Post-wildfire mudflows, in which more than half of the solids are sand or smaller, destroy the watershed environment, life, and infrastructures. The surficial soil particles turn hydrophobic due to the deposition of combusted organic matter during wildfires. Initiated by raindrops splash, runoff and erosion grow into devastating mudflows, quickly blasting obstacles on the way, and carrying large boulders and debris. The internal composition of post-wildfire mudflows has recently become of interest, intending to understand better mechanisms and transport differences between post-wildfire mudflows and non-post-wildfire mudflows. This paper shows critical new insights into how the air got entrapped during the early stage of mudflow and how air entrapment affects the properties of post-wildfire mudflows as a mixture of air bubbles, water, and hydrophobic sand. This paper proposes and experimentally investigates a new paradigm in which a significant amount of air remains entrapped in post-wildfire mudflow via hydrophobic-particle-air attraction. The mudflow mixture’s internal structure depends on the physical state of small liquid marbles, which are small air bubbles covered by hydrophobic sand particles. This paper quantifies the amount of air trapped under different sand-water volumetric concentrations, the effects of mixing speeds (energy), mixing duration, and sand particle size on the final mudflow internal structure. In addition, this paper proposes an empirical estimation of density reductions due to air entrapment in the mixture during the mixing process.