Lu-Jing Hou

Effect of neutral gas motion on the rotation of dust clusters in an axial magnetic field

Experiments are carried out to investigate the rotation of dust clusters in a radio-frequency plasma sheath with a vertical magnetic field. Our observations are in disagreement with the standard model, in which it was assumed that the neutral gas is at rest and that a steady rotation is attained when the ion-drag force is balanced by neutral friction. Here, we re-examine this basic assumption by carefully designed experiments. Our results suggest that the neutral gas is set into rotation by E×B induced ion flow through ion-neutral collisions and that the dust particles are advected by this flow. A hydrodynamic model is proposed to describe the rotation of the neutral gas and it can explain our observations.

Self-Diffusion in 2D Dusty-Plasma Liquids: Numerical-Simulation Results

We perform Brownian dynamics simulations for studying the self-diffusion in two-dimensional (2D) dusty-plasma liquids, in terms of both mean-square displacement and the velocity autocorrelation function (VAF). Superdiffusion of charged dust particles has been observed to be the most significant at an infinitely small damping rate gamma for intermediate coupling strength, where the long-time asymptotic behavior of VAF is found to be the product of t;{-1} and exp(-gammat). The former represents the prediction of early theories in 2D simple liquids and the latter the VAF of a free Brownian particle. This leads to a smooth transition from superdiffusion to normal diffusion, and then to subdiffusion with an increase of the damping rate. These results well explain the seemingly contradictory observations scattered in recent classical molecular dynamics simulations and experiments of dusty plasmas.

Trapped particles by large-amplitude waves in two-dimensional Yukawa liquids

In recent experiments of strongly coupled complex plasmas (SCCPs), trapping-like phenomena of micro-sized dust particles were observed during propagation of large-amplitude dust-density waves. A small number of particles were observed being trapped and carried along with the waves. In the present paper, we study the similar trapping phenomena in two-dimensional SCCPs by using Brownian dynamics simulation. The trapping process and its new features arising from strong-coupling effects are investigated. In particular, similarities and differences between trapping in weakly coupled plasmas and SCCPs are discussed in detail.

Microparticles deep in the plasma sheath: Coulomb “explosion”

A cloud of microparticles was trapped deep in the sheath of a radio-frequency (rf) discharge, very close to the lower (grounded) electrode of the plasma chamber. This was achieved by employing a specifically designed rf-driven segment integrated in the lower electrode, which provided an additional confinement compressing the cloud to a very high density. After switching the rf-driven segment off, the cloud “exploded” due to mutual interparticle repulsion. By combining a simple theoretical model with different numerical simulation methods, some basic properties of complex plasmas in this highly non-equilibrium regime were determined.

Effect of Centrifugal Forces on the Interparticle Distance of Two Dust Particles Confined in a Plasma

Dusty-plasma experiments with flat dust clusters are often performed in radio frequency discharges at typical gas pressures of 1 to 100 Pa. For the understanding of the structure and the dynamical behavior of dust clusters, the grain charge and the effective Debye length are of key interest. Direct measurements of these quantities are a challenging task because the dust grains are confined in the plasma-boundary layer, where plasma diagnostics is difficult. Here, we present a new approach to determine the grain charge and the horizontal screening length, which does not require prior knowledge of the plasma parameters. The method is based on a slow and controlled rotation of the neutral-gas column due to a rotating electrode. This leads to centrifugal forces to the dust grains without changing the plasma conditions and allows studying the interparticle distance of a dust cluster as a function of its rotation frequency, which directly depends on the particle charge, the screening length, and the strength of the confining potential.

A full account of compressional wave in 2D strongly coupled complex (dusty) plasmas: Theory, experiment and numerical simulation

We present a first comprehensive theoretical, experimental and numerical study of compressional wave in a two-dimensional strongly coupled complex plasma in terms of the fluctuation spectra. Full spectra for different coupling strengths are calculated by means of generalized hydrodynamics within the memory function formalism by enforcing the low-order, high-frequency sum rules that employ the static structure factor and the radial distribution function from simulation as inputs. Results are compared with those from a laboratory experiment and a self-consistent Brownian dynamics simulation. Good agreement is found between theory and experiment, not only in the peak locations of spectra, i.e., the dispersion relations, but also in the half-widths of spectra, which represent the damping of the collective modes in liquid state for a broad range of wavelengths.

Oblique interactions of dust density waves

Self‐excited dust density waves (DDWs) are studied in a striped electrode device. In addition to the usual perpendicularly (with respect to the electrode) propagating DDWs, which have been frequently observed in dusty plasma experiments on the ground, a low‐frequency oblique mode is also observed. This low‐frequency oblique DDW has a frequency much lower than the dust plasma frequency and its spontaneous excitation is observed even with a very low dust density. It is found that the low‐frequency oblique mode can exist either separately or together with the usual perpendicular mode. In the latter case, a new mode arises as a result of the interactions between the perpendicular and the oblique modes. The experiments show that these three modes satisfy the wave coupling conditions in both the frequencies and the wave‐vectors.

Expansion of complex plasma and dust particle charge

Summary form only given. Plasma expansion from a hot dense source to a low pressure environment is relevant to many subjects in science [1], [2]. In this contribution we present the expansion of dusty plasma inside a plasma sheath of an radio-frequency (rf) discharge using specifically designed segmented electrode and estimate the charge of particles at the initial stage of the expansion.

Demixing in Binary Complex Plasma: Computer Simulation

Computer simulations are employed to investigate the phase separation (demixing) phenomenon induced by interaction nonadditivity in binary complex plasmas with two different sized microparticles. The system firstly coarsens, and small particles start gathering and forming small clusters inside the large-particle phase, while there are no clear boundaries between two species. Next, sharp interfaces build up while the small-particle clusters merge into droplets and small droplets continue to merge into big ones. Finally, the two species completely separate from each other, and the small particles tend to form big droplets of nearly spherical shape, due to the surface tension.

Initial stages in phase separation of binary complex plasmas: Numerical experiments

Numerical experiments are performed to investigate the initial stages in phase separation induced by interaction non-additivity Δ in binary complex plasmas. A characteristic length scale obtained from time-dependent pair correlation functions is used to monitor the domain growth. It is found that the domain growth follows a power law with an exponent α of around 1/3, which is in a good agreement with the Lifshitz-Slyozov growth law for the initial diffusive regime of phase separation. It is also found that α is almost independent of Δ as long as the system is within the spinodal region. The coupling strength is also found to have almost no influence on α, unless the coupling is so large that the background large-particle phase starts crystallizing.