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Complex plasmas contain, in addition to the usual electrons, ions, and neutral atoms, macroscopic electrically charged (nanometer to micrometer) sized dust particles. Based on the ratio of the electrostatic potential to kinetic energy, these microparticles can exhibit gaseous, fluid, and crystal-like behavior. For this reason, complex plasmas are a unique testing ground to study multiparticle systems. The dynamics of these systems can be studied using the particle tracking velocimetry (PTV) analysis technique. The PTV technique provides a spatially resolved particle phase space distribution function, which can be used to calculate correlation functions and thermal properties of the system. There are experimental settings, such as microgravity experiments, where technical and data storage limitations make it desirable to have near real-time video analysis techniques that allow an experimenter to tune operating conditions until appropriate velocity profiles or velocity distributions are obtained. This article discusses PTV software that allows for real-time particle tracking that, we believe, can be applied to a broad range of physical systems.

Filamentary structures in low-pressure, low-temperature plasmas are produced when strong magnetic fields are applied parallel to the electric field defined by parallel electrodes. Filamentary structures are regions within the plasma that have distinct properties such as optical brightness and extend along the magnetic field lines. In our experiments, an argon, radio frequency discharge is exposed to a strong background magnetic field in the magnetized dusty plasma experiment at Auburn University. Different forms of filamentary structures emerge in the plasma such as columns, target, or spiral-like structures. To investigate the origin and the characteristics of these patterns, we have developed a three-dimensional fluid model that can reproduce the experimental observations which enables us to investigate the underlying mechanisms of the filamentation process.

First results of complex plasma experiments in microgravity with grid electrodes in the Ekoplasma facility are presented. These electrodes separate the region of plasma production from the working volume where the microparticles are injected. Grid electrodes allow a control of plasma parameters, and provide a nearly force-free environment for the microparticles. The experiments show that the grid electrode method can be applied to complex plasmas and that this technology yields a promising new experimental method for future research topics.

The last decade has seen the development of new experimental devices to explore the physics of magnetized dusty plasmas. Because of the small charge-to-mass ratio of the charged microparticles, it is necessary to operate these experiments at high magnetic fields of several Tesla in order to observe the direct effect of the magnetic forces on the transport properties of the charged microparticles. While the study of magnetized dusty plasmas is still the ultimate goal, these experiments have also provided new opportunities to studies regimes of strongly magnetized, low temperature, laboratory plasmas that have not been extensively explored. Experiments show the formation of new types of self- and imposed-ordered structures that form in both the plasma and among the microparticles. This paper summarizes recent experimental observations of plasma filamentation (in the plasma) and will discuss possible connections to 'dust gridding' phenomena that are observed in the magnetized dusty plasma experiment device.

We address an experimental observation of pattern formation in a magnetised rf plasma. The experiments are carried out in a electrically grounded aluminium chamber which is housed inside a rotatable superconducting magnetic coil. The plasma is formed by applying a rf voltage in parallel plate electrodes in push-pull mode under the background of argon gas. The time evolution of plasma intensity shows that a homogeneous plasma breaks into several concentric radial spatiotemoral bright and dark rings. These rings propagate radially at considerably low pressure and a constant magnetic field. These patterns are observed to trap small dust particles/grains in their potential. Exploiting this property of the patterns, a novel technique to measure the electric field associated with the patterns is described. The resulting estimates of the corresponding field intensity are presented. At other specific discharge parameters the plasma shows a range of special type of characteristic structures observed in certain other chemical, mechanical and biological systems., It contains four pages with four figures

Experiments with complex plasmas in a dc discharge using the PK-4 facility will be discussed, focusing on the formation of strings corresponding to an electrorheological fluid observed in recent parabolic flight experiments.

Experimental studies are presented where dust particles are suspended in the lower sheath region of an argon rf discharge at a strong vertical magnetic field from B=1.5 T up to 2.27 T. There the particles arranged in an ordered pattern imposed by the upper mesh electrode. It is observed that the particles jump to a new equilibrium position, where they exhibit self-excited vertical oscillations when illuminated by a horizontal laser beam. The dust motion is weakly damped during an upward jump and strongly damped during the return to the equilibrium after the laser is switched off. A model based on delayed charging is presented that can describe the observed behavior.

This special issue contains papers solicited at the 14th Workshop on the Physics of Dusty Plasmas, which was hosted by Auburn and Wittenberg University and held in Auburn, AL, USA, from 26 May, 2015 to 29 May, 2015. The publication of this issue follows the tradition of earlier workshops, which resulted in similar special issues in 1994, 2001, 2004, 2007, 2010, and 2013, and is intended to provide a snapshot of recent progress in the field of dusty plasma research.

It is shown that the quantitative interpretation of recent experiments to determine the ion drag force in complex (dusty) plasmas [C. Zafiu, A. Melzer, and A. Piel, Phys. Plasmas 9, 4794 (2002); 10, 1278 (2003)] is not correct. A comparison of different models of the ion drag force is carried out to illustrate the complexity of this issue and to highlight the current level of the research.

A defocusing imaging technique has been used as a diagnostic to identify binary agglomerates (dimers) in complex plasmas. Quasi-two-dimensional plasma crystal consisting of monodisperse spheres and binary agglomerates has been created where the agglomerated particles levitate just below the spherical particles without forming vertical pairs. Unlike spherical particles, the defocused images of binary agglomerates show distinct, stationary/periodically rotating interference fringe patterns. The results can be of fundamental importance for future experiments on complex plasmas.

Agglomeration of highly charged microparticles was observed and studied in complex plasma experiments carried out in a capacitively coupled rf discharge. The agglomeration was caused by strong dust density waves triggered in a particle cloud by decreasing neutral gas pressure. Using a high-speed camera during this unstable regime, it was possible to resolve the motion of individual microparticles and to show that the relative velocities of some particles were sufficiently high to overcome the mutual Coulomb repulsion and hence to result in agglomeration. After stabilising the cloud again through the increase of the pressure, we were able to observe the aggregates directly with a long-distance microscope. We show that the agglomeration rate deduced from our experiments is in good agreement with theoretical estimates. In addition, we briefly discuss the mechanisms that can provide binding of highly charged microparticles in a plasma., submitted to Phys. Plasma

Dusty plasmas have been studied in argon, radio frequency (rf) glow discharge plasmas at magnetic fields up to 2.5 T where the electrons and ions are strongly magnetized. Plasmas are generated between two parallel plate electrodes where the lower, powered electrode is solid and the upper electrode supports a dual mesh consisting of #24 brass and #30 aluminum wire cloth. In this experiment, we study the formation of imposed ordered structures and particle dynamics as a function of magnetic field. Through observations of trapped particles and the quasi-discrete (i.e., “hopping”) motion of particles between the trapping locations, it is possible to make a preliminary estimate of the potential structure that confines the particles to a grid structure in the plasma. This information is used to gain insight into the formation of the imposed grid pattern of the dust particles in the plasma.

Experiments with complex (“dusty”) plasmas in microgravity often show a particle-free region in the center of the discharge, which is the so-called “void.” The cause of this unexpected feature is assumed to be the ions streaming outward from the discharge center toward the radio-frequency electrodes and walls, dragging the microparticles with them out of the center. In several experiments onboard the International Space Station, single particles moving out of the void could be traced. The obtained trajectories reveal the distribution of the forces inside the void.

In ground-based laboratory experiments, in experiments under microgravity conditions and in parabolic flight experiments, it is often observed that individual microparticles appear sometimes to simply pass through a strongly coupled complex plasma without affecting the fabric (the structure) of the penetrated cloud. This is surprising at first sight, because the collisional mean free path in such 'plasma crystals' is of the order of the interparticle separation. We have termed this new effect 'classical tunnelling' in analogy to its quantum mechanical counterpart. To explain this anomalous transport, a 'geometrical model for charge variation' of moving and stationary ordered particles is proposed. Theoretical predictions are in good qualitative agreement with observations and numerical simulations, and show that the phenomenon can be considered as a consequence of the non-Hamiltonian character of complex plasmas. ...It's a mystery therefore I can think whatever I want ... ...Scientists have a strange tendency to be insufficiently empirical sometimes... J Whyte www.newscientist.com

A highly efficient coagulation process of microparticles was discovered during experiments that were carried out on the International Space Station. Charged microspheres injected into a low pressure neutral gas environment form mm-sized aggregates, provided the initial number density is high enough. The coagulation occurred at a rate ∼10 4 -10 5 times faster than that usually observed with uncharged grains. Theoretical estimates show that this process is so fast that it cannot even be explained by a coagulation process with charge enhanced cross-section. The observations can, however, be described by a 'gelation phase transition'. We summarize the salient measurements, outline the theoretical description and show their compatibility.