We investigate the charge separation caused by the motion of a water drop across a hydrophobic, insulating solid surface.

Sessile liquid drops have a higher vapor pressure than planar liquid surfaces, as quantified by Kelvin's equation. In classical derivations of Young's equation, this fact is often not taken into account. For an open system, a sessile liquid drop is never in thermodynamic equilibrium and will eventually evaporate. Practically, for macroscopic drops the time of evaporation is so long that nonequilibrium effects are negligible. For microscopic drops evaporation cannot be neglected. When a liquid is confined to a closed system, real equilibrium can be established. Experiments on the evaporation of water drops confirm the calculations.

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 microm to below 10 microm. From hydrophilic surfaces, the evaporation is slowed down during the last approximately 100 ms; we believe that this occurs due to flattening of the drops, which are then stabilized by interfacial forces and disjoining pressure.

When a solvent drop evaporates from a polymer surface, a characteristic structure remains. To analyze the structure formation during evaporation we deposited microdrops of ethyl acetate on planar PEMA and toluene on PS. The shape of the evaporation structures depends on the molar mass of the polymer, the number of droplets deposited, and the specific polymer/solvent combination. Crater‐like structures with a flat bottom were observed for PEMA. With PS, dot‐like protrusions were observed for low and intermediate molar masses; for $\overline M _{\rm w}z$ ≥ 210 kDa, crater‐like rims with a depression in the center were formed. These structures are interpreted based on four different processes occurring during evaporation.magnified image