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Published January 1, 1998 | public
Journal Article Open

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 efficient transport mechanism for many technological applications. This spreading process is used, for example, as the carrier mechanism in the casting of biological and organic Langmuir–Blodgett films. We have investigated the dynamics of spontaneously spreading volatile films of different vapor pressures and spreading coefficients advancing over the surface of a deep water support. Laser shadowgraphy was used to visualize the entire surface of the film from the droplet source to the leading edge. This noninvasive technique, which is highly sensitive to the film surface curvature, clearly displays the location of several moving fronts. In this work we focus mainly on the details of the leading edge. Previous studies of the spreading dynamics of nonvolatile, immiscible thin films on a deep liquid layer have shown that the leading edge advances in time as t3/4 as predicted by laminar boundary layer theory. We have found that the leading edge of volatile, immiscible spreading films also advances as a power law in time, talpha, where alpha ~ 1/2. Differences in the liquid vapor pressure or the spreading coefficient seem only to affect the speed of advance but not the value of the spreading exponent, which suggests the presence of a universal scaling law. Sideview laser shadowgraphs depicting the subsurface motion in the water reveal the presence of a single stretched convective roll right beneath the leading edge of the spreading film. This fluid circulation, likely caused by evaporation and subsequent surface cooling of the rapidly spreading film, resembles a propagating Rayleigh–Bénard convective roll. We propose that this sublayer rotational flow provides the additional dissipation responsible for the reduced spreading exponent.

Additional Information

©1998 American Institute of Physics. (Received 14 April 1997; accepted 4 September 1997) A.D.D. gratefully acknowledges support from the Ministère Francais de la Recherche postdoctoral fellowship program. S.M.T. would like to thank the National Science Foundation for financial support through a Research Initiation and CAREER award (S.M.T.) as well as a MRSEC seed grant distributed through the Princeton Materials Institute (S.M.T.). Grants from the Exxon Education Foundation and the Can Manufacturers Institute were used to purchase optical equipment for these studies.


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