Microdrops Evaporating on AFM Cantilevers

Abstract

The kinetics of evaporation or drying of microscopic, sessile drops from solid surfaces is relevant in a variety of technological processes, such as printing, painting, and heat-transfer applications, besides being of fundamental interest. Drop evaporation has been commonly observed by means of video-microscope imaging, by ultra-precision weighing with electronic microbalances or with quartz crystal microbalances (QCM). Abundant information was gained over the years with these techniques, so that the evaporation of macroscopic drops of simple liquids from inert surfaces is nowadays well understood. The same techniques are, however, not applicable to microscopic drops. Furthermore they do not directly provide a measure of the interfacial stresses arising at the contact area between liquid and solid, which are known to play a key role in the evaporation kinetics of small drops.

Here I show how the use of atomic force microscope (AFM) cantilevers as sensitive stress, mass, and temperature sensors can be employed to monitor the evaporation of microdrops of water. Starting drop diameters are always below 100 μm. The foremost interest lies in exploring the last stages of the evaporation process.