Natural Convection in Evaporating Minute Drops

Interfacial flow structures in small liquid drops evaporating on flat plates are cinematographically investigated using the methods of direct photography and laser shadowgraphy. Various liquids of relatively low boiling point were evaporated on glass and copper plates at room temperature. The laser shadowgraph records the flow patterns simultaneously at both the liquid-air interface and the liquid-solid interface, from which the evaporation rate is determined. It reveals the existence of three distinct flow structures at the liquid air interface: stable, substable, and unstable. An interfacial flow map is constructed. The direct photography is employed to study the morphology during the entire process of the unstable-interface type evaporation. The mechanism of ripple formation which enhances the evaporation rate is found to be caused by hydrophilicity of the liquid.