Numerical study of hydrophobic-island shapes with patterned wettability for pool boiling

Heat transfer during boiling is strongly affected by wettability pattern on the heater surface, and accordingly, has been actively studied. The wettability pattern comprises hydrophobic dots (islands) on a hydrophilic surface. Conventional hydrophobic dots are shaped like squares or circles. With these shapes, the contact line remains confined to the hydrophobic dot. On the other hand, by using a re-entrant dot shape, the contact line might invade the hydrophilic surface, depending upon the balance between the force required to pin the contact line to the hydrophobic dot and that required to maintain a circular contact line by surface tension. In the present study, we explore the use of crosses as the shapes of hydrophobic dots, with the aspect ratio of the arms of the cross used as a variable. The use of this re-entrant dot shape shows that the contact line does indeed invade the hydrophilic surface, as expected. The results show that the contribution of the triple-contact line to heat transfer increases with the aspect ratio. However, at low heat flux, we find a reduction in the effective nucleation area to be a side-effect of a large aspect ratio. Therefore, proper selection of the aspect ratio is needed for optimal heat transfer, depending on the amount of heat generated by the heat source.