Role of Surface Hydrophobicity in the Coalescence-Induced Bubble Detachment Kinetics

Bubble detachment plays a crucial role in preventing excessive accumulation in gas–liquid reactors and improving the mass transfer efficiency. This study investigates the effect of surface hydrophobicity on bubble detachment induced by coalescence through high-speed dynamic experiments and numerical simulations. The results show that on surfaces with low hydrophobicity (contact angles of 15 and 25°), bubbles detach because the inner contact line contracts relatively slowly, allowing the outer contact line to coalescence quickly. In contrast, on highly hydrophobic surfaces (contact angles of 35 and 45°), the simultaneous contraction of both the inner and outer contact lines prevents bubble jumping. Numerical simulations indicate that low-hydrophobicity surfaces generate sufficient kinetic energy through the pressure gradient. As hydrophobicity increases, some of the energy is dissipated through liquid-film drainage, weakening internal convection and reducing the intensity of the upward flow, which ultimately prevents bubble detachment. Energy analysis further indicates that bubble coalescence shows a weak dependence on the Bo number, while the magnitude of the adhesion energy is the key factor determining bubble jumping. When the adhesion energy exceeds approximately 20% of the released surface energy, the coalesced bubble is unable to detach.