11:00 〜 13:00
[SVC32-P14] Experimental study on the coalescence of two growing bubbles in three-dimensional space
キーワード：気泡合体、マグマ、減圧
Like a well-shaken cola, the explosivity of volcanic eruption is controlled by the amount of gas in magma that exists as bubbles. Outgassing resulting from bubble coalescence can release the gas and drastically change the eruption style from explosive to effusive activity. Thus, it is important to understand the dynamics of bubble coalescence. In the present study, I focus on the grow-driven coalescence associated with decompression, which is one of the dominant processes during magma ascent. A previous study proposed the timescale of grow-driven coalescence of two bubbles (Castro et al., 2012). However, it neglected the hydrodynamics inside a film between bubbles and has not been verified experimentally. Using a Hele-Shaw cell, Ohashi et al. (2022) performed experiments on the coalescence of two growing bubbles in quasi-two-dimensional space. They found that the hydrodynamic interaction between the bubbles determines the occurrence of coalescence. In order to investigate bubble coalescence in actual volcanic eruptions, it is required to conduct three-dimensional experiments without cell walls. Therefore, I perform laboratory experiments to observe the three-dimensional coalescence of two growing bubbles during decompression.
I used a small box-shaped desiccator to decompress bubbles in a viscous liquid. I filled the desiccator with silicone oil of 100 or 1000 Pa･s viscosity and then injected two tiny air bubbles of about 0.3-0.5 mm radius. The coalescence process was recorded with an optical microscope through a transparent side wall of the desiccator. I decompressed the interior with a vacuum pump from atmospheric pressure to 10 kPa, kept the pressure constant for 60 s, and then returned to the atmospheric pressure. The two bubbles gradually approached each other as they grow, and drained out the liquid film between them. Depending on the experimental conditions, some bubbles coalesce during decompression. The recorded movies were analyzed using Matlab.
In the present experiments, bubble growth is driven by two mechanisms: the gas expansion following the ideal gas law and the diffusional influx of dissolved air. Bubble growth rate increases with an increase of decompression rate. A striking finding is that the parallel film forms under the condition of rapid decompression and/or highly viscous liquid. The large viscous force in a liquid film deforms the bubble surface and decreases the thinning rate of the film. As a result, the bubble increases its volume by transiting from a spherical to a polygonal shape which is often seen in silicic pumice.
The finding in my experiments suggests that the rapid growth of bubbles under the high decompression rate prevents outgassing driven by coalescence. If pumice does not quench and continues expansion after fragmentation, the above mechanism may lead to the wide variation of pumice vesicularity even in one eruption event.
This research was supported by JSPS KAKENHI Grant Number JP21K14014.
I used a small box-shaped desiccator to decompress bubbles in a viscous liquid. I filled the desiccator with silicone oil of 100 or 1000 Pa･s viscosity and then injected two tiny air bubbles of about 0.3-0.5 mm radius. The coalescence process was recorded with an optical microscope through a transparent side wall of the desiccator. I decompressed the interior with a vacuum pump from atmospheric pressure to 10 kPa, kept the pressure constant for 60 s, and then returned to the atmospheric pressure. The two bubbles gradually approached each other as they grow, and drained out the liquid film between them. Depending on the experimental conditions, some bubbles coalesce during decompression. The recorded movies were analyzed using Matlab.
In the present experiments, bubble growth is driven by two mechanisms: the gas expansion following the ideal gas law and the diffusional influx of dissolved air. Bubble growth rate increases with an increase of decompression rate. A striking finding is that the parallel film forms under the condition of rapid decompression and/or highly viscous liquid. The large viscous force in a liquid film deforms the bubble surface and decreases the thinning rate of the film. As a result, the bubble increases its volume by transiting from a spherical to a polygonal shape which is often seen in silicic pumice.
The finding in my experiments suggests that the rapid growth of bubbles under the high decompression rate prevents outgassing driven by coalescence. If pumice does not quench and continues expansion after fragmentation, the above mechanism may lead to the wide variation of pumice vesicularity even in one eruption event.
This research was supported by JSPS KAKENHI Grant Number JP21K14014.