The shear viscosity of a two-dimensional liquid-state dusty plasma was measured experimentally. A monolayer of highly charged polymer microspheres, with a Yukawa interaction, was suspended in a plasma sheath. Two counterpropagating Ar+ laser beams pushed the particles, causing shear-induced melting of the monolayer and a shear flow in a planar Couette configuration. By fitting the particle velocity profiles in the shear flow to a Navier-Stokes model, the kinematic viscosity was calculated; it was of order 1 mm(2) s(-1), depending on the monolayer's parameters and shear stress applied.

There is a variety of situations in which Newton's third law is violated. Generally, the action-reaction symmetry can be broken for mesoscopic particles, when their effective interactions are mediated by a nonequilibrium environment. Here, we investigate different classes of nonreciprocal interactions relevant to real experimental situations and present their basic statistical mechanics analysis. We show that in mixtures of particles with such interactions, distinct species acquire distinct kinetic temperatures. In certain cases, the nonreciprocal systems are exactly characterized by a pseudo-Hamiltonian; i.e., being intrinsically nonequilibrium, they can nevertheless be described in terms of equilibrium statistical mechanics. Our results have profound implications, in particular, demonstrating the possibility to generate extreme temperature gradients on the particle scale. We verify the principal theoretical predictions in experimental tests performed with two-dimensional binary complex plasmas.

Dedicated experiments on melting of 2D plasma crystals were carried out. The melting was always accompanied by spontaneous growth of the particle kinetic energy, suggesting a universal plasma-driven mechanism underlying the process. By measuring three principal dust-lattice (DL) wave modes simultaneously, it is unambiguously demonstrated that the melting occurs due to the resonance coupling between two of the DL modes. The variation of the wave modes with the experimental conditions, including the emergence of the resonant (hybrid) branch, reveals exceptionally good agreement with the theory of mode-coupling instability.PACS numbers: 52.27.Lw, 52.27.Gr Strongly coupled complex (dusty) plasmas play an important role in the existing hierarchy of soft matter states [1]. Along with complex fluids and granular media as prominent examples of "regular" soft matter, complex plasmas represent natural model systems which enable remarkably broad interdisciplinary research. The characteristic length-and timescales in such systems are dramatically stretched (e.g., in complex plasmas -to hundreds of microns and tens of milliseconds, respectively). Therefore, numerous generic processes occurring in classical fluids or solids can be studied in greatest detail, at the most fundamental "atomistic" level [1][2][3]. Especially important here are 2D systems, where the complete information about all particles can be obtained at each moment of time. The current experimental capabilities make 2D complex plasmas ideal for comprehensive experimental studies of the atomistic dynamics [1,2,4,5].The investigation of atomistic processes that trigger the melting and crystallization is of particular interest. It is well known that the mechanisms resulting in the destruction of the long-range crystalline order in 2D systems can be very different from those operating in 3D crystals: The classical Kosterlitz-Thouless-HalperinNelson-Young (KTHNY) theory of 2D melting [6] predicts two consecutive phase transitions (with an intermediate, so called "hexatic phase" in between). The alternative theory [7] relates 2D melting with the formation of dislocation chains ("grain boundaries") percolating the system. These are generic melting mechanisms which can operate in very different strongly coupled systems [8][9][10][11][12].When studying generic classical phenomena occurring in regular liquids and solids, the relevance of a model system (be it colloidal suspensions, granular media, or complex plasmas) becomes crucial. Of course, careful analysis is required in the context of a given phenomenon (or, class of phenomena), but one can certainly identify essential common principles. In particular, the applicability of the Hamiltonial approach for the analysis of atomistic dynamics (for instance, to investigate kinetics of phase transitions) is one of these basic principles. This implies that non-Hamiltonian (non-conservative) behavior peculiar to model systems should play minor role at the relevant timescales [13].Non-Hamiltonial behavior of particle ensembl...

Measurements are reported for the radiation pressure and gas drag forces acting on a single melamine-formaldehyde microsphere. The radiation pressure force coefficient q, which would be unity if all incident photons were absorbed, has the value qϭ0.94Ϯ0.11. For argon, the Epstein gas drag force coefficient ␦, which would be unity if impinging molecules underwent specular reflection, has the value ␦ϭ1.26Ϯ0.13 as measured with our single-particle laser acceleration method, or ␦ϭ1.44Ϯ0.19 as measured using the vertical resonance method.

A method has been developed to heat and control temperature in a two-dimensional monolayer dusty plasma. A monolayer of highly charged polymer microspheres was suspended in a plasma sheath. The microspheres interacted with a Yukawa potential and formed a triangular lattice. Laser manipulation was used to apply random kicks to the particles. Two focused laser beams were moved rapidly around drawing Lissajous figures in the monolayer. The kinetic temperature of the particles increased with the laser power applied, and above a threshold a melting transition was observed. Characteristics of a thermal equilibrium of the laser-heated dusty plasma in solid and liquid states are discussed.