Rough surface gives drops the heave-ho

A common step in industrial cooling processes is the liquefaction of a vapor on a condenser. However, if a liquid film forms on the condenser, the cooling may be compromised. The problem can be addressed by coating the condenser with a hydrophobic material conducive to drop formation and then letting the drops slide off due to gravity. Chuan-Hua Chen of Duke University and his student Jonathan Boreyko now report a different approach. By depositing carbon nanotubes on silicon micropillars and coating both with hexadecanethiol (C₁₆H₃₄S), they engineered a rough “superhydrophobic” surface. The water drops that condensed on it were about a hundred times smaller than those on a conventional hydrophobic surface that the Duke team considered as a standard; the surface roughening offers the promise for more efficient cooling. Furthermore, as the figure and online video show, when two sufficiently large drops coalesce into a single drop, that drop literally springs off the condenser—no external prompting needed. The post-combination drop has less surface energy than do the two drops from which it forms. Most of the released surface energy is dissipated, but Chen and Boreyko observed that the vertical component of the drop’s velocity can be as much as one-sixth of the theoretical maximum. Nature has her own version of the jumping trick: Coalescence of a wet portion of a spore with a dew drop provides the energy for spore ejection in certain mushrooms. (J. B. Boreyko, C.-H. Chen, Phys. Rev. Lett. 103 , 184501, 2009 .)