Marangoni convection instability inside an evaporating droplet on an inclined substrate

•Marangoni convection instability patterns inside a tilted droplet are observed on an inclined substrate.•Transition from stationary longitudinal rolls to Bénard-Marangoni convection cells is observed during evaporation.•Instability patterns at the front region always appear later than those at the rear region.•Critical contact angles for the onset of instability patterns increase with the increase of Marangoni number.•Mechanisms of the generation and transition of thermal patterns are analyzed. To further understand Marangoni convection instability inside a tilted droplet, experimental investigations on evolution of thermal patterns on the droplet surface are conducted on an inclined substrate. Transition of Marangoni instability patterns from stationary longitudinal rolls to Bénard-Marangoni convection cells is observed during the droplet evaporation. Different from the spherical cap shaped droplet evaporating on a horizontal substrate, the hysteresis phenomena for the appearance of Marangoni instabilities are found in the droplet. The instability patterns at the front region always appear later than those at the rear region. For droplet under higher inclination angles, the total numbers of the longitudinal rolls become gradually smaller than those under smaller inclination angles. Effects of inclination angle and substrate temperature are examined and mechanisms of generation and transition of thermal patterns are analyzed.