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Abstract Most lava flows carry bubbles and crystals in suspension. From earlier works, it is known that spherical bubbles increase the effective viscosity while bubbles deformed by rapid flow decrease it. Changes in the spatial distribution of bubbles can lead to variable rheology and flow localization and thus modify the resulting lava flow structure and morphology. To understand the roles of bubble and solid phase crystal distributions, we conducted a series of analog experiments of high bubble fraction suspensions. We poured the analog lava on an inclined slope, observed its shape, calculated the velocity field, and monitored its local thickness. A region of localized rapid flow and low vesicularity, whose thickness is thinner than the surrounding area, develops at the center of the bubbly flows. These features suggest that the locally higher liquid fraction decreases the effective viscosity, increases the fluid density, and accelerates the flow. We also found that a halted particle‐bearing bubbly flow can resume flowing. We interpret this to result from the upward vertical separation of bubbles, which generates a liquid‐rich layer at the bottom of the flow. In our experiment, bubbles are basically spherical and decrease the flow velocity, while our estimate suggests that bubbles in natural lava flows could increase or decrease flow velocity. Downstream decreases in flow velocity stops the bubble deformation and can cause a sudden increase of effective viscosity. The vertical segregation of the liquid phase at the slowed flow front may be a way to generate a cavernous shelly paho’eho’e.
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This content will become publicly available on November 14, 2026
Air entrapment, internal structure, and density of hydrophobic particle–water–air mixtures
This paper investigates internal structure-driven density changes of post-wildfire and natural debris flows resulting from sand hydrophobicity and shearing. Hydrophobic sand particles entrap air by way of an armoured bubble/gas marble mechanism in water. Although individual armoured bubbles have already been broadly investigated, the effects of fluid drag and collisions in multiphase water–air–sand mixtures remain largely unexplored. The armoured bubbles’ stability in water depends on the force balance on the air bubble–particle boundary, which largely defines the mixture’s internal structure. Gravity, relative armoured bubble and fluid velocities govern the collision forces, local changes in mixture concentration, and the separation or attachment of hydrophobic particles to air bubbles in water. The initially large entrapped air volume decreases due to degassing and large armoured bubble breakdowns downstream. Experimental and theoretical approaches quantify the air entrapment under different sand-water volumetric concentrations, as well as the effects of mixing speed, duration, and sand particle size on the final mixture’s internal structure. Since hydrophobic sand particles can effectively entrap many air bubbles in the final debris flow-like mixture, the densities of debris flows that sweep over hydrophobic soil will accordingly reduce. Therefore, this paper proposes empirical estimates of density reductions resulting from air entrapment.
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- Award ID(s):
- 2025643
- PAR ID:
- 10647543
- Publisher / Repository:
- Emerald Publishing Limited
- Date Published:
- Journal Name:
- Environmental Geotechnics
- ISSN:
- 2051-803X
- Page Range / eLocation ID:
- 1 to 21
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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