Simulating the dynamics of complex plasmas

Configurational temperature of multispecies dusty plasmas

The dust charge of the two species in a binary mixture of particles in a dusty plasma has been measured using the concept of configurational temperature. There, the dust charge and the respective dust charge ratio are determined from the comparison of the instantaneous particle positions and the kinetic temperature. For that purpose, experiments of binary mixtures of melamine-formaldehyde and silica particles have been evaluated. The configurational temperature approach has also been checked against simulations. From these analyses it is found that the charge ratio of the two species can be obtained quite accurately, whereas for the determination of the absolute charge values a good knowledge of the confining potential is required.

Simulations and experiments of phase separation in binary dusty plasmas

Molecular dynamics simulations of binary dusty plasmas have been performed and their behavior with respect to the phase separation process has been analyzed. The simulated system was inspired by experimental research on phase separation in dusty plasmas under microgravity on parabolic flights. Despite vortex formation in the experiment and in the simulations the phase separation could be identified. From the simulations it is found that even the smallest charge disparities lead to phase separation. The separation is due to the force imbalance on the two species and the separation becomes weaker with increasing mean particle size. In comparison, experiments on the phase separation have been performed and analyzed in view of the separation dynamics. It is found that the experimental results are reproduced by the simulation regarding the dependency on the size disparity of the two particle species.

Penetration of a supersonic particle at the interface in a binary complex plasma

The penetration of a supersonic particle at the interface is studied in a binary complex plasma. Inspired by the experiments performed in the PK-3 Plus Laboratory on board the International Space Station, Langevin dynamics simulations were carried out. A Mach cone structure forms in the lateral wave behind the supersonic extra particle, where the kink of the cone flanks is observed at the interface. The propagation of the pulse-like perturbation along the interface is demonstrated by the evolution of the radial and axial velocity of the small particles in the vicinity of the interface. The decay of the pulse strength is determined by the friction, where the propagation distance can reach several interparticle distances for small damping rate. The dependence of the dynamics of the background particles in the vicinity of the interface on the penetration direction implies that the disparity of the mobility may be the cause of various interfacial effects.

Ion attachment rates and collection forces on dust particles in a plasma sheath with finite ion inertia and mobility

Ion attachment and ion drag to dust particles near the edge of a nonthermal plasma sheath are of interest to better understand how particles become trapped in such sheath regions. While electron-particle collisions in plasmas and sheaths can often be described by orbital motion limited theory, quantification of ion transport about dust particles in collisional sheath regions requires a distinct modeling approach. In this work, the dimensionless ion attachment coefficients and dimensionless collection forces on negatively charged particles are calculated using ion trajectory models accounting for an external electric field in a collisional sheath, ion inertia, and finite ion mobility. By considering both ion inertia and finite ion mobility, results apply for ion transport from the fully collisional regime into a regime of intermediate collisionality. Ion collection forces are calculated in two related limits; first, the nondissipative limit, wherein the dimensionless collection force function coincides with the dimensionless attachment coefficient (anticipated in the collisionless regime), and second, a dissipative limit, wherein neutral gas collisions dissipate ion momentum, which strongly affects the resulting collection force (anticipated in the fully collisional regime). We show that ion motion about a charged particle can be parametrized by the ion Stokes number, which is the ratio of ion inertia to gas resistance to motion and dimensionless electric field strength (the external field strength normalized by the electric field at the particle surface). At intermediate Stokes numbers (10^{1}-10^{2}), ions adopt trajectories that are extremely sensitive to the initial ion-particle impact parameter. Plots of the resulting collision angle at fixed Stokes number and dimensionless field strength as a function of impact parameter contain multiple discontinuities. Nonetheless, we obtain smooth curves for the ion attachment rates and collection forces in both the nondissipative and fully dissipative limits. Increasing the ion Stokes number is found to significantly decrease the dimensionless ion attachment coefficients and ion collection forces in comparison to coefficients evaluated with expressions derived in the fully collisional limit. In all instances, including the dissipative limit, we find the ion collection force acts in the direction of ion migration. Neural network fits are utilized to parametrize the resulting attachment coefficients and ion collection forces, and we apply these fits to examine the charge levels on 1-μm radius particles in external fields in the 3×10^{2}-3×10^{3}Vm^{-1} range and pressures in the 5×10^{-1}-5×10^{1} Torr (66.7-6667 Pa) range. We find the charge level is strongly sensitive to both field strength and pressure in the plasma sheath, ranging from 6 × 10^{3} to 1.8 × 10^{4} over the conditions examined. Calculations are also used to demonstrate that the ion collection force can be sufficiently strong to trap particles not only close to the bottom electrode of a parallel-plate reactor, but also close to the top electrode, with a critical ion density required for trapping.

Dynamical states in two-dimensional charged dust particle clusters in plasma medium

The formation of dynamical states for a collection of dust particles in two dimensions is shown using molecular dynamics simulations. The charged dust particles interact with each other with a Yukawa pair potential mimicking the screening due to plasma. An external radial confining force has also been applied to the dust particles to keep them radially confined. When the particle number is low (say, a few), they get arranged on the radial locations corresponding to multiple rings or shells. For specific numbers, such an arrangement of particles is stationary. However, for several cases, the cluster of dust particles relaxes to a state for which the dust particles on rings display intershell rotation. For a larger number of dust particles (a few hundred, for instance), an equilibrium state with a coherent rigid body displaying angular oscillation of the entire cluster is observed. A detailed characterization of the formation of these states in terms of particle number, coupling parameter, etc., is provided.

Experimental Demonstration of a Dusty Plasma Ratchet Rectification and Its Reversal

The naturally persistent flow of hundreds of dust particles is experimentally achieved in a dusty plasma system with the asymmetric sawteeth of gears on the electrode. It is also demonstrated that the direction of the dust particle flow can be controlled by changing the plasma conditions of the gas pressure or the plasma power. Numerical simulations of dust particles with the ion drag inside the asymmetric sawteeth verify the experimental observations of the flow rectification of dust particles. Both experiments and simulations suggest that the asymmetric potential and the collective effect are the two keys in this dusty plasma ratchet. With the nonequilibrium ion drag, the dust flow along the asymmetric orientation of this electric potential of the ratchet can be reversed by changing the balance height of dust particles using different plasma conditions.

Dynamics of nonspherical, fractal-like water-ice particles in a plasma environment

Plasmas containing small solid-state particles (also known as dust particles) are ubiquitous in nature and laboratories. Existing models typically assume that the dust particles are spherical but several observations and simulations indicate that a significant amount of dust particles are nonspherical. Because dust particles are not spherical they show different dynamics from spherical particles in a plasma environment namely, they align in the direction perpendicular to the force equilibrium line, rotate about their alignment axis due to the interaction between the dipole moment and the surrounding electric field, and show vortex motion while maintaining their alignment and rotation when they are exposed to a nonconservative drag force.

Spiral waves in driven strongly coupled Yukawa systems

Spiral wave formations are ubiquitous in nature. In the present paper, the excitation of spiral waves in the context of driven two-dimensional dusty plasma (Yukawa system) has been demonstrated at particle level using molecular-dynamics simulations. The interaction amidst dust particles is modeled by the Yukawa potential to take account of the shielding of dust charges by the lighter electron and ion species. The spatiotemporal evolution of these spiral waves has been characterized as a function of the frequency and amplitude of the driving force and dust neutral collisions. The effect of strong coupling has been studied, which shows that the excited spiral wave structures get clearer as the medium gets more strongly coupled. The radial propagation speed of the spiral wave is observed to remain unaltered with the coupling parameter. However, it is found to depend on the screening parameter of the dust medium and decreases when it is increased. In the crystalline phase (with screening parameter κ>0.58), the spiral wavefronts are shown to be hexagonal in shape. This shows that the radial propagation speed depends on the interparticle spacing.

Collective effects in vortex movements in complex plasmas

We study the onset and characteristics of vortices in complex (dusty) plasmas using two-dimensional simulations in a setup modeled after the PK-3 Plus laboratory. A small number of microparticles initially self-arranges in a monolayer around the void. As additional particles are introduced, an extended system of vortices develops due to a nonzero curl of the plasma forces. We demonstrate a shear-thinning effect in the vortices. Velocity structure functions and the energy and enstrophy spectra show that vortex flow turbulence is present that is in essence of the "classical" Kolmogorov type.