controlled switching of the wetting behavior of biomimetic surfaces with hydrogel-supported nanostructures. J Mater Chem 18, 3841 (2008).
76. A. Grigoryev, T. Tokarey, K. G. Kornev, I. Luzinov, S. Minko, Superomniphobic Magnetic Microtextures with Remote Wetting Control. J Am Chem Soc 134, 12916 (Aug 8, 2012).
77. Y. Li, L. Li, J. Q. Sun, Bioinspired Self-Healing Superhydrophobic Coatings. Angew Chem Int Edit 49, 6129 (2010).
78. H. Wang et al., Durable, Self-Healing Superhydrophobic and Superoleophobic Surfaces from Fluorinated-Decyl Polyhedral Oligomeric Silsesquioxane and Hydrolyzed Fluorinated Alkyl Silane. Angewandte Chemie 50, 11433 (2011).

79. X. L. Wang, X. J. Liu, F. Zhou, W. M. Liu, Self-healing superamphiphobicity. Chem Commun 47, 2324 (2011).
80. C. Lee, C. J. Kim, Underwater Restoration and Retention of Gases on Superhydrophobic Surfaces for Drag Reduction. Phys Rev Lett 106, (Jan 7, 2011).
81. R. Poetes, K. Holtzmann, K. Franze, U. Steiner, Metastable Underwater Superhydrophobicity. Phys Rev Lett 105, (Oct 14, 2010).
82. T. S. Wong et al., Bioinspired self-repairing slippery surfaces with pressurestable omniphobicity. Nature 477, 443 (Sep 22, 2011).
83. A. Lafuma, D. Quere, Slippery pre-suffused surfaces. Epl-Europhys Lett 96, (Dec, 2011).
84. S. Anand, A. T. Paxson, R. Dhiman, J. D. Smith, K. K. Varanasi, Enhanced Condensation on Lubricant-Impregnated Nanotextured Surfaces. Acs Nano 6, 10122 (Nov, 2012).
85. J. D. Smith et al., Droplet mobility on lubricant-impregnated surfaces. Soft Matter 9, 1772 (2013).
86. A. K. Epstein, T. S. Wong, R. A. Belisle, E. M. Boggs, J. Aizenberg, Liquidinfused structured surfaces with exceptional anti-biofouling performance. P Natl Acad Sci USA 109, 13182 (Aug 14, 2012).
87. P. Kim et al., Liquid-Infused Nanostructured Surfaces with Extreme Anti-Ice and Anti-Frost Performance. Acs Nano 6, 6569 (Aug, 2012).
88. H. A. Stone, Ice-Phobic Surfaces That Are Wet. Acs Nano 6, 6536 (Aug, 2012).
89. X. Yao et al., Adaptive fluid-infused porous films with tunable transparency and wettability. Nature Materials, in press (2013).
90. M. Nosonovsky, Materials Science Slippery When Wetted. Nature 477, 412 (Sep 22, 2011).
91. H. J. Ensikat, P. Ditsche-Kuru, C. Neinhuis, W. Barthlott, Superhydrophobicity in perfection: the outstanding properties of the lotus leaf. Beilstein J Nanotech 2, 152 (Mar 10, 2011).
92. T. Verho et al., Reversible switching between superhydrophobic states on a hierarchically structured surface. P Natl Acad Sci USA 109, 10210 (Jun 26, 2012).
93. A. Marmur, Solid-Surface Characterization by Wetting. Annual Review of Materials Research 39, 473 (2009).
94. M. Nosonovsky, B. Bhushan, Superhydrophobic surfaces and emerging applications: Non-adhesion, energy, green engineering. Curr Opin Colloid In 14, 270 (Aug, 2009).
95. E. Bormashenko, Wetting transitions on biomimetic surfaces. Philos T R Soc A 368, 4695 (Oct 28, 2010).
96. T. P. N. Nguyen, P. Brunet, Y. Coffinier, R. Boukherroub, Quantitative testing of robustness on superomniphobic surfaces by drop impact. Langmuir 26, 18369 (2010).

Author biographies Joanna Aizenberg School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA; tel. 617-495-3558; and email jaiz@seas.harvard.edu
Aizenberg is the Amy Smith Berylson Professor of Materials Science and the Director of the Kavli Institute for Bionano Science and Technology at Harvard University. She pursues a broad range of research interests that include biomimetics, self-assembly, smart materials, crystal engineering, surface science, and biooptics. She received the B.S. degree in Chemistry from Moscow State University, and the Ph.D. degree in Structural Biology from the Weizmann Institute of Science. Prior to her appointment at Harvard, Aizenberg spent a decade at Bell Laboratories. Her selected awards include: R&D 100 Award for best innovation in 2012; Fred Kavli Distinguished Lectureship in Nanoscience, MRS 2009; Ronald Breslow Award for the Achievement in Biomimetic Chemistry, ACS 2008; Industrial Innovation Award, ACS 2007; Arthur K. Doolittle Award, ACS 1999. Aizenberg is an AAAS and APS Fellow; she has served on the Board of Directors of the Materials Research Society and on the Board on Physics and Astronomy of the National Academies.

Lin Feng Department of Chemistry, Tsinghua University, Beijing, P.R. China; email fl@mail.tsinghua.edu.cn
Feng is an Assistant Professor of Chemistry, Tsinghua University. She received her Ph. D. degree in Institute of Chemistry, Chinese Academy of Sciences in 2003. She was engaged her post-doc research work at Institute of Physics, Chinese Academy of Sciences (2005) and worked as a visiting scholar in University of California, Berkeley (2009). Her research interests are focused on Biomimic functional nano-scale interfacial materials, nano-scale interfacial materials for oil/water separation and the research of nanocatalyst.

Taolei Sun State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P.R. China; email suntl@whut.edu.cn
Sun joined Wuhan University of Technology in 2009. He is currently “Changjiang Scholar” Distinguished Professor and Chair Professor at the State Key Laboratory. After earning his PhD degree at the Chinese Academy of Science in 2002 and two years of postdoctoral research, he worked in Muenster University as the Humboldt Fellow. One year later (2006), he was awarded the Sofja Kovalevskaja Award by Alexander von Humboldt Foundation in recognition of his scientific contribution on functional biointerface materials, supporting him to build the “Bio- & Nano-interface” research group working as the group leader until 2011.

Tak-Sing Wong Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA, USA; tel. 814-865-6122; and email tswong@psu.edu
Wong is an Assistant Professor of Mechanical Engineering at The Pennsylvania State University. He received his PhD degree from the University of California, Los Angeles (2009) and his B.Eng. degree from The Chinese University of Hong Kong (2003), both in mechanical engineering. He was a Croucher Postdoctoral Fellow at the Wyss Institute for Biologically Inspired Engineering at Harvard University (2010 – 2012). His research group is interested in utilizing biologically inspired strategies to design functional interfacial materials for various energy, biomedical, and industrialrelated applications. Wong is a recipient of the 2012 R&D 100 award for the invention of Slippery Liquid-Infused Porous Surfaces (SLIPS).