Particle Number Density Fluctuations Research Articles

Characterization of nanofluids in evaporating droplets by dynamic light scattering

The diffusion of nanoparticles in nanofluids in form of pendant drops and levitated droplets is characterized by dynamic light scattering. In combination with a heterodyne detection scheme, particle diffusivity in pendant drops can be determined by this method. The diffusivities therein correspond well to the values measured inside a cuvette. During evaporation, crust formation in the drop can also be identified by the method. Dynamic light scattering experiments for levitated droplets are performed with an ultrasonic acoustic levitator. Again, the dynamics of the particle number density fluctuations inside the droplet can be characterized, but the derivation of particle diffusivities is not yet possible. In both cases, the evolution of the characteristic time reflecting the diffusive process of nanoparticles can be determined until the point of crust formation. In summary, the applicability and limitation of dynamic light scattering for the characterization of diffusion of particles in evaporating drops are identified.

Measurement of atmospheric particle number density fluctuations based on moiré deflectometry

ObjectiveIn this paper, moiré deflectometry is introduced to measure the atmospheric particle number density fluctuations to expand the application field of the technology. MethodsThe relationship between the atmospheric particle number density fluctuation and the refractive index fluctuation is established, which provides a theoretical basis for obtaining the atmospheric particle number density fluctuations by moiré deflectometry. To verify the feasibility of this method, two sets of experiments which lasted two hours were executed. In experiments, the moiré fringes are recorded at a frequency of one frame per second. Therefore, 7200 arrival angles could be obtained for each experiment. ResultThe results show that the atmospheric particle number density fluctuations experimentally obtained by moiré deflectometry basically match well with those of calculated by theoretical estimation, which means that it is feasible to introduce moiré deflectometry into the measurement of atmospheric particle number density fluctuations. In a word, this research could provide some reference for studying the influence of atmospheric refractive index fluctuation on optical detection and communication.

A novel particle subgrid scale modeling of large eddy simulation for swirling particle-laden turbulent flow

How to describe the effects of subgrid scale (SGS) gas flow on particle dynamics in large eddy simulation (LES) is a challenge for modeling and simulation of particle-laden swirling turbulent flow. SGS four-way coupling interaction mechanisms of gas-to-particle, particle-to-gas, and particle-particle collision have not been considered fully. A novel particle SGS kinetic turbulent energy-granular temperature (SGS-kp-θp) model is proposed firstly to model this effect on particle motions and dispersions. Second-order moment algorithm to establish the anisotropic two-phase Reynolds stress equations and their closure correlations is utilized. Validations by experiment results and Moissette (MOB) particle dispersion model built-in OpenFoam software are performed with better agreements for mean and fluctuation velocities. Anisotropic behaviors of Reynolds stresses and particle collisions are distinctively, smaller-scale particle fluctuations originating from particle collisions lead to the redistribution and dissipation of Reynolds stresses. Power spectrum density is utilized to analyze statistically the particle number density fluctuations and Reynolds stresses, dominant frequencies are roughly 2.5 Hz and 1.0 Hz marking the internal and the corner recirculation regions. Compared to those of inlet flow regions, normal and tangential Reynolds stresses at development flow regions approximately increased by twice and decreased by 3.0 times. Modeling strategy for multiphase interactions are of great importance for capturing particle dynamics in accurate.

Trapping effects on inflation

We develop a Lagrangian approach based on the influence functional method so as to derive self-consistently the Langevin equation for the inflaton field in the presence of trapping points along the inflaton trajectory. The Langevin equation exhibits the backreaction and the fluctuation-dissipation relation of the trapping. The fluctuation is induced by a multiplicative colored noise that can be identified as the the particle number density fluctuations and the dissipation is a new effect that may play a role in the trapping with a strong coupling. In the weak coupling regime, we calculate the power spectrum of the noise-driven inflaton fluctuations for a single trapping point and studied its variation with the trapping location. We also consider a case with closely spaced trapping points and find that the resulting power spectrum is blue.

Arrest of Flow and Emergence of Activated Processes at the Glass Transition of a Suspension of Particles with Hard Spherelike Interactions

By combining aspects of the coherent and self-intermediate scattering functions, we show that the arrest of particle number density fluctuations spreads from the position of the main structure factor peak. We propose that this arrest impairs the system's ability to respond to diffusing momentum currents, leading to an enhanced resistance to flow. From the stretching of the coherent intermediate scattering functions in the glass, we read a manifestation of the undissipated thermal energy-the source of the ergodicity restoring processes that short-circuit the sharp transition to a perfect glass.

Mixing at the external boundary of a submerged turbulent jet

We study experimentally and theoretically mixing at the external boundary of a submerged turbulent jet. In the experimental study we use particle image velocimetry and an image processing technique based on the analysis of the intensity of the Mie scattering to determine the spatial distribution of tracer particles. An air jet is seeded with the incense smoke particles, which are characterized by a large Schmidt number and a small Stokes number. We determine the spatial distributions of the jet fluid characterized by a high concentration of the particles and of the ambient fluid characterized by a low concentration of the tracer particles. In the data analysis we use two approaches, whereby one approach is based on the measured phase function for the study of the mixed state of two fluids. The other approach is based on the analysis of the two-point second-order correlation function of the particle number density fluctuations generated by tangling of the gradient of the mean particle number density by the turbulent velocity field. This gradient is formed at the external boundary of a submerged turbulent jet. We demonstrate that probability density function of the phase function of a jet fluid penetrating into an external flow and the two-point second-order correlation function of the particle number density do not have universal scaling and cannot be described by a power-law function. The theoretical predictions made in this study are in qualitative agreement with the obtained experimental results.

Direct visualization of flow-induced microstructure in dense colloidal gels by confocal laser scanning microscopy

Unconstrained uniaxial compression (or squeeze flow) of high volume fraction gels of fluorescent silica particles of diameter 832 nm results in the formation of voids (at φ=0.26) and cracks (at φ=0.40) that are of scale 10–100 μm. This evidence of inhomogeneous material deformation was obtained by direction visualization of three-dimensional structure by confocal laser scanning microscopy. Flow-induced void and crack microstructures are quantified by locating all particle centroids with quantitative image processing, by performing Voronoi volume tessellation with computational geometry and by analyzing the particle number density fluctuations as a function of averaging volume. Average short-range real space structural measures, such as the pair correlation function, are little changed by the flow. However, the probability distribution of excess normalized Voronoi polyhedra volume is profoundly extended by squeeze flow, particularly at large polyhedra volumes. Comparison of the Voronoi polyhedra volume distributions and particle bond distributions indicates that: (1) in the low φ gel, large flow-induced voids are formed by the reorganization of the existing quiescent voids without significant effect on the local structure; and (2) in the high φ gel, cracks are formed by reorganization of the local structure itself. Analysis of the number density fluctuations shows that the gels respond to applied squeeze flow deformation with structural distortion on the length scale of 5–10 particle diameters.Unconstrained uniaxial compression (or squeeze flow) of high volume fraction gels of fluorescent silica particles of diameter 832 nm results in the formation of voids (at φ=0.26) and cracks (at φ=0.40) that are of scale 10–100 μm. This evidence of inhomogeneous material deformation was obtained by direction visualization of three-dimensional structure by confocal laser scanning microscopy. Flow-induced void and crack microstructures are quantified by locating all particle centroids with quantitative image processing, by performing Voronoi volume tessellation with computational geometry and by analyzing the particle number density fluctuations as a function of averaging volume. Average short-range real space structural measures, such as the pair correlation function, are little changed by the flow. However, the probability distribution of excess normalized Voronoi polyhedra volume is profoundly extended by squeeze flow, particularly at large polyhedra volumes. Comparison of the Voronoi polyhedra volume dis...

Particle number density fluctuations and pressure effects onstructure formation

We provide a covariant and gauge-invariant approach to the question ofhow a first-order pressure can be incorporated self-consistently in acosmological scenario. The approximation is relevant, in the linearregime, to weakly self-interacting or warm dark matter models. We alsoderive number density fluctuations in which new modes appear becausethe number density fluctuations are no longer simply proportional to thedensity fluctuations.

Density fluctuation dynamics in a screened Coulomb colloid: Comparison of the liquid and bcc crystal phases

Inelastic light scattering from particle number density fluctuations was carried out on a suspension of 0.109 μ diam sulfonated polystyrene microspheres exhibiting coexisting colloidal liquid and body-centered-cubic single crystalline phases. Comparison of the first cumulants of the decay of the density–density correlation function in the two phases reveals that the wave vector dependence in the liquid exhibits many features in common with that for longitudinal 110 lattice vibrations in the crystal.