Complex (dusty) plasmas: current status, open issues, perspectives

Complex (dusty) plasmas are composed of a weakly ionized gas and charged microparticles and represent the plasma state of soft matter. Complex plasmas have several remarkable features: Dynamical time scales associated with microparticles are ``stretched'' to tens of milliseconds, yet the microparticles themselves can be easily visualized individually. Furthermore, since the background gas is dilute, the particle dynamics in strongly coupled complex plasmas is virtually undamped, which provides a direct analogy to regular liquids and solids in terms of the atomistic dynamics. Finally, complex plasmas can be easily manipulated in different ways---also at the level of individual particles. Altogether, this gives us a unique opportunity to go beyond the limits of continuous media and study---at the kinetic level---various generic processes occurring in liquids or solids, in regimes ranging from the onset of cooperative phenomena to large strongly coupled systems. In the first part of the review some of the basic and new physics are highlighted which complex plasmas enable us to study, and in the second (major) part strong coupling phenomena in an interdisciplinary context are examined. The connections with complex fluids are emphasized and a number of generic liquid and solid-state issues are addressed. In summary, application oriented research is discussed.

Complex plasmas: An interdisciplinary research field

Strong correlations—cooperative behavior due to many-particle interactions—are omnipresent in nature. They occur in electrolytic solutions, dense plasmas, ultracold ions and atomic gases in traps, complex (dusty) plasmas, electrons and excitons in quantum dots and the quark–gluon plasma. Correlation effects include the emergence of long-range order, of liquid-like or crystalline structures and collective dynamic properties (collective modes). The observation and experimental analysis of strong correlations are often difficult, requiring, in many cases, extreme conditions such as very low temperatures or high densities. An exception is complex plasmas where strong coupling can be easily achieved, even at room temperature. These systems feature the strongest correlations reported so far and experiments allow for an unprecedented precision and full single-particle resolution of the stationary and time-dependent many-particle behavior. The governing role of the interactions in strongly correlated systems gives rise to many universal properties observed in all of them. This makes the analysis of one particular system interesting for many others. This motivates the goal of this paper which is to give an overview on recent experimental and theoretical results in complex plasmas including liquid-like behavior, crystal formation, structural and dynamic properties. It is expected that many of these effects will be of interest also to researchers in other fields where strong correlations play a prominent role. (Some figures in this article are in colour only in the electronic version) This article was invited by Gordon Baym.

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Complex plasmas: a laboratory for strong correlations

In this review, a systematic overview of dynamical processes in complex (dusty) plasmas is given. Complex plasmas consist of electrons, ions, neutrals, and microparticles of nanometre to micrometre size, which are responsible for the unusual properties of this kind of plasmas, such as the formation of liquid or solid phases at strong electrostatic coupling. The examples represent the progress in this field within the last five years and mostly such cases were chosen, in which experimental results could be compared with theory or where the phenomenon has a diagnostic application. The presentation begins with single particle effects, where levitation, confinement in plasma traps, charging, and oscillations are involved. It is shown that vertical oscillations can be used for particle charge measurements. Nonlinear and parametric effects as well as self-excited oscillations are discussed. Then the interaction force between the particulates is explored in few-particle systems. Scattering experiments show that the interaction in the levitation plane can be described by shielded Yukawatype potentials, while the vertical interaction is governed by additional attractive wake fields. The latter are shown to be asymmetric and lead to the formation of vertical strings. The normal modes of one-dimensional or two-dimensional particle arrangements are useful tools for diagnostics. Many-particle effects are discussed in terms of low-frequency electrostatic waves. The dust-acoustic wave, the dust ion-acoustic wave, and two types of dust lattice waves (shear and compressional) are discussed with respect to diagnostic applications, among them Mach cones in plasma crystals. Laser methods are now established for the excitation of such modes. Two novel types of instabilities, the wakefield instability and the void-forming instability, as well as first results on dust ion-acoustic shocks are presented.

Dynamical processes in complex plasmas

In dusty (complex) plasmas, containing mesoscopic charged grains, the grain–grain interaction in many cases can be well described through a Yukawa potential. In this review we summarize the basics of the computational and theoretical approaches capable of describing many-particle Yukawa systems in the liquid and solid phases and discuss the properties of the dynamical density and current correlation spectra of three- and two-dimensional strongly coupled Yukawa systems, generated by molecular dynamics simulations. We show details of the ω(k) dispersion relations for the collective excitations in these systems, as obtained theoretically following the quasi-localized charge approximation, as well as from the fluctuation spectra created by simulations. The theoretical and simulation results are also compared with those obtained in complex plasma experiments.

Dynamical correlations and collective excitations of Yukawa liquids

A unified theoretical treatment is given of longitudinal (or compressional) and transverse modes in Yukawa crystals, including the effects of damping. Dispersion relations are obtained for hexagonal lattices in two dimensions and bcc and fcc lattices in three dimensions. Theoretical predictions are compared with two recent experiments.

Longitudinal and Transverse Waves in Yukawa Crystals

The Fourier spectra of longitudinal and transverse waves corresponding to random particle motion were measured in a two-dimensional plasma crystal. The crystal was composed of negatively charged microspheres immersed in a plasma at a low gas pressure. The phonons were found to obey a dispersion relation that assumes a Yukawa interparticle potential. The crystal was in a nonthermal equilibrium, nevertheless phonon energies were almost equally distributed with respect to wave number over the entire first Brillouin zone.

http://dusty.physics.uiowa.edu/%7Egoree/papers/nunomura_PRL_2002_phonon_spectrum.pdf

Phonon spectrum in a plasma crystal.

A nonlinear time-dependent model for void formation in colloidal plasmas is proposed. For experimentally relevant initial conditions, the model describes the nonlinear evolution of a zero-frequency linear instability that grows rapidly in the nonlinear regime and subsequently saturates to form a void. A number of features of the model are consistent with experimental observations under laboratory and microgravity conditions.

Nonlinear theory of void formation in colloidal plasmas.

Low-frequency modes in two-dimensional Debye-Yukawa plasma crystals

Linear gradient drift instabilities, which are associated with type II irregularities in the equatorial electrojet, are obtained for daytime profiles using initial-value and eigenmode methods in a nonlocal theory. The methods are shown to be in agreement, thus resolving a recent discrepancy between the theories of Ronchi et al. [1989] and Wang and Bhattacharjee [1994]. A set of equilibrium profiles that are consistent with observed daytime electric and current density profiles are given and analyzed for nonlocal stability. Instabilities with horizontal wavelengths of the order of 1 km are found to be dominant over most of the electrojet, with instabilities of shorter wavelengths (of the order of 100 m) localized in regions of weak velocity shear. The predictions of linear theory are in accord with the observed preponderance of kilometer-scale irregularities in the daytime electrojet.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JA02826

S. Hu

Papers

Longitudinal and Transverse Waves in Yukawa Crystals

Phonon spectrum in a plasma crystal.

Nonlinear theory of void formation in colloidal plasmas.

Low-frequency modes in two-dimensional Debye-Yukawa plasma crystals

Linear gradient drift instabilities in the daytime equatorial electrojet