Dynamics of Suspended Dust Grains: Experimental Investigations and Implications for Protoplanetary Discs

The collective interaction of solid particulate matter with flowing gas is one of the fundamental processes occurring in planet-forming discs. Small dust grains and pebbles are comparable to or smaller than the mean free path of the gas in observed and simulated discs, where the collective force of particles on the gas has been predicted to create a fluid 'streaming instability' that can produce a turbulent flow and associated localised solid-density enhancements. Such fluid instabilities are therefore favoured amongst candidate mechanisms that concentrate solids in the initial stages of planet formation, which requires sufficiently compact mass concentrations for gravity to play its role in assembling matter, first by producing precursor planetesimals via gravitational instabilities, and subsequently via accretion. Previously, the streaming instability was studied analytically and with numerical simulations. This is the first work to test the mechanism directly in laboratory experiments. In this thesis, I show that the non-linear phase of the streaming instability is manifest, both in experiments and simulations, when fluidised particles settle against a pressure gradient and the solid-to-gas density ratio is close to unity. The experimental results set a precedent for empirical studies of scalable two-phase flows with properties similar to the fluids in protoplanetary discs. These experimental findings provide a test case to calibrate codes that include additional physical processes important for mass assembly such as collisions and charge attraction. Having established a facility to study low-pressure gas flows will enable myriad additional tests of the dynamical interaction between rarefied gas and sample materials.