  This paper shows critical new insights into how air entrapment affects the
properties of rain-induced post-wildfire mudflows as a mixture of air bubbles,
water, and hydrophobic sand. The idea of mudflows' internal structure
containing trapped air bubbles is novel. Such mixtures can flow down slopes at
incredible speeds, quickly blasting obstacles on the way and carrying large
stone boulders and objects. The surficial soil particles turn hydrophobic due
to the deposition of combusted organic matter during wildfires. Afterward,
raindrops, splash, and erosion form devastating mudflows. We propose a new
paradigm in which a significant amount of air remains entrapped in
post-wildfire mudflow via hydrophobic particle-air attraction. Specific
findings quantify the amount of air trapped within sand-water volumetric
concentrations, the effect of intermixing energy, gravity, and sand particle
size on outcome mudflow internal structure. As a result, little agglomerates of
sand particles covering air bubbles characterize the mudflow mixture's internal
structure.
