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This study investigates the phenomenon of bubble retention at gas-evolving surfaces, revealing that cascade coalescence between bubbles of unequal sizes is responsible for this unexpected behavior. High-speed imaging and numerical simulations demonstrate that when a larger bubble merges with a smaller one, it experiences a significant downward impulse that counteracts buoyancy, leading to enhanced bubble retention. The research uncovers that this mechanism can sustain bubble retention forces that exceed traditional buoyancy limits by three to four orders of magnitude, which has implications for improving mass transfer in clean energy technologies.
Cascade coalescence can generate retaining forces that defy conventional buoyancy limits, revolutionizing our understanding of bubble dynamics in energy systems.
Bubble detachment from solid surfaces governs heat, mass, and charge transport across technologies vital to clean energy, including high-current-density water electrolysis and boiling thermal management. At high gas fluxes, however, bubbles remain trapped at active surfaces despite immense buoyancy, severely restricting mass transfer and increasing energy losses. Here, we show that this unexpected surface retention originates from cascade coalescence between unequal-sized bubbles. High-speed observations around microelectrodes demonstrate that when a rising bubble merges with a smaller surface-attached successor, its trajectory abruptly reverses, accelerating toward the substrate at nearly two orders of magnitude above its rising speed. Direct numerical simulations and scaling analysis reveal that asymmetric interfacial retraction during merging generates non-canceling viscous stresses, producing a net downward impulse toward the smaller bubble. Repeated cascade coalescence events accumulate these transient impulses into a steady, time-averaged retaining force capable of opposing buoyancy three to four orders of magnitude beyond quasistatic limits. Our findings establish bubble coalescence as a previously unrecognized mechanism that dynamically sustains bubble retention under high gas flux.
