Morphological hysteresis of droplets wetting a series of triangular grooves

Abstract

Energetics and morphology of a microdroplet of varying volume settled on a surface covered with triangular grooves were investigated. The study was carried out by numerical computations performed with the Surface Evolver program. The obtained results showed that the droplet morphology underwent bifurcation into the filament-like one (F) and bulge one (B) when its volume had reached a certain value. This bifurcation point (BP) was affected by the number of grooves and the Young contact angle. At volumes smaller than that corresponding to BP, the droplet adopted the only stable morphology F, while the prediction of its morphology above the BP is a complex problem. As a result of the difference between BP and the volume of free energy equilibrium between F and B states, the droplet can assume various stable or metastable states. The actual state of the droplet was found to depend on the direction of volume changes and the possibility of overcoming a local energy barrier between the morphologies. As a result of fluctuation, the droplet can switch from one morphology to the other, hence the hysteresis of the dependence of linear droplet dimensions on its volume is observed. Morphology B was shown to be realized only if the canthotaxis condition was not exceeded otherwise, the droplet spread on the surface adjacent to the area with grooves. The appearance of the bifurcation effect depended on the number of grooves and the Young contact angle. The increase in these two parameters caused a decrease in differences between morphologies F and B, and finally the disappearance of bifurcation.