Pool boiling heat transfer on silicon surfaces with re-entrant microcavities and micropillars

High heat flux removal in advanced thermal management systems often relies on pool boiling, where surface microstructures strongly influence bubble dynamics and heat transfer. The geometry of these microstructures determines the critical heat flux (CHF) and the heat transfer coefficient (HTC). In this study, we experimentally investigate pool boiling of de-ionized water on silicon surfaces with re-entrant and vertical microcavities, re-entrant and vertical micropillars, and flat silicon surfaces. The CHF of re-entrant cavities is ≈ 142 W/cm 2 , representing ≈ 97% enhancement over vertical cavities (≈72 W/cm 2 ) and ≈ 43% over flat polished surfaces (≈99 W/cm 2 ). This improvement in CHF is attributed to the overhang geometry or hoodoo, which lowers the temperature locally at the cavity rim to sustain liquid replenishment, and accelerates bubble departure through the necked cross-section. In contrast, re-entrant pillars did not improve CHF, reaching ≈ 133 W/cm 2 versus ≈ 140 W/cm 2 for vertical pillars, as vapor trapping in re-entrant pillars promotes bubble coalescence and early transition to film boiling. Comparison of HTC values indicate that re-entrant features offer marginal benefits for cavities. However, HTC increased for micropillars because of enhanced bubble ebullition cycle. Furthermore, increasing the cap thickness from 0.5 µm to 2 µm enhanced the CHF of re-entrant microcavity surfaces from ≈ 136 W/cm 2 to ≈ 156 W/cm 2 . In contrast, the CHF of re-entrant micropillar surfaces remained unaffected when the cap thickness increased from 0.5 µm to 2 µm. Furthermore, our heat transfer measurements show that for both re-entrant cavity and re-entrant pillars, the wall superheat at the onset of nucleate boiling (ONB) decreased by ≈ 3-4 K, a result that points to the overall impact of re-entrant geometry on boiling. By elucidating the heat transfer mechanism, the new findings reported in this study provide useful insights that can guide the design of engineered surfaces for cooling next-generation power-dense electronics.