Coalescence of Growing Bubbles in Highly Viscous Liquids

Bubble coalescence in ascending magma is a key process that controls the eruption violence and the texture of volcanic pyroclasts. In the present study, we performed in situ experiments to investigate the coalescence of two growing bubbles in highly viscous liquids (102 and 1,020 Pa ⋅ s). A new experimental apparatus enables us to directly observe the drainage of the film between growing bubbles in three dimensions. We combined the experimental results and a simple scaling analysis to reveal the dynamics of film drainage in terms of a capillary number (Formula presented.) depending on the liquid viscosity η, the bubble growth rate (Formula presented.), and the surface tension σ. The capillary number represents the interplay between the viscous force arising from bubble growth and the capillary force. At Ca ≪ 1, two adjacent bubbles retain their spherical shapes until the film ruptures, and the capillary forces control the drainage timescale. In contrast, at Ca ≫ 1, bubbles largely flatten and bubble growth itself drives film drainage. We also provide a general formula for the drainage timescale over a wide range of capillary numbers (10⁻³ < Ca < 10¹). Our results highlight the importance of bubble growth in the coalescence process. The variations of bubble shape and number density during decompression can be explained by the capillary number.