Materials science

Slippery when wetted

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The slick interior of the pitcher plant has inspired a slippery material possessing self-lubricating, self-cleaning and self-healing properties. The secret is to infuse a porous material with a liquid that repels oils and water. See Letter p.443

The legendary water repellence of lotus leaves has inspired a field of research aimed at making similarly 'superhydrophobic' surfaces. But it's much more difficult to make omniphobic materials, which repel oils as well as water. On page 443 of this issue, Wong et al.1 report a radical new approach to making omniphobic surfaces that was inspired by another member of the plant family: the insect-eating pitcher plant.

An ancient Indian poem, the Bhagavad Gita, has this to say about a seeker of truth: “Having abandoned attachment, he acts untainted by evil, just as a lotus leaf is not wetted.” Here, as in many cultures, the lotus is used as a symbol of purity because of its ability to emerge clean from muddy water. Examination of lotus leaves in the 1990s using scanning probe microscopy2 revealed that this ability is a result of the leaves' surface microstructure: each leaf is covered with tiny bumps called papillae. When the surface is wetted by water, a so-called composite solid–air–liquid interface forms in which water sits atop pockets of air trapped between the papillae (Fig. 1a). This drastically reduces the solid–water contact area, so that water droplets form an almost perfect sphere and easily roll on the surface, washing away dust in the process. This superhydrophobic behaviour is often referred to as the lotus effect.

Figure 1: Forms of surface wetting.
figure1

a, The Cassie–Baxter model of surface wetting proposes that water droplets sitting on rough surfaces form a solid–air–water interface. Air pockets trapped beneath the droplet reduce the contact between the water and the surface. If the surface features are of the right size and are regular, then the surface becomes water repellent. b, In the Wenzel model, no air pockets form, and so the surface is completely wetted by the droplet. c, Wong et al.1 have made porous materials that are infused with, and wetted by, a liquid lubricant that repels water and oils. The surface features do not themselves generate water repellence, but serve to hold the lubricant in place.

Different models that connect surface roughness with water wetting have been proposed, including the Cassie–Baxter model3 (which describes a three-phase solid–air–liquid composite interface; Fig. 1a), and the Wenzel model4 (which invokes a simpler two-phase system in which no air pockets are trapped between the solid and the water; Fig. 1b). The Cassie–Baxter model explains superhydrophobicity, and has helped in the development of techniques for structuring the surfaces of different materials to mimic the lotus effect. These materials find applications in the field of tribology — the study of friction, wear and lubrication — and in other areas of engineering.

Making superhydrophobic surfaces is a challenge, but it is far more difficult to produce oleophobic surfaces that repel organic liquids such as oils. This is because oil molecules are nonpolar, and have a much lower surface energy than polar water molecules; it is therefore not energetically favourable for oil droplets to form as spheres on a solid surface. Omniphobic surfaces that are both oleophobic and hydrophobic are highly desirable for many applications — they could, for example, be used to prevent dirt from collecting on optical devices or to prevent moving parts in micrometre-scale devices from sticking to each other. Previous approaches for making oleophobic and omniphobic surfaces have involved the design of complicated surface geometries to prevent oil from penetrating into valleys between bumps5.

Wong et al.1 suggest a new approach, inspired by the surface of the insect-eating pitcher plant Nepenthes. The plant captures prey using a slippery, water-lubricated surface — insects that step on the surface at the rim of the pitcher slide down into digestive juices at the bottom6. The surface is slippery because the lubricant forms a continuous film that repels oils on the insects' feet. Although the surface has microstructures, these are irregular (unlike those of lotus-inspired surfaces), and serve only to hold the lubricant in place.

The authors mimicked pitcher-plant surfaces by making a sponge-like material and filling it with a lubricating liquid to create slippery liquid-infused porous surfaces (SLIPS). When a droplet of another liquid is placed on the material, a composite solid–lubricant–liquid interface is formed (Fig. 1c). The lubricant has a similar function to the air pockets in the lotus effect, but it also forms a continuous film, similar to that on the surfaces of pitcher plants. Unlike lotus-mimicking materials, SLIPS can be oleophobic, and the presence of a lubricant means that friction at the surfaces is very low. In fact, by choosing lubricants that are immiscible with both water and oils, the authors prepared SLIPS that have highly promising omniphobic properties. What's more, the SLIPS could withstand high pressures, were wear-resistant, and even healed themselves in the case of minor damage — all of which are advantages over lotus-mimicking materials.

The development of SLIPS typifies two themes that are likely to dominate the field of biomimetic and functional surfaces in coming years. The first is the integration of self-healing, self-lubricating and self-cleaning capabilities into surfaces. Friction and wear are usually viewed as causes of energy dissipation and material deterioration, but under certain circumstances they can lead to increased order at interfaces7. This can form the basis of the capabilities mentioned above. Indeed, this is what happens with the SLIPS — when the porous material in SLIPS is damaged by wear or impact, a combination of effects (chemical potential, concentration and pressure gradients) facilitates the lubricant's transport to the surface, restoring the materials' self-lubricating and self-cleaning properties.

The second theme is the idea that the wetting of rough surfaces can be more complex than is predicted by either the two-phase Wenzel model or the three-phase Cassie–Baxter model. Indeed, multi-phase interfaces involving a variety of components — solids, oils, water, lubricants, air and so on — have been identified, and show great promise for new applications such as underwater oleophobicity.

Not only are Wong and colleagues' SLIPS of fundamental interest, but they will probably also lead to the development of new materials for many applications — in biomedical devices, for example, or as coatings to prevent the icing or fouling of surfaces. Currently, the main weakness of SLIPS is their durability, which is limited by how long the lubricant stays in the pores without evaporating or leaking. Another problem is that there are strict limitations on the chemical properties of the lubricants: they must be immiscible with both water and oil, but they should also penetrate into the pores of the underlying material. The authors' preliminary studies into these issues are encouraging, but additional research is needed before applications will emerge.

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Correspondence to Michael Nosonovsky.

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Nosonovsky, M. Slippery when wetted. Nature 477, 412–413 (2011). https://doi.org/10.1038/477412a

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