Holly L. Capelo

Stability analysis of two-fluid protoplanetary disc models has enriched our understanding of how solids can grow into larger bodies called planetesimals. Dust particles entrained in a gas stream modify the flow, creating shear layers prone to instability. In such environments, drag occurs in the free-molecular (Epstein) regime. Recreating these two-phase flows on Earth is difficult due to gravity-driven buoyancy. Here, we use particle image velocimetry to study a low-pressure dust-gas mixture at Knudsen numbers up to 10 in microgravity. We observe a granular shear flow instability, characterized by a periodic velocity field, which can be modeled to first order as a Kelvin-Helmholtz (KH) instability. This behavior resembles a Kelvin-Helmholtz instability and provides a benchmark for two-fluid theories relevant to planet formation.

Understanding the intricate dynamics of gas-dust interactions within protoplanetary disks is crucial for unraveling the mysteries of planetary system formation. We introduce TEMPus VoLA (Timed Epstien Multi-pressure vessel at Low Accelerations), an experimental setup designed to investigate particle dynamics and rarefied gas behavior in microgravity (Capelo et al. 2022 https://doi.org/10.1063/5.0087030). Figure 1 shows the experimental setup in flight configuration, together with a rendering of the multiple pressure vessels containing low-pressure gas and dust analogue materials. Figure 1 (Reproduced from Capelo et al 2022). Left: TEMPusVoLA in flight configuration aboard Air Zero-G. Right: Multiple pressure vessels containing rarified gas and dust particle analog. Each chamber is designated to study a particular process: Shear flow, permeability, and drag forces on aggregates. Through multiple dedicated experiments, we explore the effects of collective particle-gas interaction on pressure gradients (permeability), aerodynamic drag coefficients and mechanical properties of dust aggregates, and the onset of turbulence in shear flows. Figure 2 shows an example of a periodic velocity field developing in the shear flow chamber, where dust is injected as a stream in the flow of gas at vaccum pressure. We interpret the velocity field to indicate the onset of a Kelvin-Helmholtz like instability, generated for the first time with dust as the dense phase in pure molecular flow. Figure 2 (Reproduced from Capelo et al. 2022). Particle-velocity in the stream-wise direction of the shear flow chamber, derived using particle image velocimetry. The particle velocities are not constant, but rather oscilate. Our findings, gleaned from multiple parabolic flight campaigns, support a better understanding of dust transport and dynamics in planet-forming discs and also have implications for understanding phenomena like dust emission from cometary nuclei. The presented framework offers a valuable tool for validating models and numerical simulations of collective dust particle aerodynamics in low-gravity environments.

The field of planetary system formation relies extensively on our understanding of the aerodynamic interaction between gas and dust in protoplanetary disks. Of particular importance are the mechanisms triggering fluid instabilities and clumping of dust particles into aggregates, and their subsequent inclusion into planetesimals. We introduce the timed Epstein multi-pressure vessel at low accelerations, which is an experimental apparatus for the study of particle dynamics and rarefied gas under micro-gravity conditions. This facility contains three experiments dedicated to studying aerodynamic processes: (i) the development of pressure gradients due to collective particle–gas interaction, (ii) the drag coefficients of dust aggregates with variable particle–gas velocity, and (iii) the effect of dust on the profile of a shear flow and resultant onset of turbulence. The approach is innovative with respect to previous experiments because we access an untouched parameter space in terms of dust particle packing fraction, and Knudsen, Stokes, and Reynolds numbers. The mechanisms investigated are also relevant for our understanding of the emission of dust from active surfaces, such as cometary nuclei, and new experimental data will help interpreting previous datasets (Rosetta) and prepare future spacecraft observations (Comet Interceptor). We report on the performance of the experiments, which has been tested over the course of multiple flight campaigns. The project is now ready to benefit from additional flight campaigns, to cover a wide parameter space. The outcome will be a comprehensive framework to test models and numerical recipes for studying collective dust particle aerodynamics under space-like conditions.

Forming macroscopic solid bodies in circumstellar discs requires local dust concentration levels significantly higher than the mean. Interactions of the dust particles with the gas must serve to augment local particle densities, and facilitate growth past barriers in the metre size range. Amongst a number of mechanisms that can amplify the local density of solids, aerodynamic streaming instability (SI) is one of the most promising. This work tests the physical assumptions of models that lead to SI in protoplanetary discs (PPDs). We conduct laboratory experiments in which we track the three-dimensional motion of spherical solid particles fluidised in a low-pressure, laminar, incompressible, gas stream. The particle sizes span the Stokes–Epstein drag regime transition and the overall dust-to-gas mass density ratio,ϵ, is close to unity. A recently published study establishes the similarity of the laboratory flow to a simplified PPD model flow. We study velocity statistics and perform time-series analysis of the advected flow to obtain experimental results suggesting an instability due to particle-gas interaction: (i) there exist variations in particle concentration in the direction of the mean relative motion between the gas and the particles, that is the direction of the mean drag forces; (ii) the particles have a tendency to “catch up” to one another when they are in proximity; (iii) particle clumping occurs on very small scales, which implies local enhancements above the backgroundϵby factors of several tens; (iv) the presence of these density enhancements occurs for a meanϵapproaching or greater than 1; (v) we find evidence for collective particle drag reduction when the local particle number density becomes high and when the background gas pressure is high so that the drag is in the continuum regime. The experiments presented here are precedent-setting for observing SI under controlled conditions and may lead to a deeper understanding of how it operates in nature.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics