Asymmetric Bubble Coalescence on Biphilic Surfaces in Nucleate Boiling: Suppressed Jumping and Early Release

Coalescence of neighboring bubbles is common in nucleate boiling, yet its dynamics on biphilic surfaces remain unclear. The growth, coalescence and departure of two bubbles on a biphilic surface with two hydrophobic spots were investigated by three-dimensional volume-of-fluid simulations validated against pool boiling experiments. The activation time of the second bubble was varied over the growth period of the first. Five modes were identified: symmetric, early-stage, late-stage and departure-stage coalescence, and non-coalescence. In early-stage coalescence, the coalesced bubble remains attached to both spots for many inertio-capillary times and departs soon after detaching from one of them; coalescence-induced jumping does not occur, because the contact lines pinned at the spot edges raise the adhesion energy above the released surface energy. In late-stage coalescence, the smaller bubble is absorbed into the larger one within about one inertio-capillary time, presumably owing to the Laplace pressure difference, so that its spot is released within 20% of the single-bubble period and the coalesced bubble departs earlier. The transition between the two regimes is abrupt and governed by contact line depinning. These results show that the activation timing of neighboring hydrophobic spots can advance the release of the second spot and bubble departure on biphilic surfaces.