FLUORESCENCE VISUALIZATION OF A CONVECTIVE INSTABILITY WHICH MODULATES THE SPREADING OF VOLATILE SURFACE-FILMS

The spontaneous spreading of a thin liquid film along the surface of a deep liquid layer of higher surface tension is a ubiquitous process w hich provides rapid and efficient: surface transport of organic or bio logical material. For a source of constant concentration, the leading edge of a nonvolatile, immiscible film driven to spread by gradients i n surface tension is known to advance as t(3/4) in time. Recent experi ments using laser shadowgraphy to detect the advancing front of spread ing films indicate, however, that immiscible but volatile sources of c onstant concentration spread with a reduced exponent according to t(1/ 2). Using a novel technique whereby fluorescent lines are inscribed in water, we have detected the evolution of a thermal instability beneat h the leading edge of volatile films which strongly resembles a Raylei gh-Benard roll. We propose that the increased dissipation from this ro tational flow structure is likely responsible for the reduction in spr eading exponent. This observation suggests a conceptual framework for coupling the effects of evaporation to the dynamics of spreading. (C) 1998 American Institute of Physics.