11:15 AM - 11:30 AM
[SCG46-09] Experimental study on drainage process of liquid film between bubbles during bubble growth
★Invited Papers
Keywords:bubble coalescence, magma, laboratory experiment
Bubble coalescence has attracted attention in volcanology in recent decades because it can control volcanic eruption styles. Pre- and syn-eruptive magmas involve compressive bubbles, which reduce the effective density of magma and can be a driving force of the explosion. Thus, the explosivity of dry eruptions depends on the amount of gas in magma that exists as bubbles. The interconnected bubbles formed by coalescence can help to release the gas and prevent the magma from an explosive eruption. Bubble coalescence is divided into two stages: (1) approach stage in which the distance between bubbles shorten due to bubbles growth with maintaining the round shape, and (2) drainage stage in which the liquid film between bubbles drains out until a critical thickness. Here, we focus on the drainage stage. Previous studies investigated the timescale of drainage theoretically and experimentally under gravitational and capillary force effects (e.g., Proussevitch et al., 1993; Nguyen et al., 2013). However, the drainage by bubble growth associated with decompression is poorly understood, which must be a dominant process in a conduit. Therefore, we perform laboratory experiments and investigate if the rapid growth of bubbles accelerates or decelerates the drainage of the liquid film.
We used a Hele-Shaw cell made from two glass plates separated by 0.1 mm thick spacers. We filled the cell with silicone oil of 10 Pa･s viscosity and injected a mm-size bubble by microsyringe. By penetrating the bubble with a needle, we divided it into two nearly equal-size bubbles. We then placed the cell in a transparent acrylic container sealed by O-ring. The interior of the container was decompressed with a vacuum pump from the atmospheric pressure to 5 kPa. The decompression rate was varied by a vacuum regulator. The two bubbles gradually approached each other as the bubbles grow, and drained out the liquid film between them. The drainage process was recorded with an optical microscope and analyzed using Matlab.
The experimental results show that the growth rate of a bubble during decompression is faster than that expected from the ideal gas law. This difference can be attributed to the diffusional influx of dissolved air into bubbles. As the decompression rate increases, the bubble growth rate increases, which shortens the film rupture timescale. In the rapid decompression regime, a linear plateau border forms between two adjacent bubbles. The most striking finding in our experiments is that, under the higher decompression rate, the bubbles grow larger just before the onset of film rupture than in the case under the smaller decompression rate. The large growth rate enhances the viscous resistance in the plateau border, so that the bubble growth continues until the film thickness reaches a critical value. This finding significantly suggests that the formation of the permeable network in an eruption depends not only on the void fraction but also on the decompression rate.
We used a Hele-Shaw cell made from two glass plates separated by 0.1 mm thick spacers. We filled the cell with silicone oil of 10 Pa･s viscosity and injected a mm-size bubble by microsyringe. By penetrating the bubble with a needle, we divided it into two nearly equal-size bubbles. We then placed the cell in a transparent acrylic container sealed by O-ring. The interior of the container was decompressed with a vacuum pump from the atmospheric pressure to 5 kPa. The decompression rate was varied by a vacuum regulator. The two bubbles gradually approached each other as the bubbles grow, and drained out the liquid film between them. The drainage process was recorded with an optical microscope and analyzed using Matlab.
The experimental results show that the growth rate of a bubble during decompression is faster than that expected from the ideal gas law. This difference can be attributed to the diffusional influx of dissolved air into bubbles. As the decompression rate increases, the bubble growth rate increases, which shortens the film rupture timescale. In the rapid decompression regime, a linear plateau border forms between two adjacent bubbles. The most striking finding in our experiments is that, under the higher decompression rate, the bubbles grow larger just before the onset of film rupture than in the case under the smaller decompression rate. The large growth rate enhances the viscous resistance in the plateau border, so that the bubble growth continues until the film thickness reaches a critical value. This finding significantly suggests that the formation of the permeable network in an eruption depends not only on the void fraction but also on the decompression rate.