Natural waterproofing

The natural world leaves no doubt about the marvellous things that are possible with superhydrophobic surfaces. Lotus leaves are self-cleaning because water droplets simply roll off them, picking up dirt as they go. And water striders can literally walk on water, mounting menisci like ants climbing hills¹. These are behaviours well worth mimicking — promising, in the first case, self-cleaning paints (see http://www.stocorp.com/allweb.nsf/lotusanpage), surfaces and fabrics, and in the second, water-striding micro-robots².

These properties seem to stem primarily from the micro- and nanostructured nature of the surfaces. Lotus leaves are covered with bumps called papillae, which are themselves coated with randomly oriented wax rods or tubules 100 nm or so in diameter³. The legs of water striders also have a hierarchical texture, being covered with tiny, oriented hairs with nano-grooved surfaces⁴.

The consequences of such micro-roughness have been long studied. In 1936, Robert Wenzel argued that surface roughness increases the resistance to wetting because a droplet is pinned by being impaled on the asperities⁵. And in 1944, Cassie and Baxter pointed to another effect: the trapping of air within the asperities, so that a droplet sits suspended on the peaks while the surface remains dry below⁶.

Both of these mechanisms may apply in the superhydrophobicity of micro-textured surfaces. But there has been controversy both over which situation pertains in different circumstances, and over whether the equations given by Wenzel and Cassie for predicting wetting behaviour as a function of surface contact area are correct. Lichao Gao and Thomas McCarthy argued recently that they are not, stimulating much debate⁷. Now Abraham Marmur and Eyal Bittoun propose that the experiments on which that claim was based used droplets too small — comparable to the wavelength of the roughness — for the equations to be valid anyway⁸.

Regardless of theoretical disputes, it seems clear that lotus leaves offer abundant inspiration for biomimicry. In the latest example, Bharat Bhushan and his co-workers have modelled the hierarchical structure using polymer moulds to make epoxy resin surfaces covered with pillars 10 μm across, simulating papillae, on which they deposit wax tubules self-assembled from the extracts of real plant coatings (from nasturtium and sand ryegrass)⁹. The surfaces have contact angles of about 170°: greater than those of unstructured wax films or unwaxed pillars. And the researchers can physically see Cassie's microscopic air pockets under a droplet. What's more, the hierarchical surfaces show lotus-like self-cleaning¹⁰.

But Jihua Zhang and co-workers report that Cassie dewetting can be undermined on lotus leaves by forcing water to penetrate under pressure between the papillae¹¹, something observed previously by Lafuma and Quéré¹². This forces the surface into a less hydrophobic Wenzel state, yet the transition can be reversed by drying in a stream of nitrogen. Zhang and colleagues also show how the details of the microstructure may tip the balance between Cassie and Wenzel states — an important design consideration in efforts to harness the natural engineering of wettability.