On the mechanism of own rotation of dust particles

We analyze the physical reason for own rotation of dust particles. We propose from analysis of literature data and our previous studies that own rotation of dust particles is due to azimuth-symmetric flow of ions to the particle surface, which is associated with a nonuniform distribution of the surface charge. This assumption is in conformity with the results of experiments in which the plasma flow is changed by introducing particles in the horizontal plane (horizontal cluster) and particles aligned along the discharge current (vertical cluster) and with the observation of the rotation threshold for the discharge current and the magnetic field. The experiments are performed with spherical particles using the coordinate tracing method. Our results make it possible to construct a model of spinning of charged dust tops for describing magnetic properties of a complex plasma.     

T2 relaxation induced by clusters of superparamagnetic nanoparticles: Monte Carlo simulations

Clustering strongly affects the transverse (T2) relaxation induced by superparamagnetic nanoparticles in magnetic resonance experiments. In this study, we used Monte Carlo simulations to investigate systematically the relationship between T2 values and the geometric parameters of nanoparticle clusters. We computed relaxation as a function of particle size, number of particles per cluster, interparticle distance, and cluster shape (compact vs. linear). We found that compact clusters induced relaxation equivalent to similarly sized single particles. For small particles, the shape and density of clusters had a significant effect on T2. In contrast, for larger particles, T2 relaxation was relatively independent of cluster geometry until interparticle distances within a cluster exceeded ten times the particle diameter. Results from our simulations suggest principles for the design of nanoparticle aggregation-based sensors for MRI.     

Influence of external perturbations on dynamical characteristics of dust clusters (simulation)

The results of a numerical study of the dynamics of interacting particles in cluster systems under the action of an external perturbing field on them are presented. The relaxation rates and characteristic relaxation times of a cluster to its equilibrium state are analyzed. The conditions for the formation of dynamical structures of charged particles in the field of external nonpotential forces are investigated. The peculiarities of diagnosing the pair potential of particles in nonequilibrium systems are considered. The numerical simulation conditions for the problem were close to the conditions of experiments in a dusty plasma.     

Using relaxation time approximation for collisions in laser-cluster interaction simulation

Numerical experiments were performed for cluster dynamics on a one-dimensional cluster model consisting of hydrogen atoms exposed to intense laser pulses. The algorithm deployed the relativistic equations for solving motions of both protons and electrons, respectively. In contrary to traditionally and extensively used method which treats collisions within a jumbo cluster in a statistical way, we introduced a phenomenological relaxation time parameter to cope collisions among particles in a cluster, thereby profoundly reducing computational workload while still attaining the essential physics. Positive ion’s maximum kinetic energy and maximum kinetic energy saturation with laser intensity were explored, which were found to be in fairly good accordance with experimental observations, corroborating the effectiveness of our method and inferring that this treatment is useful in numerical experiments on light-cluster interaction.     

Structure of unsupported bismuth nanoparticles

We present new results of electron diffraction experiments on unsupported nanometer-sized bismuth clusters. The high intensity cluster beam, necessary for electron diffraction, is provided by an inert-gas aggregation source. The cluster beam contains particles with average cluster sizes between 4.5 and 10 nm. When using Helium as a carrier gas we are able to observe a transition from crystalline clusters to a new structure, which we identify with that of amorphous or liquid clusters. Received 28 November 2000     

On the theory of phase transitions in magnetic fluids

Particles of magnetic fluids (ferrofluids), as is known from experiments, can condense to bulk dense phases at low temperatures (that are close to room temperature) in response to an external magnetic field. It is also known that a uniform external magnetic field increases the threshold temperature of the observed condensation, thus stimulating the condensation process. Within the framework of early theories, this phenomenon is interpreted as a classical gas-liquid phase transition in a system of individual particles involved in a dipole-dipole interaction. However, subsequent investigations have revealed that, before the onset of a bulk phase transition, particles can combine to form a chain cluster or, possibly, a topologically more complex heterogeneous cluster. In an infinitely strong magnetic field, the formation of chains apparently suppresses the onset of a gas-liquid phase transition and the condensation of magnetic particles most likely proceeds according to the scenario of a gas-solid phase transition with a wide gap between spinodal branches. This paper reports on the results of investigations into the specific features of the condensation of particles in the absence of an external magnetic field. An analysis demonstrates that, despite the formation of chains, the condensation of particles in this case can proceed according to the scenario of a gas-liquid phase transition with a critical point in the continuous binodal. Consequently, a uniform magnetic field not only can stimulate the condensation phase transition in a system of magnetic particles but also can be responsible for a qualitative change in the scenario of the phase transition. This inference raises the problem regarding a threshold magnetic field in which there occurs a change in the scenario of the phase transition.     

Species selective evaporation during surface scattering of binary van der Waals clusters

We performed both experiments and molecular dynamics simulations for the scattering of mixed from a graphite surface under conditions where evaporation of thermalized small fragments is the main channel to evacuate the excess collision energy of the cluster impact. In spite of the expected thermal nature of the scattering process, we find average temperatures for the evaporating cluster particles that are considerably higher for krypton than for argon. We discuss the possible influence of the involved binding energies and the probable role of the incident cluster structure on these new results. Received: 28 January 1999     

Statistical models for spatial patterns of heavy particles in turbulence

The dynamics of heavy particles suspended in turbulent flows is of fundamental importance for a wide range of questions in astrophysics, atmospheric physics, oceanography, and technology. Laboratory experiments and numerical simulations have demonstrated that heavy particles respond in intricate ways to turbulent fluctuations of the carrying fluid: non-interacting particles may cluster together and form spatial patterns even though the fluid is incompressible, and the relative speeds of nearby particles can fluctuate strongly. Both phenomena depend sensitively on the parameters of the system. This parameter dependence is difficult to model from first principles since turbulence plays an essential role. Laboratory experiments are also very difficult, precisely since they must refer to a turbulent environment. But in recent years it has become clear that important aspects of the dynamics of heavy particles in turbulence can be understood in terms of statistical models where the turbulent fluctuations are approximated by Gaussian random functions with appropriate correlation functions. In this review, we summarise how such statistical-model calculations have led to a detailed understanding of the factors that determine heavy-particle dynamics in turbulence. We concentrate on spatial clustering of heavy particles in turbulence. This is an important question because spatial clustering affects the collision rate between the particles and thus the long-term fate of the system.     

Charged particle correlations in\bar pp collisions at c.m. energies of 200, 546 and 900 GeV

We present data on two-particle pseudorapidity and multiplicity correlations of charged particles for non single-diffractive \(p\bar p - collisions\) at c.m. energies of 200, 546 and 900 GeV. Pseudorapidity correlations interpreted in terms of a cluster model, which has been motivated by this and other experiments, require on average about two charged particles per cluster. The decay width of the clusters in pseudorapidity is approximately independent of multiplicity and of c.m. energy. The investigations of correlations in terms of pseudorapidity gaps confirm the picture of cluster production. The strength of forward-backward multiplicity correlations increases linearly with ins and depends strongly on position and size of the pseudorapidity gap separating the forward and backward interval. All our correlation studies can be understood in terms of a cluster model in which clusters contain on average about two charged particles, i.e. are of similar magnitude to earlier estimates from the ISR.     

Family-Vicsek scaling of detachment fronts in granular Rayleigh-Taylor instabilities during sedimentating granular/fluid flows

When submillimetric particles are confined in a fluid such that a compact cluster of particles lie above the clear fluid, particles will detach from the lower boundary of the cluster and form an unstable separation front giving rise to growing fingers of falling particles. We study this problem using both experiments and hybrid granular/fluid mechanics models. In the case of particles from 50 to 500 microns in diameter falling in air, we study the horizontal density fluctuations at early times: the amplitude of the density difference between two points at a certain horizontal distance grows as a power law of time. This happens up to a saturation corresponding to a power law of the distance. The way in which the correlation length builds up to this saturation also follows a power law of time. We show that these decompaction fronts in sedimentation problems follow a Family-Vicsek scaling, characterize the dynamic and Hurst exponent of the lateral density fluctuations, respectively z ∼ 1 and ζ ∼ 0.75, and show how the prefactors depend on the grain diameter. We also show from similar simulations with a more viscous and incompressible fluid, that this feature is independent of the fluid compressibility or viscosity, ranging from air to water/glycerol mixtures.     

Distribution of the Coulomb microfield within an ion cluster

The microfield distribution function in clusters was studied by simulation using the molecular dynamics and Monte Carlo methods. The results obtained were compared with microfield distributions in infinite plasma. It was shown that the calculated distributions have the same asymptotics. However, the position of the maximum and the existence of additional extrema depend on the cluster type and size. The dependence of the microfield expectation and variance on the number of cluster particles was also studied.     

Coulomb break-up and possible evidence for cluster effects in some light nuclei

The possibility of obtaining information about cluster structures in light nuclei from Coulomb break-up experiments is discussed. A few favorable cases are pointed out. A preliminary estimate of the break-up cross section of⁷Li into alpha particles and tritons and of⁷Be into alpha particles and³He in the field of a heavy target nucleus is given. The need for more experiments as well as more refined calculations is stressed.     

Structural Recovery of High-Aspect-Ratio Nanoparticle/Polymer Nanocomposites in Simple Shear Flow

Structure formation, break-down, orientation, and recovery in a multiwalled carbon nanotube (MWCNT)-polyethylene oxide nanocomposite above the percolation threshold were investigated using oscillatory and rotational rheometric studies. It was found that above the percolation threshold a percolating cluster is formed in the mixture, which is broken down upon application of a shearing force greater than a critical value. The critical value depends on both temperature and concentration of MWCNT particles in the polymer matrix. No full recovery of the structure was observed, even after 3600 s of rest time. This was attributed to a very high tendency of the MWCNT particles to reaggregate and the high strength of the primary percolating cluster formed during the recovery process. A generalized mechanism was proposed for the breakdown and recovery of the clusters of the MWCNT particles which can explain the observations for different kinds of matrices and dispersed particles.     

Coulomb scatter of diamagnetic dust particles in a cusp magnetic trap under microgravity conditions

The effect of a dc electric field on strongly nonideal Coulomb systems consisting of a large number (~10⁴) of charged diamagnetic dust particles in a cusp magnetic trap are carried out aboard the Russian segment of the International Space Station (ISS) within the Coulomb Crystal experiment. Graphite particles of 100–400 μm in size are used in the experiments. Coulomb scatter of a dust cluster and the formation of threadlike chains of dust particles are observed experimentally. The processes observed are simulated by the molecular dynamics (MD) method.     

Contribution of slow clusters to the bulk elasticity near the colloidal glass transition

We use confocal microscopy to visualize individual particles near the colloidal glass transition. We identify the most slowly-relaxing particles and show that they form spatially correlated clusters that percolate across the sample. In supercooled fluids, the largest cluster spans the system on short time scales but breaks up on longer time scales. In contrast, in glasses, a percolating cluster exists on all accessible time scales. Using molecular dynamics simulation, we show that these clusters make the dominant contribution to the bulk elasticity of the sample.     

Diffusional coagulation of superparamagnetic particles in the presence of an external magnetic field

In this paper, we investigate the diffusional coagulation of colloidal superparamagnetic (SP) latex particles that are under the influence of an external magnetic field. The cluster size distributions (CSDs) that evolve with time were determined using an optical set-up that permits the direct visualization of particle clusters. Following the dynamic scaling analysis of van Dongen and Ernst (Phys. Rev. Lett. 54 (1985) 1396), we find that the CSDs all collapse onto a master curve when properly scaled. The bell-shape of this master curve indicates that large clusters preferentially scavenge small clusters in our system. From the time evolution of the average cluster size we infer that the reactivity between large clusters diminishes with increasing cluster size. These results are consistent with a simple mathematical formulation of the coagulation rate constant, or kernel, for the Brownian coagulation of magnetic particles. Moreover, our results support a growing body of evidence that the dynamic scaling theory developed by van Dongen and Ernst is a useful framework with which to study the microscale processes governing particle coagulation.     

Equilibrium cluster phases and low-density arrested disordered states: the role of short-range attraction and long-range repulsion

We study a model in which particles interact with short-ranged attractive and long-ranged repulsive interactions, in an attempt to model the equilibrium cluster phase recently discovered in sterically stabilized colloidal systems in the presence of depletion interactions. At low packing fractions, particles form stable equilibrium clusters which act as building blocks of a cluster fluid. We study the possibility that cluster fluids generate a low-density disordered arrested phase, a gel, via a glass transition driven by the repulsive interaction. In this model the gel formation is formally described with the same physics of the glass formation.     

Fast and slow modes of the propagation of the combustion front in heterogeneous systems

Experimental results are presented in favor of the existence of fast and slow modes of the propagation of the combustion front in diluted heterogeneous mixtures of reactive particles. A theoretical combustion model is proposed to explain the existence of these modes. The transition from the fast to the slow mode, which occurs in a narrow range of the degree of dilution of the mixture by inert powder, is associated with the break of a percolation cluster formed by reagent particles that are in direct contact with each other. After such a break of the cluster, the thermal energy of combusting particles is still insufficient to maintain the combustion wave. A sharp decrease in the front velocity in this case is associated with the necessity of heating inert regions inevitably appearing in its path.     

Dielectric constant of polarizable,nonpolar fluids and suspensions

We study the dielectric constant of a polarizable, nonpolar fluid or suspension of spherical particles by use of a renormalized cluster expansion. The particles may have induced multipole moments of any order. We show that the Clausius-Mossotti formula results from a virtual overlap contribution. The corrections to the Clausius-Mossotti formula are expressed with the aid of a cluster expansion. The integrands of the cluster integrals are expressed in terms of two-body nodal connectors which incorporate all reflections between a pair of particles. We study the two- and three-body cluster integrals in some detail and show how these are related to the dielectric virial expansion and to the first term of the Kirkwood-Yvon expansion.