Localized droplet heating by hydrophobic pins: Influence of pin area and droplet size on heat transfer

Localized heating of hydrophobic droplet is considered and the effects of restricted area heating and droplet size on heat transfer are examined. Samples composing of vertical steel pins located in Perspex holders are designed and manufactured. Sample surfaces are hydrophobized via depositing treated silica nanoparticles by dip coating method. Hydrophobicity of pin and Perspex surfaces are evaluated simultaneously securing uniform wetting state over sample surface. Droplet is heated via pin while creating a localized heating effect on droplet fluid. The experiments are conducted obtaining flow structures in droplet liquid. Simulations are also performed predicting thermal state in droplet liquid during heating while adopting conditions of the experiments. It is found that hydrophobizing of samples results in uniform contact angle (150° ± 2°) over the entire surface having hysteresis of 6° ± 3°. Enlarging pin diameter and droplet size alter flow structures inside droplet; hence, center of circulating structures changes, which modify the ratio of convection over conduction currents at droplet liquid interface. Increasing pin diameter enhances the Nusselt and the Bond numbers, which becomes more apparent as droplet volume increases.