159 research outputs found
Anisotropic shear melting and recrystallization of a two-dimensional complex (dusty) plasma
A two-dimensional plasma crystal was melted by suddenly applying localized
shear stress. A stripe of particles in the crystal was pushed by the radiation
pressure force of a laser beam. We found that the response of the plasma
crystal to stress and the eventual shear melting depended strongly on the
crystal's angular orientation relative to the laser beam. Shear stress and
strain rate were measured, from which the spatially resolved shear viscosity
was calculated. The latter was shown to have minima in the regions with high
velocity shear, thus demonstrating shear thinning. Shear-induced reordering was
observed in the steady-state flow, where particles formed strings aligned in
the flow direction.Comment: 7 pages, 8 figures, submitted to Physical Review
Supersonic dislocations observed in a plasma crystal
Experimental results on the dislocation dynamics in a two-dimensional plasma
crystal are presented. Edge dislocations were created in pairs in lattice
locations where the internal shear stress exceeded a threshold and then moved
apart in the glide plane at a speed higher than the sound speed of shear waves,
. The experimental system, a plasma crystal, allowed observation of this
process at an atomistic (kinetic) level. The early stage of this process is
identified as a stacking fault. At a later stage, supersonically moving
dislocations generated shear-wave Mach cones
Microstructure of a liquid complex (dusty) plasma under shear
The microstructure of a strongly coupled liquid undergoing a shear flow was
studied experimentally. The liquid was a shear melted two-dimensional plasma
crystal, i.e., a single-layer suspension of micrometer-size particles in a rf
discharge plasma. Trajectories of particles were measured using video
microscopy. The resulting microstructure was anisotropic, with compressional
and extensional axes at around to the flow direction.
Corresponding ellipticity of the pair correlation function or
static structure factor gives the (normalized) shear rate of the
flow.Comment: 5 pages, 6 figure
Observation of particle pairing in a two-dimensional plasma crystal
The observation is presented of naturally occurring pairing of particles and
their cooperative drift in a two-dimensional plasma crystal. A single layer of
plastic microspheres was suspended in the plasma sheath of a capacitively
coupled rf discharge in argon at a low pressure of 1 Pa. The particle dynamics
were studied by combining the top-view and side-view imaging of the suspension.
Cross analysis of the particle trajectories allowed us to identify naturally
occurring metastable pairs of particles. The lifetime of pairs was long enough
for their reliable identification.Comment: 5 pages, 4 figure
First direct measurement of optical phonons in 2D plasma crystals
Spectra of phonons with out-of-plane polarization were studied experimentally
in a 2D plasma crystal. The dispersion relation was directly measured for the
first time using a novel method of particle imaging. The out-of-plane mode was
proven to have negative optical dispersion, comparison with theory showed good
agreement. The effect of the plasma wakes on the dispersion relation is briefly
discussed.Comment: submitted to Physical Review Letter
Wave mode coupling due to plasma wakes in two-dimensional plasma crystals: In-depth view
Experiments with two-dimensional (2D) plasma crystals are usually carried out
in rf plasma sheaths, where the interparticle interactions are modified due to
the presence of plasma wakes. The wake-mediated interactions result in the
coupling between wave modes in 2D crystals, which can trigger the mode-coupling
instability and cause melting. The theory predicts a number of distinct
fingerprints to be observed upon the instability onset, such as the emergence
of a new hybrid mode, a critical angular dependence, a mixed polarization, and
distinct thresholds. In this paper we summarize these key features and provide
their detailed discussion, analyze the critical dependence on experimental
parameters, and highlight the outstanding issues
Direct experimental observation of binary agglomerates in complex plasmas
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
Supersonic dislocations observed in a plasma crystal
Experimental results on the dislocation dynamics in a two-dimensional plasma
crystal are presented. Edge dislocations were created in pairs in lattice
locations where the internal shear stress exceeded a threshold and then moved
apart in the glide plane at a speed higher than the sound speed of shear waves,
. The experimental system, a plasma crystal, allowed observation of this
process at an atomistic (kinetic) level. The early stage of this process is
identified as a stacking fault. At a later stage, supersonically moving
dislocations generated shear-wave Mach cones
Direct observation of mode-coupling instability in two-dimensional plasma crystals
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.Comment: 4 pages, submitted to Physical Review Letter
Nonlinear regime of the mode-coupling instability in 2D plasma crystals
The transition between linear and nonlinear regimes of the mode-coupling
instability (MCI) operating in a monolayer plasma crystal is studied. The mode
coupling is triggered at the centre of the crystal and a melting front is
formed, which travels through the crystal. At the nonlinear stage, the mode
coupling results in synchronisation of the particle motion and the kinetic
temperature of the particles grows exponentially. After melting of the
crystalline structure, the mean kinetic energy of the particles continued to
grow further, preventing recrystallisation of the melted phase. The effect
could not be reproduced in simulations employing a simple point-like wake
model. This shows that at the nonlinear stage of the MCI a heating mechanism is
working which was not considered so far.Comment: 6 pages, 4 figure
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