Dynamics of inertial particles in quiescent flow under microgravity.*

We investigate experimentally the deceleration of heavy spherical particles initially settling at their terminal velocity in quiescent water when gravity is abruptly suppressed. The experiment is mounted in the Dryden Drop Tower located in Portland State University which provides a step gravity reduction from g to 10-2 g in 20 milliseconds. Several particle-to-fluid density ratios and several particle diameters (i.e. particle Galileo numbers) are explored. When the gravity is suppressed, particles slow down until their velocity vanishes. We analyse this transient  relaxation at the light of the celebrated Maxey-Riley-Gatignol (MRG) equation, systematically quantifying the relative importance of the different hydrodynamic forces at play (drag, added mass, buoyancy and history forces).