Critical fluctuations and anomalous transport in soft Yukawa-Langevin systems

Simulation of a Langevin-dynamics model demonstrates emergence of critical fluctuations and anomalous grain transport which have been observed in experiments on "soft" quasi-two-dimensional dusty plasma clusters. It has been suggested that these anomalies derive from particular non-equilibrium physics, but our model does not contain such physics: the grains are confined by an external potential, interact via static Yukawa forces, and are subject to stochastic heating and dissipation from neutrals. One remarkable feature is emergence of leptokurtic probability distributions of grain displacements $\xi(\tau)$ on time-scales $\tau<\tau_{\Delta}$, where $\tau_{\Delta}$ is the time at which the standard deviation $\sigma(\tau)\equiv ^{1/2}$ approaches the mean inter-grain distance $\Delta$. Others are development of humps in the distributions on multiples of $\Delta$, anomalous Hurst exponents, and transitions from leptokurtic towards Gaussian displacement distributions on time scales $\tau>\tau_{\Delta}$. The latter is a signature of intermittency, here interpreted as a transition from bursty transport associated with hopping on intermediate time scales to vortical flows on longer time scales.Comment: 12 pages, 9 figure

Three dimensional complex plasma structures in a combined radio frequency and direct current discharge

We report on the first detailed analysis of large three dimensional (3D) complex plasma structures in experiments performed in pure rf and combined rf+dc discharge modes. Inductively coupled plasma (ICP) is generated by an rf coil wrapped around the vertically positioned cylindrical glass tube at a pressure of 0.3 mbar. In addition, dc plasma can be generated by applying voltage to the electrodes at the ends of the tube far from the rf coil. The injected monodisperse particles are levitated in the plasma below the coil. A scanning laser sheet and a high resolution camera are used to determine the 3D positions of about $10^5$ particles. The observed bowl-shaped particle clouds reveal coexistence of various structures, including well-distinguished solid-like, less ordered liquid-like, and pronounced string-like phases. New criteria to identify string-like structures are proposed.Comment: 6 pages, 7 figure

Structural properties of dense hard sphere packings

The structural properties of dense random packings of identical hard spheres (HS) are investigated. The bond order parameter method is used to obtain detailed information on the local structural properties of the system for different packing fractions $\phi$, in the range between $\phi=0.53$ and $\phi=0.72$. A new order parameter, based on the cumulative properties of spheres distribution over the rotational invariant $w_6$, is proposed to characterize crystallization of randomly packed HS systems. It is shown that an increase in the packing fraction of the crystallized HS system first results in the transformation of the individual crystalline clusters into the global three-dimensional crystalline structure, which, upon further densification, transforms into alternating planar layers formed by different lattice types.Comment: 4 pages, 5 figure

Instantaneous shear modulus of Yukawa fluids across coupling regimes

The high frequency (instantaneous) shear modulus of three-dimensional Yukawa systems is evaluated in a wide parameter range from the very weakly coupled gaseous state to the strongly coupled fluid at the crystallization point (Yukwa melt). This allows us to quantify how shear rigidity develops with increasing coupling and inter-particle correlations. The radial distribution functions (RDFs) needed to calculate the excess shear modulus have been obtained from extensive molecular dynamics (MD) simulations. MD results demonstrate that fluid RDFs appear quasi-universal on the curves parallel to the melting line of a Yukawa solid in accordance with the isomorph theory of Roskilde-simple systems. This quasi-universality allows us to simplify considerably calculations of quantities involving integrals of the RDF (elastic moduli represent just one relevant example). The calculated reduced shear modulus grows linearly with the coupling parameter at weak coupling and approaches a quasi-constant asymptote at strong coupling. The asymptotic value at strong coupling is in reasonably good agreement with the existing theoretical approximation. © 2020 Author(s).The work leading to this publication was partly supported by the German Academic Exchange Service (DAAD) with funds from the German Aerospace Center (DLR). We would like to thank Hubertus Thomas for reading the manuscript

Freezing and melting of 3D complex plasma structures under microgravity conditions driven by neutral gas pressure manipulation

Freezing and melting of large three-dimensional complex plasmas under microgravity conditions is investigated. The neutral gas pressure is used as a control parameter to trigger the phase changes: Complex plasma freezes (melts) by decreasing (increasing) the pressure. Evolution of complex plasma structural properties upon pressure variation is studied. Theoretical estimates allow us to identify main factors responsible for the observed behavior.Comment: Phys. Rev. Lett. (in press); 4 pages, 4 figure

Fluid-solid phase transitions in 3D complex plasmas under microgravity conditions

Phase behavior of large three-dimensional complex plasma systems under microgravity conditions onboard the International Space Station is investigated. The neutral gas pressure is used as a control parameter to trigger phase changes. Detailed analysis of structural properties and evaluation of three different melting/freezing indicators reveal that complex plasmas can exhibit melting by increasing the gas pressure. Theoretical estimates of complex plasma parameters allow us to identify main factors responsible for the observed behavior. The location of phase states of the investigated systems on a relevant equilibrium phase diagram is estimated. Important differences between the melting process of 3D complex plasmas under microgravity conditions and that of flat 2D complex plasma crystals in ground based experiments are discussed.Comment: 13 pages, 10 figures; submitted to Phys. Rev.

Scale-free vortex cascade emerging from random forcing in a strongly coupled system

The notions of self-organised criticality (SOC) and turbulence are traditionally considered to be applicable to disjoint classes of phenomena. Nevertheless, scale-free burst statistics is a feature shared by turbulent as well as self-organised critical dynamics. It has also been suggested that another shared feature is universal non-gaussian probability density functions (PDFs) of global fluctuations. Here, we elucidate the unifying aspects through analysis of data from a laboratory dusty plasma monolayer. We compare analysis of experimental data with simulations of a two-dimensional (2D) many-body system, of 2D fluid turbulence, and a 2D SOC model, all subject to random forcing at small scales. The scale-free vortex cascade is apparent from structure functions as well as spatio-temporal avalanche analysis, the latter giving similar results for the experimental and all model systems studied. The experiment exhibits global fluctuation statistics consistent with a non-gaussian universal PDF, but the model systems yield this result only in a restricted range of forcing conditions

Effect of fortification of fresh cow milk with coconut milk on the proximate composition and yield of warankashi, a traditional cheese

Cheese is a concentrated dairy product produced by acid or rennet coagulation or curdling of milk, stirring and heating the curd, draining off the whey, collecting and pressing the curd. The effect of partial substitution of fresh cow milk with coconut milk on the yield and proximate composition of cheese was examined. Extracted coconut milk was mixed with fresh raw cow milk at varying proportions of 5%: 95%, 10%: 90%, 15%: 85%, 20%: 80%, 25%:75%, 70%: 30% and the control (0%:100%) to produce cheese. The control and the partially substituted cheeses were stored in a refrigerator and examined for sensory quality, percentage yield, total titrable acidity, and proximate analysis. The yield of cheese showed significant (p< 0.05) decrease from 26.71% (control sample) to 13.55% as the level of coconut milk increased. The total titrable acidity of cheese was found to be between the ranges of 0.20% - 0.29% which displayed a significant increase from 0.20% - 0.29%. The protein content of the cow-coconut cheese blends showed a significant difference (p<0.05) and an increase of 14.05%-15.33% (at 5%-30% substitution of coconut milk), with the control sample having 13.75%. There was also an increase in fat content from 9.20% - 9.64% (5% - 30% substitution of coconut milk, with the control sample having 8.94%. There was a decrease in the carbohydrate content of the cheese blends which ranged between 8.23% -2.82%, with the control sample having 9.60%. There was a significant decrease (p<0.05) in the ash content of the cow-coconut cheese blends, with the control sample having 1.02%. Significant difference (p<0.05) was observed in the colour, aroma, taste, texture, and overall acceptability as influenced by varying proportions of added coconut milk. The blend with 5% coconut milk and 95% cow milk was most acceptable by panellists. The work showed the potential of coconut as an alternative source of milk in cheese making with improved nutritional value and consumer acceptability