As a droplet evaporates, the curvature becomes greater, which leads to a greater pressure inside the droplet. When the droplet rests on a patterned surface, this leads to a sudden transition to a state in which the water penetrates between the ‘nails’. This is known as a Cassie-to-Wenzel transition.
The video below shows a suspended-to-collapsed transition during droplet evaporation from a patterned substrate.
Analysis of droplet evaporation on a super-hydrophobic surface
G. McHale, S. Aqil, N.J. Shirtcliffe, M.I. Newton and H.Y. Erbil, Langmuir 21 (24) (2005) 11053-11060.
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