Marangoni convection instability in an evaporating droplet deposited on volatile liquid layer

Abstract

The Marangoni convection patterns in an evaporating droplet deposited on volatile liquid layer were experimentally investigated in this work. Two types of Marangoni instabilities were observed on the surface of the methanol droplet under two different equilibrium state. The hydrothermal waves which travel along azimuthal direction with a source and a sink appear in the liquid lens fully floating on liquid layer. With evaporation, the wave number decreases and follows a power law with the dimensionless radius of lens and the temperature difference between the substrate and the ambient. For droplet partially floating on thin liquid layer, the surface pattern is arranged in an n-fold structure with serval cold bands linking the droplet apex with the contact line during the early period of evaporation. After that, the Bénard-Marangoni convection cells occur near the contact line and they move inwards along radial direction, whose dynamic behavior is entirely opposite to that of the droplet on solid surface. Several gaps appear at the contact line and the number of which increases with evaporation and decreases sharply at the end of evaporation process. These gaps are always located at the junction of adjacent BM cells in the liquid layer and they are actually the origins of the BM cells in droplet but not the really breakage of the contact line. The influence of the substrate temperature on the instability patterns is studied and the critical thicknesses of liquid layer for the onsets of them are determined. The physical reasons underlying these phenomena are analyzed.