Oscillating particles in the sky

Publication of the Physics Laboratory in the journal Physical Review Letters on January 19, 2024. News by CNRS Physics on January 23, 2024. Tiny particles such as ice crystals or ashes tend to oscillate when deposited in the atmosphere. By combining theory and experiments, a Franco-German-Swedish scientific collaboration involving the Physics Laboratory (CNRS/ENS de Lyon) has built a model that can help refine forecasts of the wind transport of atmospheric pollutants and weather forecasts. The orientation of nonspherical particles in the atmosphere, such as volcanic ash and ice crystals, influences their residence times and the radiative properties of the atmosphere. Here, we demonstrate experimentally that the orientation of heavy submillimeter spheroids settling in still air exhibits decaying oscillations, whereas it relaxes monotonically in liquids. Theoretical analysis shows that these oscillations are due to particle inertia, caused by the large particle-fluid mass-density ratio.