Adhesion of ultrafine particles—A micromechanical approach

In particle processing and product handling of fine , ultrafine and nanosized particles , the well-known flow problems of dry cohesive powders in process apparatuses or storage and transportation containers include bridging, channelling, widely spread residence time distribution associated with time consolidation or caking effects, chemical conversions and deterioration of bioparticles. Avalanching effects and oscillating mass flow rates in conveyors lead to feeding and dosing problems. Finally, insufficient apparatus and system reliability of powder processing plants are also related to these flow problems. Thus, it is very essential to understand the fundamentals of particle adhesion with respect to product quality assessment and process performance in particle technology.The state-of-the-art in constitutive modelling of elastic, elastic-adhesion, elastic-dissipative, plastic-adhesion and plastic-dissipative contact deformation response of a single isotropic contact of two smooth spheres is briefly discussed. Then the new models are shown that describe the elastic-plastic force-displacement and moment-angle behaviour of adhesive and frictional contacts.Using the model “stiff particles with soft contacts”, a sphere-sphere interaction of van der Waals forces without any contact deformation describes the “stiff” attractive term. A plate-plate model is used to calculate the “soft” micro-contact flattening and adhesion. Various contact deformation paths for loading, unloading and contact detachment are discussed. Thus, the varying adhesion forces between particles depend directly on this “frozen” irreversible deformation. Thus, the adhesion force is found to be load dependent. Their essential contribution on the tangential force in an elastic-plastic frictional contact with partially sticking within the contact plane and microslip, the rolling resistance and the torque of mobilized frictional contact rotation is shown.     

Surfactant adsorption rather than “shuttle effect”?

In the absence of chemical reaction, mass transfer enhancement by suspended particles (mostly activated carbon) at the gas/liquid interface has been frequently reported and is usually explained by a “shuttle mechanism” exerted by particles with a high adsorption capacity for the transfer component. A major problem of this model is that unrealistic enrichment of the solids at the interface as compared to the bulk concentration has to be assumed. A comprehensive study has been carried out in a stirred tank in a wide range of the stirring speed (0-) with 9 different powdered solids suspended in water. With a flat gas/liquid interface, moderately hydrophobic solids significantly increased the mass transfer rates at low solid loadings (0.1-). However, the effect is not limited to particles with a high adsorption capacity for the gas (e.g. activated carbon) but it is observed also for non-porous particles (e.g. graphite or sulphur). When the particles are removed by rinsing, the absorption rates remain high. When the system is kept very clean (surfactant free), the enhancement effect is not observed. Based on these findings, it is concluded that adsorption of surfactants on hydrophobic solids cleans the interface resulting in higher mass transfer coefficients k_L.     

Modeling and verification of fluid-responsive polymer pumps for microfluidic systems

The fundamental principles necessary to create and model a pump for microfluidic systems using fluid-responsive polymer particles are described. The pump is “activated” by the addition of water to the particles, which induces a significant particle volume expansion and pushes a stored fluid from an adjacent reservoir at a predicted flow rate. Two particle systems were investigated to examine how polymer properties affect the rate and amount of fluid delivered. Poly (acrylic acid) (PAA) particles obtained from Pampers^® diapers yield the best micropump swelling characteristics for delivering fluid at pressures above 1100 Pa, whereas the softer potassium-neutralized PAA particles from Aldrich are best only at lower pressures. The maximum flow rates produced by the Pampers^®and Aldrich particles with minimal backpressure are 0.5 and of PAA, respectively. The experimental results demonstrate good agreement with an analytical model describing equilibrium and dynamic polymer swelling coupled with pressure-driven flow through cylindrical channels under conditions in which gel-blocking was not important. Fluid-responsive polymer micropumps could provide an inexpensive and lightweight method for driving fluid flow in microfluidic and other applications.     

Ultrafine cohesive powders: From interparticle contacts to continuum behaviour

Continuum mechanical models and appropriate measuring methods to determine the material parameters are available to describe the flow behaviour of cohesive powders. These methods are successfully applied to design process equipment as silos. In addition, “microscopic” studies on the particle mechanics can give a better physical understanding of essential “macroscopic” constitutive functions describing a powder “continuum”. At present, by means of the discrete element method (DEM), a tool is available that allows one to consider repulsive and frictional as well as attractive adhesion forces in detail. Within the framework of Newton's equations of motion, each particle in the system is tracked, and reacts to the forces acting.The knowledge of the interaction forces between particles is thus a prerequisite for understanding (via DEM) the stability and flow of particulate systems and other phenomena. In this study, macroscopic cohesion and friction are related to their microscopic counterparts, adhesion and contact-friction. The macroscopic cohesion is found to be proportional to the maximal microscopic adhesion force, and the macroscopic friction coefficient is a non-linear function of the contact friction, dependent (or independent) on the preparation procedure for yielding (or steady-state flow).One of the few methods available for the direct measurement of surface and contact-forces is the atomic force microscope (AFM) and, related to it, the so-called particle interaction apparatus (PIA). A contact model for ultrafine cohesive particles (average radius ) is introduced, based on such experiments. Plugged into DEM, consolidation, incipient yielding, and steady-state flow of the model powders are studied. Also the dynamic formation of the shear zone is examined and compared with experimental observations. Eventually, the shear experiments with volumetric strain measurements in a translational shear cell are used for validation.     

Mechanism of enhanced gas absorption in presence of fine solid particles. Effect of molecular diffusivity on mass transfer coefficient in stirred cell

Desorption of oxygen and hydrogen from various liquids (water, 0.8 molar sodium sulphate solution) containing suspended particles of activated carbon at various solid loading was investigated. The desorption was used to avoid supersaturation effect which was observed during oxygen and hydrogen absorption into liquid saturated with nitrogen. Experiments were carried out in a stirred cell with flat gas-liquid interface at and atmospheric pressure. An increase of k_L upon addition of the particles was observed. Enhancement factor increases with increasing contact time of the particles with liquid reaching maximum steady-state value of approx. 3 after sufficiently long time (a few hours) regardless of solid loading , agitator frequency and solute gas (O₂,H₂). The results fit the correlation (e is specific power dissipated by agitator in liquid and D is molecular diffusivity of gas absorbed) with the exponent for liquids without and for the liquids with the particles. It indicates that the interface is rigid in absence of particles and hinders the motion of liquid along the interface forming boundary layer while in the presence of particles the interface is completely mobile and surface renewal proceeds according to the penetration model. These results confirm a finding of Kaya and Schumpe (2005) that the enhancement of mass transfer in the cell at the presence of hydrophobic solids is due to clean-up of the interface from surfactants by their adsorption on hydrophobic solids rather than by a “shuttle mechanism” exerted by particles with a high adsorption capacity for the transfer component.     

Study of limestone particle impact attrition

Impact attrition of limestone particles was investigated at temperatures from 25 to and 1 atm pressure. Impacts changed the particle size distribution and mean particle diameter significantly for conveying gas velocities of 20-100 m/s. With increasing temperature less attrition occurred due to a decrease in particle impact velocity and an increase in the threshold particle impact velocity. The activation energy for impact attrition was . The mean limestone particle diameter decreased with increasing number of impacts and increasing impact velocity. Two empirical equations give good agreement with the experiments. Based on the experimental observations and correlations, an impact mechanism is suggested, where the area of new surface generated is proportional to the total kinetic energy consumed, to the number of impact cycles and an exponential decrease with temperature. When particles break, each particle generally splits into 2-3 daughter particles. The threshold particle velocities for breaking limestone particles were found to be at , similar to the reported literature values.     

Local shear and skin friction on particles in three-phase fluidized beds

An extension of the electrochemical shear-rate measurement technique is carried out in this work to evaluate the friction force and the shear stress on a particle in two and three phase fluidized beds. Using this technique, the skin friction on a sphere has first been validated for single phase flow. In two- and three-phase fluidized bed, the significance and the direction of the velocity gradient at the wall are discussed.In the case of three phase fluidization, glass spheres (2 mm in diameter, ) and plastic spheres (5 mm in diameter, ) were used. This choice provides very different bubbly flows due to different balances of coalescence and break-up of bubblesThe contribution of the frictional force is more important in “coalescent” fluidized beds than in “break-up” fluidized beds. The effect of gas injection is depending on the fluidized particle effect on bubble coalescence and break-up. Correlations have been developed linking frictional force to gas hold-up.The correlations recommended for frictional force in fluidized beds for both systems, (i.e., coalescence and break-up) are as follows:

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Glass spheres (2 mm diameter, coalescence regime):

     

Dynamics of diffusion with reversible binding in microscopically heterogeneous membranes: General theory and applications to dermal penetration

Essentially all biological membranes and tissues exhibit microscopic heterogeneity in the form of cellular, lamellar or other organization, and molecular diffusion in these materials is frequently slowed by binding to elements of the microstructure (“trapping”). This paper addresses situations where binding is describable as a linear reversible process at the microscale, with forward (“on”) and reverse (“off”) rate constants k_f(x) and k_r(x) that vary with position. Very commonly it is tacitly assumed that the macroscopically observable binding behavior should follow the same rate law with the substitution of appropriate effective (tissue-average) rate constants and . This assumption is probed theoretically for spatially periodic microstructures using a judicious application of numerical calculations and asymptotic analysis to prototypical one-dimensional transport problems. We find that smooth microscopic variations produce an anomalous macroscopic exchange between free and bound solute populations that is not well described by a single pair of forward and reverse rate constants, i.e., violates the usual paradigm. In contrast, discontinuous variations (as in two-phase composite media) are evidently well described by the usual paradigm. For the latter case we derive simple and general algebraic equations giving and , and generalize them to any three-dimensional unit cell representing the tissue microstructure. Validity of the formulas is demonstrated with reference to a concrete example describing molecular diffusion through the stratum corneum (barrier) layer of skin, comprising lipid (intercellular) and corneocyte (cellular) phases. Our analysis extends coarse-graining (homogenization, effective transport) theory for irreversible trapping systems to the reversible case.     

Development and performance test of a thermo-denuder for separation of volatile matter from submicron aerosol particles

In this study we designed and evaluated a home-made thermo-denuder (TD) both experimentally and numerically. Sodium chloride (NaCl) particles, toluene gas, and carbon black particles were used for the performance evaluation of the TD. The TD was evaluated for various set-point air temperatures and particle sizes using the following three parameters: the temperature profile, penetration efficiency, and gas adsorption efficiency. At , the temperature was nearly uniform, remaining within of the set-point temperature, in the heating section and decreased to the temperature of ambient air in the cooling section. The particle penetration efficiencies were 93–96% at and 58–67% at for particle sizes of 20–60 nm. The gas adsorption efficiency was nearly unity until the breakthrough time of 65 h, and the total amount of toluene adsorbed by activated carbon particles was 72 mg-toluene/g-activated carbon particles. From size distribution measurements of dry carbon black and toluene enriched carbon black particles, the mode diameter measured at the set-point temperature of was found to be 48.6 nm, which agreed with the one obtained from the dry particle measurements. The overall number concentration obtained after particle losses were compensated was lower than that measured without using the TD by 35.6%, which was caused by gas adsorption in the TD.     

Bubble sizes in agitated solvent/reactant mixtures used in heterogeneous catalytic hydrogenation of 2-butyne-1,4-diol

Catalytic hydrogenations reactions are frequently conducted in “dead-end” multiphase stirred reactors with the reactant dissolved either in an alcohol, or in water or a mixture of the two. In such systems, the rate of gas-liquid mass transfer, which depends on bubble size, may well be the overall rate-limiting step. However, a study of bubble sizes across the whole range of solvent compositions from entirely water to entirely organic has not been reported. Here, for the first time, a systematic investigation has been made in a 3 L, closed vessel simulating a “dead-end” reactor containing 1% by volume of air which is dispersed by a Rushton turbine in water, isopropanol (IPA) and mixtures of the two, with and without 2-butyne-1,4-diol simulating a reactant. Mean specific energy dissipation rates, , up to have been used and bubbles size distributions and mean size were measured using a video-microscope-computer technique. In the single component solvents (water, ; IPA, though the interfacial tensions are very different, irregular, relatively large bubbles of similar sizes were observed ( in IPA, and in water) with a wide size distribution. In the mixed aqueous/organic solvents, and especially at the lower concentrations of IPA (1%, 5%, 10%), the bubbles were spherical, much smaller (d₃₂ from 50 to ) with a narrow size distribution. The addition of the reactant (0.2 M 2-butyne-1,4-diol) to the mixed solvents had little effect on the mean size, shape or distribution. However, addition to water (thus producing a mixed aqueous/organic liquid phase) led to small spherical bubbles of narrow size distribution. Neither Weber number nor surface tension was suitable for correlating bubble sizes since σ decreased steadily from pure water to IPA whilst bubble size passed through a minimum at around 5% IPA. For any particular fluid composition, the functionality between d₃₂ and was similar, i.e. . The above observations are explained in terms of the polarisation of bubble surfaces in miscible mixed aqueous/organic liquids caused by preferential directional adsorption at low concentrations of the organic component with its hydrophilic part directed into the aqueous phase and its hydrophobic part into the gas phase. As a result, coalescence is heavily suppressed in the low-concentration miscible alcohol (or diol)/aqueous systems whilst strong coalescence dominates bubble sizes in water and the alcohol and at high concentrations of the latter.     

Using CFD to predict the behavior of power law fluids near axial-flow impellers operating in the transitional flow regime

A computational fluid dynamics (CFD) model of flow in a mixing tank with a single axial-flow impeller was developed with the Fluent^TM software. The model consists of an unstructured hexagonal mesh (158,000 total cells), dense in the region from the surface of the impeller. The flow was modeled as laminar and a multiple reference frame approach was used to solve the discretized equations of motion in one-quarter of a baffled tank. A solution of 0.1% Carbopol in water, a shear-thinning fluid, was found to be clear enough to measure impeller discharge angles using laser Doppler velocimetry. This is the first time that impeller discharge angles have been reported in the literature for a shear-thinning fluid with a hydrofoil impeller. Rheological measurements indicated that the Carbopol solution can be characterized by the power law (K=9,n=0.2) under the range of shear conditions (0.1- expected near the impeller in the mixing tank. The CFD model accurately predicted the dependence of power number and discharge angle on Reynolds number (as predicted by Metzner and Otto), for an A200 (pitched blade turbine or PBT) and an A315 (Hydrofoil) impeller operating in the transitional flow regime (Reynolds numbers: 25-400) with glycerin and 0.1% Carbopol solutions. Subsequently, the results of a systematic CFD study with power law fluids indicated that the power number and discharge angle of an axial-flow impeller in the transitional flow regime depends not only on the Reynolds number (as determined by Metzner and Otto's method) but also on the flow behavior index n. Consequently, an alternative to Metzner and Otto's method was pursued. The results of converged CFD simulations indicate that the near-impeller “average shear rate” increases not only with increasing RPM (as proposed by Metzner and Otto), but also with decreasing flow behavior index (n) and discharge angle in the transitional flow regime. Considering this result, an improved method of estimating the power number and discharge angle for power law fluids in the transitional flow regime is proposed.     

Twenty-first century research needs in electrostatic processes applied to industry and medicine

From the early 20th century Nobel Prize winning (1923) experiments with charged oil droplets, resulting in the discovery of the elementary electronic charge by Robert Millikan, to the early 21st century Nobel Prize (2002) awarded to John Fenn for his invention of electrospray ionization mass spectroscopy and its applications to proteomics, electrostatic processes have been successfully applied to many areas of industry and medicine. Generation, transport, deposition, separation, analysis, and control of charged particles involved in the four states of matter: solid, liquid, gas, and plasma are of interest in many industrial and biomedical processes. In this paper, we briefly discuss some of the applications and research needs involving charged particles in industrial and medical applications including: (1) generation and deposition of unipolarly charged dry powder without the presence of ions or excessive ozone, (2) control of tribocharging process for consistent and reliable charging, (3) thin film powder coating and powder coating on insulative surfaces, (4) fluidization and dispersion of fine powders, (5) mitigation of Mars dust, (6) effect of particle charge on the lung deposition of inhaled medical aerosols, (7) nanoparticle deposition, and (8) plasma/corona discharge processes. A brief discussion on the measurements of charged particles and suggestions for research needs are also included.     

A vibrating membrane bioreactor (VMBR): Macromolecular transmission—influence of extracellular polymeric substances

The vibrating membrane bioreactor (VMBR) system facilitates the possibility of conducting a separation of macromolecules (BSA) from larger biological components (yeast cells) with a relatively high and stable macromolecular transmission at sub-critical flux. This is not possible to achieve for a static non-vibrating membrane module. A BSA transmission of 74% has been measured in the separation of 4 g/L BSA from 8 g/L dry weight yeast cells in suspension at sub-critical flux . However, this transmission is lower than the 85% BSA transmission measured for at pure 4 g/L BSA solution. This can be ascribed to the presence of extracellular polymeric substances (EPS) from the yeast cells. The initial fouling rate for constant sub-critical flux filtration of unwashed yeast cells is 3-4 times larger than for washed yeast cells . At sub-critical flux, an EPS transmission of around 32% is measured for a pure yeast cell suspension. Thus, EPS and BSA are “competing” in being transmitted which might explain the lowered BSA transmission in the presence of yeast cells. Additionally, EPS heavily foul the membranes, leading to a 86% permeability drop and a fouling resistance 6 times larger than the membrane resistance after 5  h of constant sub-critical flux filtration of pure 8 g/L dry weight yeast cell suspensions. Thus, the addition of hydraulic resistance caused by EPS might also explain the lowered BSA transmission, in the presence of yeast cells, since the membrane pores might be narrowed or partly blocked. EPS is, furthermore, able to cause a relatively large permeability drop even on a membrane module pre-fouled by EPS.     

Fractal crystal growth of poly(ethylene oxide) crystals from its amorphous monolayers

The fractal crystal growth process of the PEO monolayer with a molecular weight and a distribution has been followed on the substrate of the silicon wafer using AFM equipped with a hot stage. A depletion zone between the ramified crystals and the viscous amorphous layer was found in the AFM height images. The formation of the depletion zone shows that the molecules have to “break up” with the amorphous layer and then diffuse through the depletion zone to join the crystals. The diffusion process further means the diffusion-controlled mechanism resulting in the fractal crystal pattern with a fractal dimension D_f ≈ 1.63. The linear feature of the crystal pattern radius growth with time means that the surface kinetic process plays a key role in the crystal growth.     

Use of numerical modeling in design for co-firing biomass in wall-fired burners

Co-firing biomass with coal or gas in the existing units has gained increasing interest in the recent past to increase the production of environmentally friendly, renewable green power. This paper presents design considerations for co-firing biomass with natural gas in wall-fired burners by use of numerical modeling. The models currently used to predict solid fuel combustion rely on a spherical particle shape assumption, which may deviate a lot from reality for big biomass particles. A sphere gives a minimum in terms of the surface-area-to-volume ratio, which impacts significantly both motion and reaction of a particle. To better understand the biomass combustion and thus improve the design for co-firing biomass in wall-fired burners, non-sphericity of biomass particles is considered. To ease comparison, two cases are numerically studied in a long gas/biomass co-fired burner model. (1) The biomass particles are assumed as solid or hollow cylinders in shape, depending on the particle group. To model accurately the motion of biomass particles, the forces that could be important are all considered in the particle force balance, which includes a drag for non-spherical particles, an additional lift due to particle non-sphericity, and a “virtual-mass” force due to relatively light biomass particles, as well as gravity and a pressure-gradient force. Since the drag and lift forces are both shape factor- and orientation-dependent, coupled particle rotation equations are resolved to update particle orientation. To better model the reaction of biomass particles, the actual particle surface area available and the average oxygen mass flux at particle surface are considered, both of which are shape factor-dependent. (2) The non-spherical biomass particles are simplified as equal-volume spheres, without any modification to the motion and reaction due to their non-sphericity. The simulation results show a big difference between the two cases and indicate it is very significant to take into account the non-sphericity of biomass particles in order to model biomass combustion more accurately. Methods to improve the design for co-firing biomass in wall-fired burners are finally suggested.