Multicellular structures in thin free liquid films induced by thermocapillary effect

Hypothesis: Multicellular convective structures that are induced in a fluid exposed to temperature difference are commonly observed in nature and in daily life. Different types of basic flow patterns are induced in a free liquid film by thermocapillary effect, whereas the formation of such multicellular structures has not been hitherto unravelled. Experiments: A thin film of high-Prandtl-number liquid is prepared in a rectangular aperture of the order of 0.1 mm in thickness sustained by its surface tension. A designated temperature difference is imposed between the end surfaces of the aperture to generate a thermocapillary-driven convection in the free liquid film. We monitor the induced thermal flow patterns to evaluate the cell numbers and their wavelength by experimental and numerical approaches. Findings: The multicellular structure is established by the thermocapillary effect in the free liquid films. The cell number increases in a stepwise manner as the liquid-film width increases. When the cell number increases, another pair of the cells always newly emerges. We determine the wavelength in a non-dimensional manner, and present the variation of the wavelength against the aspect ratio corresponding to the liquid-film width. The results are compared to those of convectional Marangoni-Bénard convection.