Evaporative cooling of sessile water microdrops measured with atomic force microscope cantilevers

Abstract

We monitored the evaporation kinetics of drops from solid surfaces and the resulting cooling of the surfaces. To do this we deposited water drops with diameters smaller than 100 µm onto atomic force microscope (AFM) cantilevers. Due to the surface tension of the liquid, the Laplace pressure inside the drop, the change of the interfacial stress at the solid–liquid boundary and evaporative cooling, the cantilevers are deflected by typically some hundred nanometers. We used pure silicon and gold-coated silicon cantilevers in order to be able to separate and quantify the cooling effect from that due to the surface tension. We measured the bending of the cantilevers along their longitudinal axis versus time with an AFM-like setup, and monitored the contact angle and radius of the evaporating drops with video microscopy. We developed a FEM model for the bending of a cantilever as a result of surface forces and evaporative cooling. Experimental results are reproduced by FEM simulations.