| The enhancement of heat transfer of nucleate boiling on a hydrophilic-hydrophobic patterned surface when compared to that on a hydrophilic surface or hydrophobic surface was studied in this thesis. Hydrophobic islands on hydrophilic networks were micro fabricated as patterns on a 1 mm x 1 mm indium-tin-oxide (ITO) glass heater with an insulated layer. The hydrophobic islands with size of 100 microm x 100 microm and pitch distance of 200 microm are selfassembled monolayers (SAM) of (1H, 1H, 2H, 2H)-Perfluorodecyldimethylchlorosilane (FAS) with contact angle of 100°. Water at room temperature was used as the working fluid and the input heat flux for the heater ranges up to 300 W/cm2.;It was found that the critical heat flux (CHF) on the wettability patterned surface was enhanced by 90% in comparison with a hydrophilic surface. Patterns increase the CHF by moderating the Helmholtz instabilities, preventing the formation of an insulating vapor layer. Viewed from beneath the substrate, bubble behavior on the patterned surface was captured and analyzed for better understanding of how hydrophobic islands worked as localized and controlled nucleate sites even with a relatively small heater. The visualization results show that the mechanism for enhancing heat transfer coefficient (HTC) of a wettability patterned surface is caused by increasing the active nucleation sites (more than 12 times higher), decreasing the bubble departure diameter (about 1/3 of the size on the homogeneous surface), remaining residual bubbles when departed, and activating more bubble interactions. Bubble interactions and instability of big bubbles resulted in both a larger critical heat flux and higher heat transfer coefficient. The latent heat of the trapped droplet under bubbles having unique shapes on the biphilic surface and the local convective microflow surrounding the bubbles contributed to the enhancement. |
