Drops hop from hydrophilic surfaces (Vol. 46 No. 1)

The impact of liquid droplets on solid surfaces is ubiquitous in many natural and industrial settings. It is now well known that drops can bounce on super-hydrophobic surfaces such as the leaf of a lotus plant or a patterned engineered surface, coated to repel water. Furthermore, it is commonly thought that when impacting hydrophilic substrates, for instance a glass window, drops will splash or spread but never bounce. Here, this assumption is shown to be incorrect - drops do in fact bounce on smooth and defect-free hydrophilic substrates such as very clean glass, silicon wafers, or the surface of cleaved mica.

And they do so by never actually contacting the surface.

Direct experimental evidence supports a surprising mechanism for drop rebound. If the velocity of the drop is relatively low, on impact it spreads over a thin film of air, approaching to within 10 nanometres of the surface, but never truly contacting it; then the drop retracts and detaches from the surface. If, however, the impacting drop exceeds a threshold impact velocity, the nanometre-thick air film breaks down and liquid-solid contact initiates.

J. M. Kolinski, L. Mahadevan and S. M. Rubinstein, “Drops can bounce from perfectly hydrophilic surfaces”, EPL 108, 24001 (2014)