Heat Transfer and Fluid Flow Characteristics in a Sessile Droplet on Oil-Impregnated Surface under Thermal Disturbance

The present study examines the flow field and heat transfer inside a sessile droplet on oil-impregnated surface when subjected to a small temperature difference at the droplet-oil interface. Temperature and flow fields inside the droplet are predicted and the flow velocities predicted are validated through the data obtained from a particle image velocimetry (PIV). Several images of droplets in varying sizes are analyzed and the droplet geometric features and experimental conditions are incorporated in the simulations. A polycarbonate wafer is used to texture the surface via incorporating a solvent-induced crystallization method. Silicon oil is used for impregnation of the textured surfaces. It is found that two counter-rotating circulation cells are formed in the droplet because of the combined effect of the Marangoni and buoyant currents on the flow field. A new dimensionless number (Merve number (MN)) is introduced to assess the behavior of the Nusselt and the Bond numbers with the droplet size. The Merve number represents the ratio of the gravitational force over the surface tension force associated with the sessile droplet and it differs from the Weber number. The Nusselt number demonstrates three distinct behaviors with the Merve number; in which case, the Nusselt number increases sharply for the range 0.8 ≤ MN ≤ 1. The Bond number increases with increasing the Merve number, provided that its values remain less than unity, which indicates the Marangoni current is dominant in the flow field.