Superhydrophobic surfaces
Introduction
Many surfaces in nature are highly hydrophobic and self-cleaning. Examples include the wings of butterflies [1] and the leaves of plants such as cabbage and Indian cress [2••]. Some undesirable plants such as gorse (Ulex europeaus) introduced into New Zealand, and many common yard weeds have waxy leaf surfaces that make wetting them with water-based herbicides very difficult. The trisiloxane superwetters [3] were developed for their remarkable ability to wet such hard-to-wet surfaces and enhance herbicide efficacy. The best known example of a hydrophobic self-cleaning surface is the leaves of the lotus plant (Nelumbo nucifera). Electron microscopy of the surface of lotus leaves shows protruding nubs about 20–40 μm apart each covered with a smaller scale rough surface of epicuticular wax crystalloids [4•]. Numerous studies have confirmed that this combination of micrometer-scale and nanometer-scale roughness, along with a low surface energy material leads to apparent WCAs > 150°, a low sliding angle and the self-cleaning effect [5•]. Surfaces with these properties are called “superhydrophobic”. However, some natural examples (the wings of some insects) do not exhibit two length scales and many studies (for example, [6••], [7]) in which nanostructured surfaces were prepared have found large contact angles and low sliding angles calling into question the necessity for a double length scale.
Since the group at Kao [8••] first demonstrated artificial superhydrophobic surfaces in the mid-1990s, a very large number of clever ways to produce rough surfaces that exhibit superhydrophobicity have been reported. Besides water repellency, other properties such as transparency and color, anisotropy, reversibility, flexibility and breathability have also been incorporated into superhydrophobic surfaces. The intention of this article is to provide readers the current status of studies on superhydrophobic surfaces, concentrating mainly on publications appearing in the past year. Useful recent reviews have been published by Nakajima et al. [5•] and Sun et al. [9•]. Quere [10••] critically discussed the surface chemistry of non-sticking surfaces from the original papers by Cassie and Wenzel to the present.
Before we go into the details, it is worth pausing to recall that the lotus plant achieves an apparent WCA > 160° and nil sliding angle using paraffinic wax crystals containing predominantly –CH2– groups. Nature does not require the lower surface energy of –CH3 groups or fluorocarbons to achieve these effects. This plainly demonstrates that extremely low surface energy is not necessary to achieve non-wetting. Rather, the ability to control the morphology of a surface on micron and nanometer length scales is the key. This decoupling of wetting from simple surface energy opens up many possibilities for engineering surfaces.
Controlling the wetting of surfaces is an important problem relevant to many areas of technology. The interest in self-cleaning surfaces is being driven by the desire to fabricate such surfaces for satellite dishes, solar energy panels, photovoltaics, exterior architectural glass and green houses, and heat transfer surfaces in air conditioning equipment. Non-wettable surfaces may also impart the ability to prevent frost from forming or adhering to the surface. The fact that liquid in contact with such a surface slides with lowered friction suggests applications such as microfluidics, piping and boat hulls. Most of these applications involve solid surfaces, but the emergence of flexible membrane forms should lead to uses in garments and barrier-membranes—both Cassie and Wenzel were originally involved in work on waterproofing textiles [11], [12]. The non-wettable character has been claimed in biomedical applications ranging from blood vessel replacement to wound management. We assume that unexpected applications will emerge as the technology to make non-wettable surfaces matures—nature uses this property in all known ecosystems from polar bears to ducks to butterflies and water-walkers to plant leaves.
Techniques to make superhydrophobic surfaces can be simply divided into two categories: making a rough surface from a low surface energy material and modifying a rough surface with a material of low surface energy.
Section snippets
Fluorocarbons
Fluorinated polymers are of particular interest due to their extremely low surface energies. Roughening these polymers in certain ways leads to superhydrophobicity directly [13], [14], [15]. For example, Zhang et al. [13] reported a simple and effective way to achieve a superhydrophobic film by stretching a poly(tetrafluoroethylene) (Teflon®) film. The extended film consisted of fibrous crystals with a large fraction of void space in the surface which was believed responsible for the
Making a rough substrate and modifying it with low surface energy materials
Methods to make superhydrophobic surfaces by roughening low surface energy materials are mostly one-step processes and have the advantage of simplicity. But they are always limited to a small set of materials. Making superhydrophobic surfaces by a totally different strategy, i.e., making a rough substrate first and then modifying it with a low surface energy material, decouples the surface wettability from the bulk properties of the material and enlarges potential applications of
Conclusions and perspectives
This is an active research field with many publications appearing each month dealing with methods to make superhydrophobic surfaces and theoretical understanding of the relationship between surface morphology and wettability/sliding. Theoretical understanding is maturing with models appearing capable of dealing with more complex forms of surface morphology. Techniques to make non-wettable solid surfaces using polymers or sol–gel chemistry have been widely documented. Other types of surfaces,
Acknowledgments
This research was supported in part by the U.S. Army through the Institute for Soldier Nanotechnologies, under Contract DAAD-19-02-D-0002 with the U.S. Army Research Office. The content does not necessarily reflect the position of the Government, and no official endorsement should be inferred. The authors would like to acknowledge the support of Dow Corning Corporation and many useful discussions with Gregory Rutledge.
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