On the rapid growth and departure of nucleate bubbles in boiling on hydrophobic surfaces.

HYPOTHESIS: Recent experiments have shown that tuning the initial wetting state of a hydrophobic surface before boiling gives rise to bubble behavior resembling that on hydrophilic surfaces. This contrasts with typical bubble behavior on hydrophobic surfaces, where bubble growth initiates from a residual vapor seed and results in slow growth and delayed departure. Here, we hypothesize that such wetting-state tuning modifies the near-surface liquid-vapor interfacial structure beneath the bubble, possibly through the formation of a near-surface thin liquid film, thereby altering bubble dynamics on hydrophobic surfaces. METHODS: Synchrotron X-ray imaging with high spatial resolution (2.44 μm) and a large field of view (∼5 × 5 mm) was employed in pool boiling experiments to visualize the near-surface liquid-vapor interface and bubble evolution under different surface initial wetting states. Non-dimensional analysis of bubble growth and departure was performed to characterize bubble dynamics. FINDINGS: We identify a unique bubble mode on the PDMS-coated hydrophobic surface in which the near-surface liquid-vapor interfacial structure resembles that on hydrophilic surfaces, suggesting the possible presence of a near-surface thin liquid film. In addition, the associated non-dimensional parameters indicate rapid bubble growth similar to that observed during microlayer evaporation-assisted growth on hydrophilic surfaces. Notably, the bubble departs rapidly and smoothly, maintaining an oblate shape and a nearly constant apparent contact angle during departure without obvious hysteresis. A capillary relaxation timescale defined from bubble-shape deformation is comparable to the actual departure duration, suggesting that capillary forces contribute significantly to the rapid bubble departure.