Transition in the Evaporation Kinetics of Water Microdrops on Hydrophilic Surfaces

We describe a technique that allows measurement of the mass and shape of sessile liquid microdrops during evaporation. Therefore, the microdrops are deposited by an inkjet onto a silicon microcantilever, and the bending and the shift in resonance frequency are monitored. From hydrophobized surfaces, microscopic water drops evaporate with the same kinetics as macroscopic drops; we verify the validity of known evaporation laws to drops with diameters from 100 mu m to below 10 mu m. From hydrophilic surfaces, the evaporation is slowed down during the last similar to 100 ms; we believe that this occurs due to flattening of the drops, which are then stabilized by interfacial forces and disjoining pressure.