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

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 $\pm 45^{\circ}$ to the flow direction. Corresponding ellipticity of the pair correlation function $g({\bf r})$ or static structure factor $S(\bf{k})$ gives the (normalized) shear rate of the flow.Comment: 5 pages, 6 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

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

Strong acceleration of glacier area loss in the Greater Caucasus between 2000 and 2020

An updated glacier inventory is important for understanding glacier behaviour given the accelerating glacier retreat observed around the world. Here, we present data from a new glacier inventory for two points in time (2000, 2020) covering the entire Greater Caucasus (Georgia, Russia, and Azerbaijan). Satellite imagery (Landsat, Sentinel, SPOT) was used to conduct a remote-sensing survey of glacier change. The 30 m resolution Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Model (ASTER GDEM; 17 November 2011) was used to determine aspect, slope, and elevations, for all glaciers. Glacier margins were mapped manually and reveal that in 2000 the mountain range contained 2186 glaciers with a total glacier area of 1381.5 ± 58.2 km2. By 2020, the area had decreased to 1060.9 ± 33.6 km2 a reduction of 23.2 ± 3.8 % (320.6 ± 45.9 km2) or −1.16 % yr−1 over the last 20 years in the Greater Caucasus. Of the 2223 glaciers, 14 have an area > 10 km2, resulting in the 221.9 km2 or 20.9 % of total glacier area in 2020. The Bezengi Glacier with an area of 39.4 ± 0.9 km2 was the largest glacier mapped in the 2020 database. Glaciers between 1.0 and 5.0 km2 accounted for 478.1 km2 or 34.6 % in total area in 2000, while they accounted for 354.0 km2 or 33.4 % in total area in 2020. The rates of area shrinkage and mean elevation vary between the northern and southern and between the western, central, and eastern Greater Caucasus. Area shrinkage is significantly stronger in the eastern Greater Caucasus (−1.82 % yr−1), where most glaciers are very small. The observed increased summer temperatures and decreased winter precipitation along with increased Saharan dust deposition might be responsible for the predominantly negative mass balances of Djankuat and Garabashi glaciers with long-term measurements. Both glacier inventories are available from the Global Land Ice Measurements from Space (GLIMS) database and can be used for future studies

The First Rock Glacier Inventory for the Greater Caucasus

Rock glaciers are an integral part of the periglacial environment. At the regional scale in the Greater Caucasus, there have been no comprehensive systematic efforts to assess the distribution of rock glaciers, although some individual parts of ranges have been mapped before. In this study we produce the first inventory of rock glaciers from the entire Greater Caucasus region—Russia, Georgia, and Azerbaijan. A remote sensing survey was conducted using Geo-Information System (GIS) and Google Earth Pro software based on high-resolution satellite imagery—SPOT, Worldview, QuickBird, and IKONOS, based on data obtained during the period 2004–2021. Sentinel-2 imagery from the year 2020 was also used as a supplementary source. The ASTER GDEM (2011) was used to determine location, elevation, and slope for all rock glaciers. Using a manual approach to digitize rock glaciers, we discovered that the mountain range contains 1461 rock glaciers with a total area of 297.8 ± 23.0 km2. Visual inspection of the morphology suggests that 1018 rock glaciers with a total area of 199.6 ± 15.9 km2 (67% of the total rock glacier area) are active, while the remaining rock glaciers appear to be relict. The average maximum altitude of all rock glaciers is found at 3152 ± 96 m above sea level (a.s.l.) while the mean and minimum altitude are 3009 ± 91 m and 2882 ± 87 m a.s.l., respectively. We find that the average minimum altitude of active rock glaciers is higher (2955 ± 98 m a.s.l.) than in relict rock glaciers (2716 ± 83 m a.s.l.). No clear difference is discernible between the surface slope of active (41.4 ± 3°) and relict (38.8 ± 4°) rock glaciers in the entire mountain region. This inventory provides a database for understanding the extent of permafrost in the Greater Caucasus and is an important basis for further research of geomorphology and palaeoglaciology in this region. The inventory will be submitted to the Global Land Ice Measurements from Space (GLIMS) database and can be used for future studies

Retardation of nanoparticles growth by doping

The process of doping of CdS nanoparticles with Mn during colloidal synthesis is analyzed by EPR and optical studies. Analysis of EPR results demonstrated that Mn2+ ions are successfully incorporated into the nanoparticles and occupy the crystal sites both in the bulk of a NP and near the surface of a NP. Optical absorption measurements revealed the retardation of absorption edge shift during the growth for Mn-doped CdS NPs as compared to the undoped CdS NPs. It was concluded that the presence of Mn in the solution leads to the inhibition of NPs growth