Thermocapillary convection inside a stationary sessile water droplet on a horizontal surface with an imposed temperature gradient

The present study reports the fluid flow inside a stationary water droplet placed on a hydrophobic surface subjected to a temperature gradient. The development of temperature gradient substrate surface is accomplished using two thermoelectric coolers. Confocal microscale PIV and IRT measurements have been carried out for measurement of internal convection flow field and substrate temperature, respectively. The substrate temperature gradient is set equal to 1.8 degrees C/mm, and droplet diameter is varied between 1.20 and 1.94 mm. The convection pattern inside the droplets changes with droplet size. For small droplets, the fluid flows from hot side to cold side along the contact line leading to formation of two recirculation bubbles. In contrast, the flow direction near the substrate surface becomes unidirectional from cold side to hot side of the droplet for large droplet. The buoyancy effect becomes predominant for large droplets, and the fluid flows from hot side to cold side along the apex of the droplet. The present experimental results demonstrate the presence of complex three-dimensional flow structures, indicating the importance of three dimensionality in droplet fluid dynamics on a temperature gradient surface contrary to various simulation results reported in the literature based on lower-dimensional lubrication and two-dimensional models.