Atomic Force Microscope Cantilevers Used as Sensors for Monitoring Microdrop Evaporation

abstract

For studying the evaporation of millimetre-sized drops of liquids techniques such as video-microscope imaging and ultra-precision weighing with electronic microbalances or with quartz crystal microbalances have been employed in the past decades. Similar techniques are, however, hardly applicable to microscopic drops. Moreover, they do not provide a measure of the interfacial stresses arising at the contact area between liquid and solid. Here we demonstrate the use of atomic force microscope (AFM) cantilevers as sensitive stress, mass, and temperature sensors for monitoring the evaporation of microdrops of water from solid surfaces. Starting from considerations on drops in equilibrium, we will further discuss evaporating drops and details of the experimental technique. We will show how the evaporation of water microdrops on a hydrophobic surface differs from the evaporation on a hydrophilic surface, and how this difference becomes more pronounced towards the end of evaporation. We further show that one-side metal-coated cantilevers, acting as bimetals, allow measuring the average temperature of an evaporating microdrop. Finally, we will discuss two further applications of microdrops evaporating on cantilevers, namely testing the local cleanliness of cantilevers’ surfaces and calibrating cantilevers’ spring constants.