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.