CAPILLARITY EFFECTS ON VISCOUS GRAVITY SPREADINGS OF WETTING LIQUIDS

Highly viscous silicone oils within a wide range of volumes were left to spread on smooth horizontal substrates to investigate the effects o f capillarity on viscous gravity spreadings under the condition of com plete wetting. The study was centered on the intermediate asymptotic b ehavior, where the details of the initial liquid distribution are irre levant. We detected small but appreciable departures from the well-kno wn solution without surface tension forces (viscous gravity self simil ar solution, or base solution). Two stages are clearly identified in t he spreadings. During the first, which is usually rather brief, capill arity does not play an appreciable role on the dynamics of the spreadi ng, i.e., the base solution is a very good approximation, The head of the current displays a wheel-like profile progressively decreasing in size; when the size becomes on the order of the capillary length, this stage ends. The wheel-like configuration cannot be associated either with the rheological behavior of the fluid used or with the initial co nditions. To observe this first stage without the influences due to pe culiarities of the initial conditions, a proper release of the spreadi ng is needed. The second stage is characterized by a spreading rate be low the base solution; the slowing is associated with a change of the current head shape, which takes the form of a wedge as a consequence o f capillarity. The rate of advance of the front may still be well appr oximated by the same power law as given by the base solution, but with a smaller prefactor. In this work we measure the parameters that char acterize the flow during both stages; besides, for the second stage, w e develop a heuristic calculation which shows that the wedge-like shap e of the current head gives place to a higher viscous dissipation rate , thus explaining the observed slowing of the spreading. (C) 1996 Acad emic Press, Inc.