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  1. Abstract

    The mechanical processes that convert an initially fluffy chondrule fine-grained rim (FGR) into a more compact structure remain poorly characterized. Given the presence of shocks in protoplanetary disks, we use numerical simulations to test the hypothesis that dust-laden shocks in the solar nebula contributed to FGR modification. We use the iSALE2D shock physics code to model the collision of dusty nebular shock fronts (which we term “dust clouds”) into chondrule surfaces that host a porous FGR. In our simulations, dust particles are modeled as dunite disks. The dust radii follow the Mathis–Rumpl–Nordsieck distribution of interstellar grains. Chondrules are modeled as rectangular dunite slabs. We vary the impact speedvimp, the fractional abundancefcloudof dust grains in the impacting shock, and the fractional abundancefFGRof dust grains in the pre-existing FGR. We thus compute dust temperatures and pressures resulting from the collisions, as well as the net mass accretion of dust by the FGRs. Dust temperatures increase upon impact, depending on the kinetic energy of the dust cloud and onfFGR. Dust rims with a higherfFGRheat up more than those with a lowerfFGR, with possibly important implications for the composition and structure of FGRs. Maximum impact pressures increase withfcloud. Fine-grained rims can experience mass gain from the impacting cloud, but in some instances, mass is lost from the rim. We find qualitative similarities in the topography of the FGR–chondrule interface between our simulations and petrographic analyses of the Paris CM chondrite by other authors.

     
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  2. This study examines the structure and stability of filamentary dusty plasmas using data from the Plasmakristall-4 (PK-4) facility on board the International Space Station. Under the action of a polarity-switched DC electric field, the dust particles in the PK-4 discharge have been found to organize into field-aligned extended filaments, which has been compared to the filamentary state in electrorheological (ER) fluids. Here we discuss how, in addition to an ER-type structural transition, the PK-4 dusty plasmas exhibit structural states reminiscent of those observed in liquid crystals (LCs) with rod-shaped molecules. We find that dust particles within the filaments are strongly coupled in a crystalline-like structure, while the coupling of particles across filaments is liquid-like. In addition to a common orientation along a director axis (nematic behavior), the dust filaments also appear to align in large-scale nested structures, or shells (smectic behavior). Finally, these filaments are found to further arrange in hexagonal patterns within the plane orthogonal to the director axis, suggesting the possibility for smectic-B and smectic-C structural states. As the observed ER and LC features of the filamentary dusty plasma states are sensitive to variations in the PK-4 discharge conditions, we argue that these dusty plasmas can provide a controlled analogous system for the study of fundamental phenomena in soft matter, such as the origins of pattern formation and universality of phase transitions. 
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  3. The microgravity environment of the Plasmakristall-4 experiment on the International Space Station provides a laboratory for exploring plasma-mediated interactions among charged dust grains in fully three-dimensional space. Away from the strong influence of Earth's gravity, the dust grains can levitate in the bulk of the plasma, where they have been observed to form extended filamentary structures aligned with the discharge tube axis. These structures can be used as a macroscopic analogue for other self-organizing systems, including electrorheological fluids and liquid crystals, and the success of the analogy depends on a thorough understanding of the mechanisms guiding the dust interaction potential. Here we present the results from molecular dynamics simulations of the ion flow past isolated dust chains within the dust cloud and the dust cloud macrostructure. Although dust grains are known to respond on the millisecond timescale, analysis reveals that periodic variations of plasma conditions on the microsecond timescale significantly affect dust structure formation. In addition to the expected formation of filamentary dust chains in the dust cloud macrostructure, dust grains in a large cloud are also observed to organize into ordered positions on the surface of nested cylinders, in agreement with experimental observations. 
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  4. Abstract Nonlinear frequency response analysis is a widely used method for determining system dynamics in the presence of nonlinearities. In dusty plasmas, the plasma–grain interaction (e.g. grain charging fluctuations) can be characterized by a single-particle non-linear response analysis, while grain–grain non-linear interactions can be determined by a multi-particle non-linear response analysis. Here a machine learning-based method to determine the equation of motion in the non-linear response analysis for dust particles in plasmas is presented. Searching the parameter space in a Bayesian manner allows an efficient optimization of the parameters needed to match simulated non-linear response curves to experimentally measured non-linear response curves. 
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    The PK-4 system is a micro-gravity dusty plasma experiment currently in operation on-board the International Space Station. The experiment utilizes a long DC discharge in neon or argon gases. We apply our 2D particle-in-cell with Monte Carlo collisions discharge simulation to compute local plasma parameters that serve as input data for future dust dynamics models. The simulation includes electrons, Ne+ ions, and Nem metastable atoms in neon gas and their collisions at solid surfaces including secondary electron emission and glass wall charging. On the time scale of the on-board optical imaging, the positive column appears stable and homogeneous. On the other hand, our simulations show that on microsecond time scales the positive column is highly inhomogeneous: ionization waves with phase velocities in the range between 500 m s−1 and 1200 m s−1 dominate the structure. In these waves, the electric field and charged particle densities can reach amplitudes up to 10 times of their average value. Our experiments on ground-based PK-4 replica systems fully support the numerical findings. In the experiment, the direction of the DC current can be alternated, which has been found to favor dust particle chain formation. We discuss possible mechanisms for how the highly oscillatory plasma environment contributes to the dust particle chain formation. 
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  7. Dust kinetic temperature is a measure of the energy of the stochastic motion of a dust particle and is a result of the combination of the Brownian motion and the fluctuations in the dust charge and confining electric field. A method using the equilibrium value of the mean square displacement was recently introduced to obtain the dust kinetic temperature experimentally. As a follow up, this paper investigates the relationship between the dust kinetic energy derived from the mean square displacement technique and a technique using the probability distribution of the displacements obtained from random fluctuations of the dust particle. The experimental results indicate that the harmonic confinement potential acting on the dust particle can be obtained by combining the two methods, allowing the nonlinear effect of the confining force to be investigated. The thermal expansion in a 1-D vertical chain is discussed as a representative application as it is related to the nonlinear confinement force, or the asymmetric confinement potential. 
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  8. Abstract

    In this paper, we report the first experimental observation of internal resonance in a dusty plasma, which shows the intrinsic nonlinearities of dust interactions in plasmas. When driving a system of vertically aligned dust particle pairs in the vertical direction, the horizontal motion is found to be excited during onset of internal resonance when the higher-frequency horizontal sloshing mode is nonlinearly coupled to the vertical breathing mode through the 1:2 commensurable relation. A theoretical model of the nonlinear interaction of dust particles in plasma is also provided and the results of the theoretical model are shown to match experimental observations.

     
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