Coalescence of Growing Bubbles in Highly Viscous Liquids

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 23; no. 11
• Main Authors: Ohashi, Masatoshi, Maruishi, Takafumi, Toramaru, Atsushi
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.11.2022; Wiley
• Subjects: bubble coalescence; Bubbles; Coalescence; Decompression; Dimensions; Drainage; film drainage; Growth rate; Lava; Liquids; Magma; Percolation; Scaling; Surface tension; Viscosity; Volcanic eruptions
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010618

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 Ca=ηṘ/σ $Ca=\eta \dot{R}/\sigma $ depending on the liquid viscosity η, the bubble growth rate Ṙ $\dot{R}$, 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. Plain Language Summary The explosivity of a volcanic eruption depends on the amount of gas that exists as bubbles in the rising magma. Because interconnected bubbles formed by coalescence can release gas from the magma, it is valuable to reveal the coalescence process. In this study, we focused on the growth driven coalescence of bubbles, which is poorly understood, particularly in a highly viscous liquid like magma. To address this knowledge gap, we performed in situ experiments where we could observe the coalescence of two growing bubbles in three dimensions. Our results indicate that the coalescence process is divided into capillary‐and growth‐dominated regimes depending on a non‐dimensional number called the capillary number. The normalized drainage timescale increases as both the bubble growth rate and the liquid viscosity increase. The long drainage timescale may increase the percolation threshold and inhibit vesiculated magma from outgassing. This suggests that even low‐viscosity magma like basaltic magma can erupt explosively without gas loss if it is decompressed rapidly in a volcanic conduit. Key Points We conducted in situ experiments to investigate the growth driven coalescence of bubbles in three dimensions We found that the coalescence process is scaled by a dimensionless capillary number We obtained a film drainage timescale that is consistent with experiments over a wide range of capillary numbers