Effect Of Particle Polydispersity Research Articles

Viscoelastic focusing of polydisperse particle suspensions in a straight circular microchannel

Flow cytometry is a technique for the analysis of cells and particles by coupling a sheath flow assisted focusing microfluidic device to an optical or electrical reader. In this context, polymer solutions can be employed to drive particle and cell focusing on the centreline of simple straight microfluidic channels. However, the change of the focusing efficiency due to polydispersity, which is a rule rather than the exception across cell populations, has not been extensively studied yet. In this work, the effect of particle polydispersity on the viscoelastic focusing in a straight cylindrical microchannel was studied by preparing two particle suspensions, containing different concentrations of particles with average diameters of 10,upmu hbox {m}, 15,upmu hbox {m} and 20,upmu hbox {m}, suspended in three solutions of polyethylene oxide with mass concentrations of 0.1 wt%, 0.25 wt% and 0.5 wt%. When the fluid explored the constant-viscosity region of the rheological curve, up to 95% of the particles were aligned on the channel centreline. When the fluid explored the shear-thinning region of the rheological curve, centreline focusing efficiency decreased, with the highest value of aligned particles being of 60%. For both mixtures, it was also observed that the fraction of aligned particles in a polydisperse system was not equivalent to that derived from the estimate of independent experiments with monodisperse particles.

Effects of particle polydispersity on radiative heat transfer in particle-laden turbulent flows

This study considers the effects of polydispersity of particles on thermodynamic and hydrodynamic behaviour of solid particle solar receivers. To this end we consider a canonical setting of a semi-periodic domain and perform DNS of turbulent flows interacting with point particles subject to a heating source. The turbulent flow is seeded with polydisperse particles that have a specific cumulative distribution function of particle sizes that is tuned based on available measurements. A thermodynamically equivalent monodisperse particle size is calculated, in a sense that the mass loading ratio of particles to gas and the total frontal area of particles available for radiation absorption are matched between the polydisperse and monodisperse particles. Our results show that the effective heat transfer rate between the two phases is significantly impacted by both particle clustering and polydispersity. We compare our DNS results to the predictions of a semi-analytical 1D model that assumes uniform particle concentration and gas properties per streamwise cross-section. Gas and particle temperatures predicted by this 1D model show least agreement with DNS at a Stokes number close to unity, where particles are most preferentially concentrated. Capturing the quantitative effects of particle preferential concentration on the effective heat transfer leads to a closure problem in the context of the 1D model. Using the DNS data, we examine such closure models and assess whether closure corrections inferred from monodisperse DNS can be applied to polydisperse mixtures viewed as superposition of multiple monodisperse systems.

Effect of Particle Polydispersity on Flow and Reaction Behaviors of Methanol-to-Olefins Fluidized Bed Reactors

A reactor model has been first developed with the objective to fundamentally comprehend the qualitative influence of the solid polydispersity on the features of a methanol-to-olefins (MTO) fluidized bed reactor. Since coke deposition on the MTO catalyst significantly affects the reaction rate, with its formation usually taking dozens of minutes to achieve the desired coke content, the optimum average coke content was first obtained based on a filtered computational fluid dynamic (CFD) model. Furthermore, comparison of predictions through a CFD method and a reaction engineering approach was conducted. Predictions basically match the experimental data. Subsequently, the influence of the solid particle size distribution on reactor characteristics was comprehensively explored through a filtered CFD method coupled with the population balance model. Results suggested that the particle breakage performed a significant function in MTO fluidized bed reactors, causing particle size to evidently decrease.

Effect of particle polydispersity on micromechanical properties and energy dissipation in granular mixtures

A series of numerical tests was conducted to study the micromechanical properties and energy dissipation in polydisperse assemblies of spherical particles subjected to uniaxial compression. In general, distributed particle size assemblies with standard deviations ranging from 0% to 80% of the particle mean diameter were examined. The microscale analyses included the trace of the fabric tensor, magnitude and orientation of the contact forces, trace of stress, number of contacts and degree of mobilization of friction in contacts between particles. In polydisperse samples, the average coordination numbers were lower than in monodisperse assemblies, and the mobilization of friction was higher than in monodisperse assemblies due to the non-uniform spatial rearrangement of spheres in the samples and the smaller displacements of the particles. The effect of particle size heterogeneity on both the energy density and energy dissipation in systems was also investigated.

Effect of particle polydispersity on the structure and dynamics of complex formation between small particles and large polymer

The effect of particle polydispersity on the structure and dynamics of small silica particle–large polymer chain mixtures at low polymer concentration has been investigated. Two types of silica particles were used as model systems, having similar mean size, specific surface area and surface properties but differing substantially in size polydispersity. Mixtures of PEO with the polydisperse silica particles showed strong shear-thickening upon shaking, while mixtures of the same composition with the monodisperse silica did not. Fluorescence correlation spectroscopy revealed that larger flocs were formed in the polydisperse particle–PEO systems. Particle polydispersity was identified as the major factor accounting for these differences and a plausible mechanism was proposed based on the spatial distribution of the silica particles within the complexes. This work suggests that the rheological behavior of colloid–polymer mixtures can be dramatically modified by a simple change in particle polydispersity, rather than more involved surface modifications or the use of additives.

Effect of particle polydispersity on the irreversible adsorption of fine particles on patterned substrates

We performed extensive Monte Carlo simulations of the irreversible adsorption of polydispersed disks inside the cells of a patterned substrate. The model captures relevant features of the irreversible adsorption of spherical colloidal particles on patterned substrates. The pattern consists of (equal) square cells, where adsorption can take place, centered at the vertices of a square lattice. Two independent, dimensionless parameters are required to control the geometry of the pattern, namely, the cell size and cell-cell distance, measured in terms of the average particle diameter. However, to describe the phase diagram, two additional dimensionless parameters, i.e., the minimum and maximum particle radii, are also required. We find that the transition between any two adjacent regions of the phase diagram solely depends on the largest and smallest particle sizes, but not on the shape of the distribution function of the radii. We consider size dispersions up to 20% of the average radius using a physically motivated, truncated, Gaussian-size distribution, and focus on the regime where adsorbing particles do not interact with those previously adsorbed on neighboring cells to characterize the jammed state structure. The study generalizes previous exact relations on monodisperse particles to account for size dispersion. Due to the presence of the pattern, the coverage shows a nonmonotonic dependence on the cell size. The pattern also affects the radius of adsorbed particles, where one observes preferential adsorption of smaller radii, particularly at high polydispersity.

Effect of particle polydispersity on particle concentration measurement by using laser Doppler anemometry

Measurement of particle concentration by laser Doppler anemometry (LDA) is studied on a vertical air jet seeded by a powder disperser with controlled particle and air flow rates. Particle arrival rate is utilized to retrieve particle number densities from conventional LDA operation. The effect of polydisperse nature of the particles is assessed. Comparisons between measured and estimated particle number densities suggest that only a certain portion of the particle population with a particle size to fringe spacing ratio around unity can be detected. Results indicate that reliable measurement of absolute particle concentration is possible for a particle population of narrow size distribution with an average diameter equivalent to fringe spacing. Present number density measurement technique which is useful for practical purposes with conventional LDA systems is found to yield physically reasonable profiles in both laminar and turbulent regimes.

Effect of cake thickness and particle polydispersity on prediction of permeate flux in microfiltration of particulate suspensions by a hydrodynamic diffusion model

We present a modification of the concentration polarization model with shear-induced diffusion which corrects the problem of flux overestimation at the membrane inlet and takes into account the presence of a particle cake layer of a finite thickness and the effect of particle polydispersity. The resistance of the cake is calculated from the Kozeny–Carman equation, while the cake porosity and particle concentration on the cake surface are determined from permeametric experiments. The model calculates the local cake thickness, its mean porosity and specific resistance, the shear rate and permeate flux as a function of distance from the inlet by equating the flux through the cake given by Darcy’s law and the flux given by the concentration polarization model. The cake is assumed to form at a distance from the inlet when the hydraulic flux through the clean membrane starts to exceed the flux predicted by the concentration polarization model. The characteristic particle diameter for diffusion and the mean particle diameter in the cake were determined by comparing predictions of the model with a set of experimental data on microfiltration of CaCO 3 suspensions through 0.1 μm pore size hollow fibers. The mean diffusion diameter, at 8.2 μm, was found, from the model, to be larger than the mean surface-volume diameter (4.65 μm), while the cake was found to be predominantly formed of smaller particles (mean diameter = 1–2 μm). This result was confirmed by granulometric analysis of the cake formed on the membrane. The predictions of our model are very close to those from the Romero and Davis model (J. Membr. Sci. 39 (1988) 157–185). With these particle diameters, our model correctly predicts the variations of permeate flux with transmembrane pressure, velocity and particle concentration when the comparison is made with different experimental data. This model, which describes only external fouling, should be more realistic than previous models when the cake thickness is not negligible in comparison with the membrane lumen.

Effects of particle polydispersity and shape factor during dust cake loading on air filters

The formation and structure of the dust cake on the surface of air filters have been investigated theoretically and experimentally. A novel experimental setup was developed to perform in situ measurement and monitoring of the dust cake height, pressure drop, and the challenge aerosol concentration and size distribution. Test aerosols included the Arizona Road Dust (AC fine), alumina particles, and tale particles. A CCD camera was used to observe and measure the increase of cake thickness with time. The cake mass at a specific time was inferred from the measurement of the final cake mass and the total collection time. A new analytical equation, including the effects of polydispersity and shape-factor of the particles, was derived to correlate all the parameters involved. The equation gives dust cake structure (porosity) as a function of operating conditions (pressure drop and filtration velocity), particle characteristics (particle size distribution and particle shape), and the cake layer thickness. Results for different particle types used in the experiments show a good correlation using the derived equation. The study finds significant effect of polydispersity and shape-factor of dust particles on filter loading.

A theoretical study on the evaporation of dry ammonium chloride and ammonium nitrate aerosols

A theoretical model has been developed to investigate possible mechanisms and parameters which may influence the evaporation behavior of dry ammonium chloride and ammonium nitrate aerosols. The model results compared well with the experimental observations available in the literature. Effects of particle polydispersity, gas-particle interaction, flow pattern of the system, particle number concentration, and accommodation coefficient on the evaporation of dry ammonium chloride and ammonium nitrate were quantitatively determined. The particle size distribution has a significant effect on the overall evaporation rate. A negligence of the size distribution and inappropriate use of the evaporation theory would lead to departures from experimental observations.