Drop mobility on superhydrophobic microstructured surfaces with wettability contrasts

Manipulation of drop motion has attracted considerable attention recently as it is pertinent to industrial/biological applications such as microfluidics. Wettability gradients/contrasts applied to microtextured, superhydrophobic surfaces are probable candidates for engineering drop motion by virtue of their wettability controllability and low contact angle hysteresis. In the present work, we present a systematic study of drop mobility induced via wettability contrasts. A millimetre-sized water drop, placed on the boundary between two surfaces with distinct, uniform arrays of pillars, immediately moved toward the surface more densely populated with asperities, which was relatively more hydrophilic. The velocity of the motion was found to increase proportionally with the difference in pillar densities on each surface, in circumstances where the rear side surface had sufficiently small contact angle hysteresis. To elucidate the underlying mechanism of drop motion, we implemented a surface energy analysis for each motion event. Motion was initiated by the excess surface free energy due to drop deformation and directed in favour of energy minimisation. Lastly, we propose a theory to predict the direction of the drop which at the same time acts as the criterion for the motion to ensue.