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The manipulation of suspended particles using ultrasonic fields remains an active area of research due to the many potential applications of acoustophoresis. While prior research has focused primarily on the acoustophoretic behavior of particles in response to an acoustic field of a single wavelength, relatively little work has been done to study the acoustophoretic behavior of subject to two or more concurrent frequencies of sound. The presence of multiple concurrent, resonant frequencies can lead to more complicated particle behavior spatially across the chamber than the simple node/anti-node attraction/repulsion behavior present in single-frequency chambers. In this study, we investigate the behavior of spherical particles (both single phase and core-shell) in multi-frequency chambers in both the Rayleigh (long wavelength) limit and cases where the particle size is the same order of magnitude as the wavelength of the background field. A mathematical description of the phenomena is developed in terms of the ratios of the pressure amplitudes for the individual background frequencies, including the criteria for continuously moveable nodes. Attention will also be paid to the implications of more than one simultaneous frequency on the motion of particles undergoing a change in size.

Many sorbents demonstrate exceptionally high absorbance capacity on a per unit mass basis, with some sorbents capable of absorbing hundreds of times their own mass. However, the volumetric capacity...

Porous hydrogels with controlled morphology were successfully prepared from polymerizing soy-protein-isolate (SPI)-stabilized high internal phase emulsions (HIPEs). The ability of SPI to act as a surfactant in an oil-in-water HIPE containing acrylic acid or acrylamide monomer was investigated. The void and window sizes in the polyHIPEs were tailored by adjusting SPI and/or monomer concentration. Ultrasonication treatment was applied to vary the physical properties of the SPI. Although this treatment weakens the emulsifying efficiency of SPI, the HIPEs were stable enough to create polyHIPEs with larger pores and windows than polyHIPEs from untreated counterparts. The formation of polyHIPEs with interconnected, open-cell morphologies indicated that SPI is not a typical Pickering emulsifier. The performance of the hydrogels to capture heavy metal (e.g. lead (II)) ions was also explored. The highly interconnected polyHIPE structure with large voids revealed an enhanced absorption behavior compared with non-porous hydrogels.

Polymer foams have broad application as structural and functional materials in a variety of technological applications. Polymerization of monomers present in the continuous phase of water-in-oil hi...

Fluid transport behavior inside polymer foams (polyHIPEs) templated from high-internal-phase emulsions (HIPEs) has been studied. Morphological properties of the foams were controlled through shearing their precursor emulsions for varying durations. Two types of flow, Darcy flow through the filled foam, and capillary imbibition into unfilled foams, have been studied. From permeability measurement, the size of the interconnections (windows) between the voids of the polyHIPE was found to provide the appropriate characteristic length to describe Darcy flow. For capillary imbibition, liquid/foam interaction effects were studied under two wetting conditions: silicone oil in which the oil readily spreads (at a vanishing contact angle) on the struts within the foam; and, using water and foams coated with various amount of surfactant where the liquid only partially wets the surface. For capillary imbibition, the dominant morphological feature was also the window size when the internal surfaces of the foam have a macroscopically uniform wettability for the liquid. The Lucas–Washburn analysis was found to well describe imbibition rates in this case. However, when the internal surfaces of the foam have a non-uniform wettability (due to a non-uniform deposition of surfactants), the concentration and distribution of surfactants plays a large role in determining imbibition kinetics.

Fabrication of flexible strain sensors incorporating superlight and robust conductive skeletons in a facile and environmentally friendly manner poses a number of processing challenges. Herein, we report a novel, low-density, three-dimensional conductive filler skeleton consisting of natural psyllium and conductive carbon nanostructure (CNS, i.e. branched carbon nanotubes). After infiltration of polydimethylsiloxne (PDMS) into the skeleton, a composite with excellent electrical conductivity and superior piezoresistive sensitivity was obtained. Psyllium assists the formation of a conductive CNS network, thus the composite made by the skeleton exhibits electrical conductivity up to 6.6 S/m for only 0.2 wt% CNS. Skeletons composed with 0.2 wt% CNS and various content of psyllium infiltrated with PDMS have a linearly responsive gauge factor between 2.3 and 4.5. Continuous psyllium- CNS pathways not only enhance electron transport within composites, but the psyllium also provide extra mechanical support to ensure stable and reproducible strain sensing in the composite. Potential applications of the composites as control switches and for motion detection are reported.

The ultrasonic degradation of locust bean gum (LBG) in aqueous solutions has been studied at 25 °C for ultrasonication times up to 120 min. Although LBG is not a polyelectrolyte, the degradation extent and kinetics were found to be somewhat sensitive to the ionic conditions in solution, and this is attributed to changes in molecular conformation that can occur in different salt environments. Ultrasonic degradation was tracked by rheological measurements that lead to the determination of intrinsic viscosity for the LBG molecules. A kinetic model was also developed and successfully applied to characterize and predict the degradation results.

Porous polymer monoliths were prepared by emulsion templating. The relationship between emulsion behavior and cellular morphology and mechanical properties after polymerization of the monomeric continuous phase was investigated. Porous poly(High Internal Phase Emulsion)s were synthesized with the same strut material and relative density but different morphology and mechanical properties, achieved by varying the surfactant system and concentration. The range of morphologies spanned from cellular solids with hierarchical porosity and millimeter-scale voids to highly interconnected, micron-scale voids resembling Kelvin cell structures. The difference in mechanical properties was evaluated as a function of morphology and polymer crosslinking density. According to the Gibson-Ashby relative modulus – density relationship, morphology was not the sole indicator of mechanical properties. Variance in crosslinking density was inferred from swelling and thermal degradation, indicating that the interfacial film in the emulsion stage influences the polymerization reaction and the degree of crosslinking. A mixed surfactant system of cationic and nonionic surfactant showed the best ability to support small droplets and limit coalescence. However, the magnitude of the interfacial tension alone is not an indicator of the final properties.

The compressive behavior of poly(HIPE) foams was studied using the developed micromechanics based computational model. The model allowed identifying the morphological parameters governing the foam compressive behavior. These parameters comprise: (i) foam density, (ii) Sauter mean diameter of voids calculated from the morphological analysis of the polydispersed microstructure of poly(HIPE), and (iii) polymer/strut characteristic size identified as the height of the curvilinear triangular cross-section. The model prediction compared closely with the experiments and considered both the linear and plateau regions of the compressive poly(HIPE) behavior. The computational model allows the prediction of structure–property relationships for poly(HIPE) foams with various relative densities and open cell microstructure using the input parameters obtained from the morphology characterization of the poly(HIPE). The simulations provide a pathway for understanding how tuning the manufacturing process can enable the optimal foam morphology for targeted mechanical properties.

The influence of the pyruvate group on the efficiency of ultrasonic degradation of xanthan gum in aqueous solution has been studied. Blends of natural and pyruvate-free xanthan gums were ultrasonicated at 20 °C for up to 30 min and evaluated for molecular-weight degradation by viscometry. Solutions of pure pyruvate-free xanthan exhibited the highest stability to degradation among all blend ratios studied. Removing the pyruvate group is believed to enable the molecular chains to adopt a more compact conformation, which renders the polymer less susceptible to ultrasonication. In addition, the effects of salt on ultrasonic degradation efficiency were studied by using 0.1, 10 −2 , or 10 −4 M of NaCl or Na 2 SO 4 in solution prior to ultrasonication. A degradation kinetics model was developed to quantify the degradation behavior. The absence of pyruvate groups renders the xanthan gum less sensitive to the influence of salt, and hence decreases the ultrasonic degradation efficiency.

The effectiveness of ultrasound to degrade the molecular weight of xanthan gum in aqueous solutions was investigated for sonication times up to 60 min at 20 °C and for polymer concentrations up to 0.1 g/dl. The Huggins equation was found to be applicable to the intrinsic viscosity of xanthan gum prior to sonication, while a truncated form was found to be adequate for estimating the intrinsic viscosity of the degraded xanthan. To better understand the influence of salting-in and salting-out salts (classified on the basis of the Hofmeister series) on degradation, xanthan-gum solutions were pre-mixed with 0.1, 10 −2 , 10 −3 , or 10 −4 M NaCl or Na 2 SO 4 , prior to ultrasonication. A kinetic model was developed and successfully applied to quantify and predict the degradation rates and efficiency. The various reaction rate constants and reaction orders were found to correlate with the different salt species and concentrations used, suggesting that salting-in and salting-out salts could increase or inhibit ultrasonic degradation by adjusting the molecular conformation of the xanthan.

The surface functionalization of silica by alkoxysilane can improve its dispersion into polymers by beneficially modifying interparticle and particle–polymer interactions. Thermogravimetric analysis (TGA) has been used to study the kinetics of binding a bifunctional alkoxysilane onto silica over the temperature range of 50 to 110 °C. Derivative TGA curves of the reacted silica show distinct peaks, which correspond to the alkoxysilane bound to one or two surface silanol sites. Binding at two silanol sites is postulated to occur in a two-step series reaction model. An Arrhenius analysis for the alkoxysilane binding reactions shows that the activation energies of the two reaction steps are similar, which is expected because the alkoxysilane contains two identical binding groups. This work demonstrates the suitability of the TGA technique to investigate the reaction kinetics for modifying the surface of silica.

Thermogravimetric analysis (TGA) has been used to study the kinetics of bound-rubber formation on the surface of silica modified with a bi-functional alkoxysilane coupling agent (CA) over the temperature range of 110 to 150 °C. The bound-rubber formation is attributed to the direct binding of a polymer chain with a number of nonpolar interactive sites provided by the CA molecules on the silica surface. A kinetic model developed for the reaction process suggests that the bound-rubber morphology is altered at temperatures above 110 °C.

A model for the evolution of torque requirements in the batch mixing of silica agglomerates in styrene-butadiene rubber has been developed. The analysis considers the rheology of suspensions as modeled by the Krieger-Dougherty equation but with modifications so that it can apply to a wider range of solids loading, and combines that with a separate model that expresses the kinetics of the dispersion of silica agglomerates. The model shows good agreement with the trends of experimental data, and thus can be used to make predictions about torque requirements for compounding commercial grades of silica into rubber, as related to dispersion, under practical mixing conditions.

An approach for investigating the microstructural features that govern the capillary-pressure driven infiltration of liquid within beds of particles has been developed. Simultaneous measurement of the rate of liquid mass uptake and the volume occupied by that liquid within the particle bed provides information about the fraction of pore volume that participates in liquid infiltration as well as the effective size of the geometric unit that governs infiltration kinetics. Experiments involving the infiltration of squalane within different grades of silica and titanium dioxide illustrate the technique. The method is sensitive enough to enable interpretation of how variations in the morphology of the particles comprising the bed, or small changes in packing density within the particle bed, can lead to large changes in infiltration behavior.

The effects of carbon nanotube (CNT) inclusion on cyclic fatigue behavior and the tensile properties of polyurethane (PU) composites have been studied. Tension–tension cyclic fatigue tests were conducted at various load levels (30–50 MPa) to establish the relationship between stress and the number of cycles to failure (S–N curves). The tensile energy to break PU composites was enhanced up to 38% by using a low amount of CNTs. In addition, the incorporation of CNTs increased the fatigue life of PU in the high-stress amplitude, low-cycle regime by up to 248%. Micrographs show highly dispersed CNTs and indicate the key mechanisms for enhancement in fatigue life such as CNT crack-bridging and pull-out. The tensile-tensile fatigue properties obtained in this work show that PU systems can outperform epoxy systems widely used in structural applications.

The effects of carbon nanotube (CNT) inclusion on cyclic fatigue behavior and the residual mechanical properties of epoxy composites after different degrees of fatigue have been studied. Tension–tension cyclic fatigue tests were conducted at various load levels (25–50 MPa) to establish the relationship between stress and the number of cycles to failure (S–N curves). The residual strength and modulus were measured after loading at 30 MPa for 5000, 15,000, and 25,000 cycles. The incorporation of a small amount of CNTs increased the fatigue life of epoxy in the high-cycle, low-stress-amplitude regime by 1550%. Micrographs indicate the key mechanisms for enhancement in fatigue life such as CNT crack-bridging and pullout. POLYM. ENG. SCI., 52:1882–1887, 2012. © 2012 Society of Plastics Engineers

The effects of the addition of eight different block copolymers on the dispersion stability of multi-walled carbon nanotubes (MWCNTs) are reported. Suspensions of CNTs in different components of an epoxy system have been prepared using a tip sonicator and different amounts of block copolymers. The resistance to sedimentation of MWCNTs in various media was systematically investigated by using a centrifugation technique. Block copolymers that result in dispersions of MWCNTs in epoxy and hardener stable for more than 1 week have been obtained. Dispersions using a single or a combination of two different dispersing agents have been used for the fabrication of MWCNT nanocomposites. The effect of different preparation routes and use of block copolymers on the tensile properties and surface resistivity of the composites have been evaluated. The results obtained have been related with the dispersion stability of the MWCNTs in the epoxy components.

The effects of eight different dispersants on the stability of multiwalled carbon nanotubes (MWCNTs) in vinyl ester (VE) resin systems have been systematically analyzed. Suspensions incorporating different amounts of dispersants relative to the concentration of MWCNTs in the VE have been prepared. The resistance to sedimentation of MWCNTs in the resin was investigated by using a centrifugation technique. One dispersing agent (B60H) was found to impart long-term stability to suspensions of MWCNTs in VE. VE-based composites reinforced with MWCNTs were prepared using different amounts of the dispersing agent. Tensile tests were performed to analyze the effect of different concentrations of MWCNTs and the use of the dispersing agent in the composites. Fracture surfaces of VE nanocomposites have been analyzed using scanning electron microscopy (SEM) to elucidate the reinforcing effect of the CNTs. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers

A process for dispersing agglomerates or clusters of particles utilizing pressure generated from volatilization of an interstitial liquid. More particularly, the method relates to infusing the particles with a first liquid, placing the infused particles in a second liquid or fluid having a higher boiling point than the first liquid and heating the composition to a temperature above the boiling point of the first liquid thereby resulting in breakage of the particles. Compositions including particles dispersed by interstitial liquid vaporization are also disclosed.

A model for the coalescence of the oil phase within aqueous emulsions driven by the application of a low-intensity, resonant ultrasonic field has been developed. Under the application of a resonant ultrasonic field, the density and compressibility difference between the dispersed and continuous phases results in a net force that pushes droplets toward pressure antinodes where coalescence subsequently occurs. A mathematical model that determines the relative transport of two individual droplets under the action of acoustic and other relevant forces was recently published. That approach is utilized here in the development of a population balance model which predicts global coalescence rates and the evolution of the droplet size distribution under the influence of the ultrasonic field. These results are experimentally validated and good qualitative agreement between model predictions and experimental observations of the evolution of the droplet size distribution is observed. Discrepancies between the experimental and modeling results are attributed to spatial non-uniformities in the acoustic field utilized in the experiments and the associated lateral radiation forces. The contribution of these forces to overall coalescence rates is described in terms of an effective strength of the acoustic field within the acoustic chamber.

The dispersion of particle agglomerates is affected by the presence of any chemical additives incorporated within the agglomerate structure. Traditional additives change the particle-particle and particle-matrix interactions to a degree that depends primarily on the chemical nature of the additive and its concentration within the agglomerate. A new generation of additives, based on polymeric systems that respond to external stimuli, allows the adjustment of interparticle interactions by manipulating the variable to which the additive is responsive. In this study, dispersion tests in simple shear flow are used to examine the effect of a thermally responsive additive, poly(N-isopropyl acrylamide) (PNIPAM), on the dispersion behavior of silica agglomerates. The temperature sensitivity of the dispersion mechanisms and kinetics for silica clusters treated with either linear or crosslinked PNIPAM has been examined.

The dispersion behavior of clusters of hydrophilic and hydrophobic silica treated with a thermoresponsive polymer has been investigated. The influence of the thermoresponsive interfacial chemistry was assessed by performing dispersion studies in poly(dimethylsiloxane) (PDMS) at 25 and 68 degrees C for agglomerates of powders incorporating poly(N-isopropyl acrylamide) (PNIPAM), a thermoresponsive polymer, and water. While the dispersion of clusters of hydrophilic silica was found to be temperature independent, dispersion of hydrophobic silica exhibited a temperature dependency. The response of the polymer upon temperature variation brings about modifications in the interparticle interactions of the PNIPAM-treated powders thus leading to changes in the cluster cohesivity. The dependence of the cluster cohesivity on temperature was evidenced through changes in the dispersion mechanism and kinetics of the treated silica clusters at both tested temperatures. In addition, optical microscopy of silica suspensions with and without PNIPAM at 25 and 60 degrees C, illustrated the effect of the polymer presence and configuration on the particle-particle interaction forces. While micrographs of the particles at 25 degrees C show less evidence of particle aggregation, the micrographs at 60 degrees C illustrate an increase in aggregation suggesting an enhancement in the attractive forces between particles.

The total force of interaction induced by a viscous binder between two equal-sized spheres had been previously analyzed. In this work, we extend the modeling approach to the interaction of dissimilar particles via a harmonic mean approximation used to compute the effective size of the interacting entities. We test the validity of this approximation to extrapolate these earlier analytical models to the practical processing scenario of particle size distributions generated during dispersive mixing. Additionally, we use an experimental methodology to simulate typical processing flow geometries and conditions ranging from the simple shear to dynamic shear to extensional flows. This enables application of the previously established algorithm, which is based on constant velocity interactions, to a range of realistic mixing modeling and simulation efforts.

A method to aid the separation of the oil phase from aqueous emulsions using a low-intensity, resonant ultrasonic field has recently been developed. The density and compressibility difference between the dispersed and continuous phases within the emulsion results in a net force on the oil drops that pushes them toward the pressure antinodes of the standing-wave field, where coalescence subsequently occurs. A trajectory model is developed to predict the relative motion of drops subjected to the acoustic field. Such trajectories are sensitive to the physical properties and relative size of interacting drops, the initial configuration of the drops, and acoustic field parameters. Model predictions are validated by comparing experimentally observed trajectories with those predicted by the model. The modeling approach is then extended to determine the temporal evolution of the size of the region surrounding a target drop cleared by coalescence as a function of physical and acoustic field parameters. These results form the basis of a population balance model that attempts to track the size-evolution of a drop population coalescing under the influence of an acoustic field.

A unified approach to predict the tendency for dispersion of particle agglomerates, inclusive of a wide range of particle and agglomerate properties, is presented. This framework is applied to analyze the behavior of three prototypical materials (fumed silica, calcium carbonate and titanium dioxide) across a range of agglomerate packing densities. Simulations of dispersion phenomena, which employ our previously developed solution for liquid–bridge interactions for wet interparticle contacts and the Rumpf model for the tensile strength of the dry and wet portions of the agglomerate, have been performed. Various mechanisms of dispersion are predicted for various conditions of agglomerate density and extent of fluid infiltration. These range from an adhesive mechanism at the wet–dry interface for sparse materials to a cohesive mechanism by erosion as agglomerate density increases. The results correspond well with the results of earlier experimental studies involving the same materials.

The strain-dependent dynamic storage modulus of a poly(dimethyl-siloxane-co-methylvinyl-siloxane-co-methylphenyl-siloxane)-based silicone elastomer (PVMQ), which is reinforced with fumed silica and crosslinked with peroxide, is investigated. The time dependence of the dynamic storage modulus on the magnitude of the mean strain at a particular test condition is investigated. The dynamic modulus results are shown to depend on the time of cycling as well as the relative magnitudes of the dynamic and mean strains. The relaxation of the force required to maintain the mean strain is observed to depend on the magnitude of the dynamic strains and the data are shown to be consistent with static stress relaxation experiments in the limit of zero dynamic strain. Recovery of the dynamic modulus from the exposure to higher strain cycling is seen to be facilitated by dynamic cycling with higher cycling strains yielding faster recovery rates. The observed phenomena are interpreted in terms of the role of entanglements in the polymer phase on the dynamic behavior of the elastomer material. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2197–2204, 2007

The size distribution within and electrokinetic properties of aqueous perovskite (LaCoO3) suspensions, have been characterized as a function of processing conditions. Submicron–sized perovskite particles have been obtained using a cavitation technique in which the suspension is passed through a series of small orifices under extreme driving pressure drops. When no additives were used, the zeta potential of the particles was found to be positive over the entire pH range studied. Use of an acrylic copolymer surfactant with multiple negatively charged sites during the cavitation processing was found to improve dispersion stability. The observed variations in zeta potential and particle size for the suspensions are explained in terms of electrostatic interactions between particles, the tendency for the surfactant to adsorb onto the particles, and the degree of steric stabilization provided by the surfactant.

A modeling approach to predict and enhance understanding of the dispersion phenomenon is presented. The discrete/distinct element method is adopted to study the behavior of single spherical agglomerates, immersed in a simple shear flow field, in response to shearing under steady or dynamic/oscillatory flow conditions. The effects of hydrodynamic forces, which result from both the straining and rotating components of the flow, and cohesive forces of interaction, comprised of short-range van der Waals attractive and Born repulsive forces, are considered. The results of simulated distortion and dispersion of nano-size silica agglomerates in response to steady shearing are used to demonstrate the functionality of the three-dimensional simulation. Simulated results are found to be in good agreement with reported experimental trends. The current model allows us to probe and predict the dispersion phenomenon as a function of processing conditions, agglomerate structure/morphology, and material properties and interaction forces.

The formation of submicron TiO2 smoke (a gas-phase suspension) from titanium tetrachloride in a low-pressure hydrolysis reaction in a simple reactor configuration has been studied. Particle size distribution, particle morphology and degree of crystallinity have been characterized as a function of reaction conditions. Highly crystalline anatase TiO2 particles with narrow size distribution and smaller particle size were formed at high reactor temperature, while larger, amorphous particles were found at lower reactor temperatures. These results are consistent with literature studies. At 817 °C, small (450 nm), spherical, unagglomerated particles could be produced. A gas-phase dispersion of these particles is intended for use as seeds in subsequent kinetic studies of titanium dioxide formation reactions involving a rapid compression methodology.

A model for the erosion kinetics of particle agglomerates in simple shear flows has been developed. The erosion rate is taken to be proportional to the difference between hydrodynamic and cohesive forces and to the rotation velocity of the dispersing agglomerate. For dispersion under identical hydrodynamic conditions, the model predicts faster erosion for larger agglomerates. Moreover, at equivalent hydrodynamic stress, erosion is enhanced at higher shear rates. Dispersion experiments using silica agglomerates of various densities and liquid low molecular weight polymers (e.g., poly(dimethyl siloxane)) of different viscosities were conducted in an oscillatory shear flow device. The experimental results validate the erosion model; the general shape of the erosion kinetic curve, and the effect of viscosity, shear rate and agglomerate size on dispersion kinetics are well predicted. The model can also predict erosion for agglomerates with fractal structure.

A modeling approach to study effects of liquid infiltration on the dispersion mechanism of particle agglomerates is presented. Specifically, the locus and the corresponding mechanism of failure are predicted using a discrete element method for solving the relative displacement of segments of the agglomerate as influenced by the various forces acting within the system. The phenomenon of adhesive failure is demonstrated by the simulation of the agglomerate behavior under specific extents of infiltration and the associated structural rearrangement that can occur due to capillary pressure effects. These modeling results are related to experimental observations of dispersion of fumed silica agglomerates wherein the infiltrated outer layer detaches from the dry core at reduced levels of applied stress.

This paper presents an analysis of a filtration technique that uses ultrasound to aid the collection of small particles (tens of microns in diameter) from suspension. In this method, particles are retained within a porous mesh that is subjected to a resonant ultrasonic field, even though the pore size of the mesh is two orders of magnitude greater than the particle diameter. The role of acoustic forces in driving the retention phenomena has previously been studied on a micro-scale, which included modeling and experimental verification of particle motion and trapping near a single element of the mesh. Here, we build on this work to develop an overall transport model to predict macroscopic performance criteria such as breakthrough times and the dynamics of the filtration performance. Results from this model compare favorably to experimental studies of the filtration phenomena; simulation results scale appropriately with experimental results in which inlet feed concentration and flow rate are varied.

Hydrodynamic dispersion of particle agglomerates occurs whenever the applied shear stresses can break the interparticle bonds responsible for the cohesivity of the agglomerate. Various mechanisms of hydrodynamic dispersion have been demonstrated for silica agglomerates infiltrated to different extents by the suspending fluid. In some cases, hydrodynamic forces are sufficient to induce the removal of incompletely infiltrated fragments from the parent agglomerate (dry cohesive failure) or the breakage of wetted fragments from the infiltrated portion of the parent agglomerate (wet cohesive failure). Dispersion can also occur such that a portion of the fracture surface originates at the interface between the infiltrated periphery of the agglomerate and its dry core (adhesive failure). To elucidate the tendencies for dispersion via various modes, a hydrodynamic analysis of the forces acting on and within the agglomerate has been performed for both uninfiltrated and partially infiltrated structures. This analysis reveals that the size of the region on which the hydrodynamic stress bears is sensitive to the degree of infiltration, which is consistent with the observed shifts in dispersion mechanism.

The strain-dependent dynamic storage modulus of a poly(dimethyl-siloxane-co-methylvinyl-siloxane-co-methylphenyl-siloxane) based silicone elastomer, which is reinforced with fumed silica and crosslinked with peroxide, is investigated. The dependence of the resulting dynamic storage modulus on the duration of cycling at a particular test condition is investigated and compared to static stress relaxation measurements in the same strain range. The dynamic modulus results are shown to depend on the time of cycling at the current test conditions as well as the time of cycling at prior conditions of lower strains. The relaxation is shown to be related to the time of cycling rather than the number of cycles performed. The pattern of behavior of the relaxation of the dynamic modulus with respect to peak strain amplitude is different than that observed in a static stress relaxation test, both of which show significant nonlinear effects in strain. The observed phenomena are interpreted in terms of the role of the polymer phase on the dynamic behavior of the elastomer material. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1001–1009, 2005

A model for the motion of particles driven by acoustic and hydrodynamic effects in the vicinity of a cylindrical collector has been previously reported. This trajectory model was developed to describe the essential physics that underlies an ultrasonically aided particle-filtration process in which a porous mesh is used to capture particles two orders of magnitude smaller than the pore size. To validate this trajectory model, experiments were performed to elucidate the detailed motion of particles in the neighborhood of a single cylindrical collector. Images of 54-μm-diameter polystyrene particles in aqueous suspension responding to acoustic and hydrodynamic forces were analyzed. Particle trajectories, calculated using only experimentally measured parameters as model inputs, well predicted the experimental observations. Adjustment of the local magnitude of the acoustic field, which accounts for spatial nonuniformities in the field, results in improvements in the correspondence between the trajectory predictions and the experimental observations. © 2005 American Institute of Chemical Engineers AIChE J, 2005

The effect of peroxide crosslinking on the dynamic modulus of a silica-reinforced silicone [poly(dimethylsiloxane)] elastomer was investigated. Three different peroxides (t-butyl peroxide, t-butyl perbenzoate, and benzoyl peroxide) were employed at various practical loadings and differences in the nonlinear behavior of the dynamic modulus were found. Results are discussed with respect to changes in crosslinking density and the identity of the peroxide. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1504–1512, 2005

A colloidal processing approach combining the phenomena of depletion flocculation and steric stabilization has been used in the preparation of high-quality coatings (monolayer and thicker) of silicon particles on silicon carbide fibers. Grafting procedures for the steric additive and the use of the depletion flocculation method to achieve dispersions of relatively high solids concentration are described. The impact of processing parameters on coating quality is demonstrated through electron microscopic images. The relationship between optimal processing parameters and the size and shape characteristics of the solid particles used is also discussed.

The dispersion behavior of agglomerates of several grades of fumed silica in poly(dimethyl siloxane) liquids has been studied as a function of particle morphology and applied flow conditions. The effects of primary particle size and aggregate density and structure on cohesivity were probed through tensile and shear strength tests on particle compacts. These cohesivity tests indicated that the shear strength of particle compacts was two orders of magnitude higher than the tensile strength at the same overall packing density. Experiments carried out in both steady and time-varying simple-shear flows indicate that dispersion occurs through tensile failure. In the steady-shear experiments,enhanced dispersion was obtained at higher levels of applied stress and, at comparable levels of applied stress, dispersion was found to proceed faster at higher shear rates. Experiments conducted in time-varying flows further corroborated the results obtained in tensile cohesivity tests. Experiments in which the mean and maximum stresses in the time-varying flows were matched to the stresses produced in steady shear flows highlight the influence of flow dynamics on dispersion behavior.

A novel method for recovering the oil phase from aqueous emulsions has been developed. The method applies a low-intensity, resonant ultrasonic field within a rectangular chamber, which is optionally filled with a highly porous medium. Oil droplets dispersed in water have negative acoustic contrast factor and thus are driven to the pressure antinodes of the standing wave field under the influence of acoustic radiation forces. Subsequent coalescence and/or wetting onto the internal surfaces of the chamber occur. Three types of porous media (an unconsolidated bed of 3-mm glass beads, aluminum mesh or reticulated polyester mesh) having pore sizes two to three orders of magnitude larger than droplets being collected were used. The oil collection was found to be sensitive to the natural affinity between the oil and the porous medium as well as its porosity. Of the three media studied, the polyester mesh was found to be the best in terms of the percentage oil collection while the bed of glass beads performed the poorest. The oil collection was found to be highly sensitive to the residence time of the emulsion in both the porous medium and acoustic field. Oil collection also showed expected trends with applied electrical power, but it was not found to be strongly dependent on the internal surface area of the mesh for the range of feed concentration tested. These experiments enable a preliminary understanding about the mechanisms underlying the separation process.

A fractionation method applicable to different populations of cells in a suspension is reported. The separation was accomplished by subjecting the suspension to a resonant ultrasonic field and a laminar flow field propagating in orthogonal directions within a thin, rectangular chamber. Steady, laminar flow transports the cell suspension along the chamber, while the ultrasonic field causes the suspended cells to migrate to the mid-plane of the chamber at rates related to their size and physical properties. A thin flow splitter positioned near the outlet divides the effluent cell suspension into two product streams, thereby allowing cells that respond faster to the acoustic field to be separated from those cells that respond more slowly. Modeling of the trajectories of individual cells through the chamber shows that by altering the strength of the flow relative to that of the acoustic field, the desired fractionation can be controlled. Proof-of-concept experiments were performed using hybridoma cells and Lactobacillus rhamnosus cells. The two populations of cells could be effectively separated using this technique, resulting in hybridoma/Lactobacillus ratios in the left and right product streams, normalized to the feed ratio, of 6.9 +/- 1.8 and 0.39 +/- 0.01 (vol/vol), respectively. The acoustic method is fast, efficient, and could be operated continuously with a high degree of selectivity and yield and with low power consumption. (c) 2004 Wiley Periodicals, Inc.

The dispersion of spherical carbon black agglomerates suspended in polydimethyl siloxane liquid and subjected to simple shear flows has been studied in a cone-and-plate shearing device. Sets of dispersion experiments were carried out for agglomerates of various size and packing density. For agglomerates of equal density and under conditions of equivalent cone rotation rates, the dispersion rates of small agglomerates were smaller than those observed for larger agglomerates. Since the agglomerates occupy a significant fraction of the flow domain, the magnitude and distribution of shear stress acting on the agglomerate at a fixed cone rotation rate depends on the ratio of agglomerate size relative to the size of the gap between the cone and plate. To investigate whether this effect could cause the observed variation in dispersion behavior, we performed three-dimensional simulations of the flow fields within the cone-and-plate device and calculated the resulting stress fields acting on spherical agglomerates. These results helped guide additional experiments in which the peak stress acting on agglomerates of various sizes was matched. However, even under matched stress conditions, the dispersion kinetics was found to vary according to the agglomerate size. In addition, the dispersion kinetics for identical sized agglomerates was found to depend on their processing history. Both of these results lead to the conclusion that some other effect, likely the infiltration of the processing fluid within the agglomerate structure, also influences the dispersion behavior.

Lattice Boltzmann methods (LBM) for one- and two-phase flows are used to model the capillary pressure-driven infiltration of fluids into porous spheres and rectangular beds. This simulates the soaking of fine particle agglomerates and powder compacts, respectively. The sensitivity of the infiltration behavior to physical parameters (such as porosity, permeability, degree of saturation and dimensions of the porous solid, as well as the dynamic viscosity and surface tension of the fluid) was investigated. Results of the LBM approach show good consistency with those from continuum models describing infiltration behavior. Additionally, the one-phase LBM was applied to investigate the influence of pore network structure on the infiltration behavior.

The effect of surface treatment on silica agglomerate dispersibility was investigated. Precipitated silica powders were treated with a commercially available coupling agent Bis-(triethoxysilylpropyl)-tetrasulphane (TESPT) in a blender. Spherical agglomerates of known density were prepared and dispersed in styrene butadiene rubber (SBR) under controlled flow conditions. The erosion kinetics was monitored by measuring the reduction in size of parent agglomerate with time. Silica dispersibility was greatly enhanced upon surface treatment. The coupling agent used in surface treatment is known to reduce filler—filler interactions and therefore affects the intrinsic cohesivity of the powder. However powder surface treatment can also induce changes in agglomerate morphology and filler-liquid interactions. The work presented in this paper evidences such changes.