Nucleate Boiling of Dielectric Liquids on Hydrophobic and Hydrophilic Surfaces

An experimental study was conducted to investigate the effect of hydrophobic, hydrophilic and mixed hydrophobic/hydrophilic surfaces in nucleate boiling heat transfer. A dielectric liquid, HFE-7100, was used as the working fluid in the saturated boiling tests. A total of 12 test samples were used in this study, featuring four types of boiling surfaces with a common copper substrate; (1) plain, smooth copper surface (as reference), (2) hydrophobic patterned or fully-covered surface, (3) hydrophilic patterned or fully-covered surface, and (4) mixed hydrophobic/ hydrophilic patterned surface. All test samples were prepared on 10 mm × 10 mm × 2 mm copper substrates with matching size thick film resistors attached onto the opposite side, to generate heat and simulate high heat flux electronic devices. The fabrication of hydrophobic surfaces involved common photolithography techniques to apply 100 μm thick Teflon layer. Hydrophilic surfaces were prepared by depositing a TiO₂ layer through a two-step process involving layer by layer self-assembly (L-B-L) and liquid phase deposition (L-P-D) techniques. Test samples with the mixed hydrophobic/hydrophilic surfaces were obtained by first applying Teflon hydrophobic patterns, and then by covering the remaining substrate area with hydrophilic coating. The effect of pattern and pitch size was investigated by varying the circular pattern dimensions between 40, 100 and 250 μm and corresponding pitch dimensions between 80, 200 and 500 μm. The results indicated that hydrophobic and hydrophilic surfaces have distinct benefits, and mixed hydrophobic/hydrophilic surfaces offer an optimum performance enhancement, providing: (a) early transition to boiling regime with no temperature overshoot at boiling incipience, (b) up to 10.6 kW/m²°C HTC (representing 82% increase), and (c) up to 28 W/cm² CHF level (representing 47% increase). The studied enhanced surfaces therefore demonstrated a practical surface modification method for heat transfer enhancement in immersion cooling applications.