Droplet heat transfer on micropost arrays with hydrophobic and hydrophilic characteristics

Heat transfer analysis for a water droplet on micropost arrays is carried out while mimicking the environmental conditions. Since the micropost arrays spacing size alters the state of the hydrophilicity of the surface, the size of the micropost arrays spacing is varied and the resulting heat transfer characteristics are analyzed. Spreading rate of water droplet on the micropost arrays is considered and the adhesion force for the pinning of the water droplet on the micropost arrays is presented. Temperature and flow fields are predicted and the predictions of flow velocity inside the water droplet are validated through the particle image velocimetry (PIV). The Nusselt number variation for various sizes of the micropost arrays is obtained for two droplet volumes. It is found that reducing the solid fraction of micropost array beyond Ø_s=0.25, the Cassie and Baxter state of the surface changes to the Wenzel state; in which case, hydrophobic characteristics changes to hydrophilic characteristics for the water droplet. Heat transfer from the droplet bottom gives rise to development of the buoyancy and the Marangoni currents, which in turn generate two counter rotating circulation cells. The center of circulation cells moves further in the droplet upper part for the hydrophobic droplet case. The Nusselt number attains high values for the hydrophobic droplet at micropost array spacing size b=10 lm and hydrophobic droplet at spacing size b=50 lm due to fin effects of the micropost arrays.