Evaporation kinetics of wettability-moderated capillary bridges and squeezed droplets

Abstract

We experimentally and theoretically probe the evaporation kinetics of liquid capillary bridges and squeezed droplets for different heights and wettability conditions. Water droplets of uniform volume were squeezed between hydrophilic and superhydrophobic (SH) flat substrates, and the relative spacing between them is varied to understand the role of bridge height on its evaporation behavior. The transient evolution of bridge profile during the evaporation is monitored using optical imaging technique, with thermal imaging aid. Our observations reveal augmented evaporation rate for smaller relative spacing between the hydrophilic surfaces due to increased curvature of the liquid–vapor interface. For SH surfaces, the vapor entrapment phenomenon plays a significant role in addition to interfacial curvature, especially for small relative spacing. For mixed wettability bridge, the shape of the bridge exhibits a transition in curvature, which governs its temporal evaporation rate. We show that the bridge stability and contact line dynamics during evaporation is strongly dictated by the capillary pressure and the geometry of the liquid–vapor interface. For smaller relative spacing, the contact line de-pins at early stages and the liquid bridge remains stable for a longer duration. Whereas, bridges pinch off at early stages for higher spacing conditions via rapid evaporation of the neck. To model the evaporative mass loss rate, the transient bridge interfacial profile under influence of gravity is quantified using the principal of energy minimization, and the curvature driven evaporative flux is summed up over this interface to obtain the evaporation rate. The model predictions conform well to the experimental observations.