Steady thermocapillary flows of thin liquid layers. II. Experiment

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Abstract
The steady thermocapillary flow of nonvolatile layers, nonuniformly heated from below, is examined. The interactions among viscous forces, thermocapillarity, and hydrostatic effects give rise to the steady‐state dimpling of the interface. The steady dimpling of nonuniformly heated silicone–oil layers with mean thicknesses ranging from 0.125 to 1.684 mm is studied experimentally. The temperature distribution in the substrate is monitored by thermocouples and the interface shapes by a mechanical impedance probe. Measured steady shapes and theoretical predictions agree within 20% for moderate heating when the film is not close to rupture. When the heating rate causes the film to ‘‘dry out’’ above the hottest point on the substrate, the long‐wave theory delivers a parametric index, involving thermocapillary and hydrostatic effects, which is an excellent predictor of rupture. Nonlinear long‐wave theories of the type discussed here have never been tested experimentally, until now. The confirmation of this thermocapillary theory is suggestive of the validity of the previous long‐wave analysis [Phys. Fluids A 2, 313 (1990)] of unsteady, evaporating/condensing liquid layers.