Directional wetting transitions during evaporation on microcavity surfaces

Abstract

A range of physico-chemical factors such as substrate surface energy and geometry, thermal distributions, and surface tension of the liquid affect the wetting states and their transitions. Recently, it was reported that the transitions from Cassie to Wenzel states may not be instantaneous or uniform. In this work, we investigate the mechanisms and spatio-temporal distribution of wetting transitions with respect to the surface energy and structure of closed microcavity surfaces. Patterned microcavity surfaces used for the study were fabricated using deep reactive ion etching (DRIE) process. The surface energy of the patterns was varied using diamond-like carbon (DLC), Fluorine incorporated diamond-like carbon (FDLC) and polytetrafluoroethylene (PTFE) coatings. We showed that wetting transitions can be tuned to be either instantaneous or spatio-temporally distributed, by controlling the surface energy and geometry. Importantly, we showed that when the coated surfaces were hydrophobic, evaporating drop under some specific conditions would undergo directional wetting transition, where the cavities near to the 3-phase contact line are filled first. We showed that such transitions were not related to the Laplace pressure. We highlight the role of internal flow dynamics of the drop, resulting from the differential mass transfer of the evaporating drop surface, in the directional wetting transitions.