Marangoni convection instabilities in an evaporating droplet on a non-isothermal substrate

• The maximum number of LRs is smaller than that on the isothermal substrate • The instability patterns preferentially occur at the lower temperature side • LRs continuously propagate along azimuthal direction towards cold side • Propagation speed increases with increase of the temperature gradient • Critical contact angles for the onset of LRs decrease with increasing Ma The Marangoni convection instabilities are investigated in a sessile ethanol droplet as it evaporates on a non-isothermal substrate. The instability structures consistently, preferentially occur at the lower temperature side of substrate. Drifted by the azimuthal thermocapillary flow under externally imposed temperature gradient, the longitudinal rolls are found to propagate from the hot side to the cold side for the droplet, while a radial propagation of the BM cells directly from the cold side to the hot side along the droplet diameter occurs at the last period of evaporation process. The azimuthal propagation speed of longitudinal rolls increases with the increase of temperature gradient, but is independent of the mean temperature of the substrate. Effects of the mean temperature of substrate and the temperature gradient on the Marangoni instabilities are investigated, and the critical conditions for the onset of longitudinal rolls are determined.