Entrainment by a plume of microscopic particles falling in air

While multiphase plumes are widespread in natural and industrial settings, particle plumes remain scarcely studied and poorly understood, in particular in gas-solid systems. Their dynamics is instead crucial to environmental issues associated to, for example, mining operations and volcanic eruptions. We perform experiments on 30-micrometer glass spheres falling at high concentrations into otherwise quiescent air. Using high-speed particle image velocimetry and particle tracking velocimetry, we characterize the plume velocity and spreading rate, highlighting fundamental differences with single-phase plumes. Moreover, using multiple camera views, we track the plume interface and measure the surrounding air motion to calculate the entrainment rate. The latter informs a one-dimensional model based on mass and momentum conservation of both phases, which compares favorably with the experimental observation. The measurements reveal that, over the considered range of parameters and unlike single-phase shear flows, entrainment is largely driven by engulfment over large billows formed by an interface that does not exhibit fractal properties.