Dynamics of non-spherical solid particles in free fall in air at Reynolds number close to unity*

Non-spherical particles are ubiquitous in the atmosphere, and their orientation dynamics play an important role in many physical processes, such as, the enhanced reflectivity of ice clouds, the transport of pollutants, e.g, dust and microplastics, and the dispersion of volcanic ash. Due to the non-isotropic shape of such particles, their preferred orientations and free-fall velocities are affected. However, there is still no consensus in the scientific community on the preferred orientations of such particles, in particular at low particle Reynolds number (Re) and at high particle-to-fluid density ratio, which is the parameter space for most particles in the atmosphere. Here we present laboratory experiments and numerical simulations of free-falling particles in quiescent air, that describe the orientation dynamics of ellipsoidal particles at low Re and high density ratio. The particles studied, which vary widely in shape (elongated and/or flattened ellipsoids) and size (less than 1mm), are produced by two-photon polymerization techniques with sub-micrometer resolution. They are injected into a glass shielded parallelepiped tower and allowed to settle in a quiescent environment at a high density ratio of about 1000 and at a particle Reynolds number of about 5, while being imaged with four high-speed cameras. We have observed a large variability in the onset of the asymptotically stable orientation, in the amplitudes and frequencies of the transient oscillations, and in the free-fall dynamics. The experimental data are verified and studied in detail using particle-resolved direct numerical simulations based on the Lattice-Boltzmann Immersed Boundary Method. Our results show that the transient dynamics does not depend on Re and shape alone, but is fundamentally affected by the particle-to-fluid density ratio.