2D Yukawa Liquids Research Articles

Non-invasive determination of the parameters of strongly coupled 2D Yukawa liquids

We derive a method for identifying the Yukawa model parameters of two-dimensional (2D) strongly coupled dusty plasmas. The method is based on non-invasive observation of the dust component of the plasma and comparison with a large body of simulation data in the form of simple analytical formulas. This approach requires only input from a time-series of configuration snapshots and particle velocities with no recourse to additional information about the system. In addition, we derive an effective coupling parameter for 2D Yukawa systems based on the height of the first maximum of the pair distribution function. Two variants—one valid in the high-coupling range, the other for arbitrary couplings of the liquid—are derived. Comparison to previous approaches to Yukawa coupling parameters shows that the present concept is more general and more accurate. Our results should be directly applicable as a simple, yet reliable diagnostic method for a variety of experiments, including dusty plasmas, colloidal suspensions, and ions in traps, and can be employed to facilitate comparisons between experiments, theory, and simulations.

Time-correlation functions and transport coefficients of two-dimensional Yukawa liquids

The existence of coefficients for diffusion, viscosity, and thermal conductivity is examined for two-dimensional (2D) liquids. Equilibrium molecular dynamics simulations are performed using a Yukawa potential and the long-time behavior of autocorrelation functions is tested. Advances reported here as compared to previous 2D Yukawa liquid simulations include an assessment of the thermal conductivity, using a larger system size to allow meaningful examination of longer times, and development of improved analysis methods. We find that the transport coefficient exists for diffusion at high temperature and viscosity at low temperature, but not in the opposite limits. The thermal conductivity coefficient does not appear to exist at high temperature. Further advances in computing power could improve these assessments by allowing even larger system sizes and longer time series.

Non-Gaussian statistics and superdiffusion in a driven-dissipative dusty plasma

Particle random motion can exhibit both anomalous diffusion and non-Gaussian statistics in some physical systems. Anomalous diffusion is quantified by a deviation from alpha=1 in a power law for a particle's mean-square displacement, MSD proportional, variant(Deltat)alpha. A deviation from Gaussian statistics for a probability distribution function (PDF) is quantified by fitting to a kappa function or Tsallis distribution, with a fit parameter q. We report an experiment and simulations to test a theory that connects anomalous diffusion and non-Gaussian statistics. In the experiment, a single-layer dusty plasma, which behaved as a two-dimensional (2D) driven-dissipative system, had a non-Gaussian PDF. By adjusting an externally applied laser heating, q was varied over a wide range. A correlation between the deviations from Gaussian statistics and normal diffusion for a 2D liquid was found in the experiment. This correlation indicates a connection between anomalous diffusion and non-Gaussian statistics. However, such a connection is lacking in equilibrium 2D Yukawa liquids, as demonstrated in numerical simulations.

SHEAR VISCOSITY OF STRONGLY-COUPLED TWO-DIMENSIONAL YUKAWA LIQUIDS: EXPERIMENT AND MODELING

This paper reviews experimental and modeling efforts aimed at the determination of the shear viscosity of strongly-coupled Yukawa liquids. After briefly reviewing prior work on three-dimensional (3D) systems, recent experimental and computer simulation studies of two-dimensional (2D) settings are presented in detail. In the experiments two counterpropagating laser beams were used to perturb a dusty plasma monolayer and monitoring of the velocity field reconstructed from particle trajectories allowed the determination of the shear viscosity with the aid of an analytical model. Subsequent computer simulations based on the molecular dynamics approach resulted in velocity profiles which are in very good agreement with the experimental ones. Further simulation studies of idealized 2D Yukawa liquids (in which gas friction is neglected) gave results for the shear viscosity over a wide range of system parameters and demonstrated the existence of the shear thinning effect (non-Newtonian behavior) of the liquid at high shear rates.

Phonons in Yukawa lattices and liquids

The understanding of the theoretical structure of phonon dispersion in Yukawa lattices and the relationship between these perfect lattice phonons on the one hand, and the excitations in the disordered and liquid states on the other, is an important issue in analysing experimental and simulation results on plasma crystals. As the first step in this programme, we have numerically calculated the full phonon spectrum for 2D triangular Yukawa lattices, for a wide range of κ (screening parameter) values and along different propagation angles. Earlier calculations of the excitation spectra of the 2D and 3D Yukawa liquids were based on the quasilocalized charge approximation (QLCA), whose implicit premise is that the spectrum of an average distribution (governed by the isotropic liquid pair correlation function) is a good representation of the actual spectrum. To see the implications of this model more clearly, we compare the high r (near crystallization) QLCA phonon spectra with the angle-averaged phonon spectra of the lattice phonons.