Credit: © 2009 RSC

Superhydrophobic surfaces that repel liquids have several applications in self-cleaning surfaces and protective clothing. These surfaces are known to perform well with cool water (around 25 °C) owing to their unique surface roughness, but whether they can repel hot liquids remains unexplored. Now, researchers at Hong Kong Polytechnic University and University of Minnesota show that superhydrophobic surfaces cannot repel hot liquids effectively because the wettability of the surface changes when hot water comes in contact with them1.

Yuyang Liu, John Xin and colleagues compared the repellent characteristics of a natural lotus leaf, an artificial leaf with similar micro- and nanostructures, and Teflon (a well-known hydrophobic coating manufactured by DuPont) by spraying them with water and measuring how much wetting occurred on the surfaces. All the surfaces repelled cool water (∼25 °C) effectively, but not hot water (>55 °C). The figure shows cool water (left) and hot water (right) on lotus leaves. Further experiments indicated that surface roughness has an important role in repelling cool liquids, but the surface energy of a substrate is more important when dealing with hot water.

The lower surface tension of hot water also means that droplets can enter pores and fissures on rough surfaces more easily and displace the 'air cushion' that normally helps support cool water droplets. For this reason, it is suggested that to repel hot liquids surfaces should be designed to trap air.