DYNAMICS OF SPONTANEOUS SPREADING WITH EVAPORATION ON A DEEP FLUID LAYER

The spontaneous spreading of a thin volatile film along the surface of a deep fluid layer of higher surface tension provides a rapid and eff icient transport mechanism for many technological applications. This s preading process is used, for example, as the carrier mechanism in the casting of biological and organic Langmuir-Blodgett films. We have in vestigated the dynamics of spontaneously spreading volatile films of d ifferent vapor pressures and spreading coefficients advancing over the surface of a deep water support. Laser shadowgraphy was used to visua lize the entire surface of the film from the droplet source to the lea ding edge. This noninvasive technique, which is highly sensitive to th e film surface curvature, clearly displays the location of several mov ing fronts. In this work we focus mainly on the details of the leading edge. Previous studies of the spreading dynamics of nonvolatile, immi scible thin films on a deep liquid layer have shown that the leading e dge advances in time as t(3/4) as predicted by laminar boundary layer theory. We have found that the leading edge of volatile, immiscible sp reading films also advances as a power law in time, t(alpha), where al pha similar to 1/2. Differences in the liquid vapor pressure or the sp reading coefficient seem only to affect the speed of advance but not t he value of the spreading exponent, which suggests the presence of a u niversal scaling law. Sideview laser shadowgraphs depicting the subsur face motion in the water reveal the presence of a single stretched con vective roll right beneath the leading edge of the spreading film. Thi s fluid circulation, likely caused by evaporation and subsequent surfa ce cooling of the rapidly spreading film, resembles a propagating Rayl eigh-Benard convective roll. We propose that this sublayer rotational flow provides the additional dissipation responsible for the reduced s preading exponent. (C) 1998 American Institute of Physics.