OPTICAL CATASTROPHE APPLIED IN STUDY OF SPREADING OF EVAPORATING LIQUID DROPS ON A SOLID SURFACE

摘要

Optical catastrophe occurring in a far field of waves of a drop-refracted laser beam implies a wealth of information about drop spreading. The present paper reveals, based on catastrophe optics, that the far-field caustic images can successfully detect the interface instability of sessile drops, measure the contact angles of the drops, estimate the drop-foot height, visualize capillary flows in the drops, and investigate the spreading characteristics of evaporating liquid drops, such as the drop profile at the three-phase line, the effects of evaporation and capillary flaws on the spreading, etc. All of these obviously are very important for the study of spreading of evaporating liquid drops. As an example of the applications, three typical volatile liquid drops were tested using the far-field caustic images of the drop-refracted laser beam. The effects of evaporation and thermocapillary convection on the spreading are revealed and discussed.