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

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 (T-w) from those lower than room temperature (T-a) to those higher than it. For T-w > T-a, the Benard-Marangoni convection cells are observed and the cell patterns vary from the quasi-steady state to irregular oscillatory state with the droplet evaporation. For T-w < T-a, the irregular oscillatory Benard-Marangoni cells are observed when T-w is higher than 10.6 degrees C. The cell size is larger while there are less cell numbers compared to that of T-w > T-a. When T-w is higher than 14.3 degrees C three kinds of convection mode, i.e. the travelling hydrothermal waves, the coexistence of hydrothermal waves and irregular oscillatory Benard-Marangoni convection, and the irregular oscillatory Benard-Marangoni convection, occur successively with droplet evaporating. The critical contact angles for the incipience of these Marangoni instabilities are determined. (C) 2018 Elsevier Ltd. All rights reserved.