Influence of substrate temperature on Marangoni convection instabilities in a sessile droplet evaporating at constant contact line mode

Abstract

Marangoni instabilities of a sessile droplet of 0.65 cSt silicone oil evaporating at constant contact line mode are experimentally investigated in a wide range of the substrate temperatures (Tw) from those lower than room temperature (Ta) to those higher than it. For Tw > Ta, the Bénard-Marangoni convection cells are observed and the cell patterns vary from the quasi-steady state to irregular oscillatory state with the droplet evaporation. For Tw < Ta, the irregular oscillatory Bénard-Marangoni cells are observed when Tw is higher than 10.6 °C. The cell size is larger while there are less cell numbers compared to that of Tw > Ta. When Tw is higher than 14.3 °C three kinds of convection mode, i.e. the travelling hydrothermal waves, the coexistence of hydrothermal waves and irregular oscillatory Bénard-Marangoni convection, and the irregular oscillatory Bénard-Marangoni convection, occur successively with droplet evaporating. The critical contact angles for the incipience of these Marangoni instabilities are determined.