Settling of actively buoyant particles

Not all particulate matter carried by fluid flows has constant buoyancy; in some cases, the buoyancy of a particle can change dynamically based on the surrounding flow. We refer to such an effect as "active buoyancy." Actively buoyant particles are found throughout nature. Examples include firebrands in wildfires, whose effective buoyancy is modulated by conductive and convective heat transfer, or some marine microorganisms, which can vary their density to move up and down the water column. Although they are common, the dynamics of actively buoyant particles are not well understood. Thus, to begin to understand this complex problem, we will describe the results of a series of experiments that were conducted to investigate the effect of active buoyancy on particle settling. We find that particles that are closer to neutrally buoyant with high Grashof number have increased mean settling velocity by up to 1.6 times their expected mean settling velocity. Additionally, we observe a nonmonotonic relationship between normalized settling velocity and Grashof number across a range of particle to fluid density ratios.