Shear-induced Aggregation Research Articles

GPVI-mediated thrombus stabilization of shear-induced platelet aggregates in a microfluidic stenosis

Supraphysiological shear rates (>2000 s−1) amplify von Willebrand factor unfurling and increase platelet activation and adhesion. These elevated shear rates and shear rate gradients also play a role in shear-induced platelet aggregation (SIPA). The primary objective of this study is to investigate the contributions of major binding receptors to platelet deposition and SIPA in a stenotic model. Microfluidic channels with stenotic contractions ranging from 0% to 75% are fabricated and coated with human type I collagen at 100 μg/mL. Fresh human blood is reconstituted to 40% HCT and treated to stain platelets. Platelet receptors αIIbβ3, GPIb, or GPVI are blocked with inhibitory antibodies or proteins to reduce platelet function under flow at 500, 1000, 5000, or 10,000 s−1 over 5 min of perfusion. Additional validation experiments are performed by dual-blocking receptors and performing coagulability testing by rotational thromboelastometry. Control samples exhibit SIPA correlating to increasing shear rate and increasing stenotic contraction. Inhibition of αIIbβ3 or GPIb receptors causes a nearly total reduction in platelet adhesion and a loss of aggregation at >1000 s−1. GPVI inhibition does not notably reduce platelet adhesion at 500 or 1000 s−1 but affects microthrombus stability at 5000–10,000 s−1 following aggregation formation in 50%–75% stenotic channels. Inhibition of von Willebrand factor-binding receptors completely blocks adhesion and aggregation at shear rates >1000 s−1. Inhibition of GPVI reduces platelet adhesion at 5000–10,000 s−1 but renders thrombi susceptible to fragmentation. This study yields further insight into mechanisms regulating rapid growth and stabilization of arterial thrombi at supraphysiological shear rates.

Monitoring effects of hydrodynamic cavitation pretreatment of sodium oleate on the aggregation of fine diaspore particles through small-angle laser scattering

Hydrodynamic cavitation (HC) enhanced fine particle aggregation could be largely due to the generation of tiny bubbles and their role in bridging particles. However, the lack of adequate characterizations of aggregates severally limits our further understanding of the associated aggregation behaviors. In this study, the aggregation of fine diaspore particles was comparatively investigated in sodium oleate (NaOl) solutions with and without HC pretreatment through the small-angle laser scattering (SALS) technique in a shear-induced aggregation (SIA) system. Results showed that HC pretreatment caused the formation of bulk nanobubbles (BNBs), which significantly modified the particle interactions and thereby modified the size and mass fractal dimension (Df) of aggregates under different SIA conditions. Although HC pretreatment did not noticeably alter the gradual change trend of aggregate size and structure characteristics under specific variables, BNBs bridging facilitated the aggregation process towards the diffusion-limited cluster aggregation model, resulting in the formation of larger but looser aggregates. This effect was more pronounced under relatively high NaOl concentrations. Apart from BNBs, the aggregation was also affected by cavitation bubbles formed during shear cavitation, which was more significant under high stirring intensity conditions (i.e., 1800 rpm) than the low stirring intensity conditions (i.e., 600 rpm).

Vortex-assisted electroosmotic mixing of Carreau fluid in a microchannel.

Pertaining to the mixing of the non-Newtonian Carreau fluid under electrokinetic actuation inside a plane microchannel, we propose a new design of micromixer that involves inserting a two-part cylinder bearing zeta potential of the same sign but different magnitude in the upstream and downstream directions. We numerically solve the transport equations to predict the underlying mixing characteristics. We demonstrate that a substantial momentum difference between the microchannel's plane wall and cylinder leads to the development of a vortex in the flow pathway, which in turn, enhances mixing substantially. As shown, for a fluid having a highly shear-thinning nature, the vortex-assisted convection mixing strength increases with diffusivity of the candidate fluids. Moreover, it is shown that for the higher shear-thinning nature of the candidate fluid, an increase in cylinder radius enhances mixing efficiency and flow rate simultaneously, resulting in a "quick and efficient" mixing condition. Additionally, the fluid rheology significantly alters the kinetics of shear-induced binary aggregation. Our findings show that the shear-induced aggregation characteristic time sharply increases with increasing shear-thinning behavior of the fluid.

Brownian dynamics simulations of shear-induced aggregation of charged colloidal particles in the presence of hydrodynamic interactions

HypothesisIn spite of the abundant literature on Brownian simulations of the aggregation behavior of colloidal suspensions both under quiescent conditions and in the presence of shear, few works performed simulations including the effect of hydrodynamic interactions. Even fewer works have investigated the effects of shear on the aggregation of electrostatically-stabilized colloidal suspensions. SimulationsIn this work, we employed Brownian dynamics simulations implementing the Rotne-Prager-Yamakawa approximation to account for hydrodynamic interactions and investigated the aggregation kinetics of electrostatically-stabilized colloidal suspensions exposed to simple shear, for various Péclet number values, particle volume fractions and surface potential values. ResultsThe increase in Péclet number (i.e., in the shear rate), leads to an overall increase in the aggregation rate and the formation of large aggregates that, for sufficiently high volume fractions, rapidly grow, leading to either breakup and restructuring phenomena or percolation of the system. In some cases, a bimodal distribution of the cluster population was observed. Our simulations further indicate that at the highest Péclet, the aggregation dynamics is independent of the energy barrier and entirely controlled by shear. A comparison with a simple BD method reveals that neglecting long-range hydrodynamic interactions leads to a substantial underestimation of the aggregation rate.

Shear-Induced Aggregation and Distribution in Photocatalysis Suspension System for Hydrogen Production

Shear-Induced Aggregation and Distribution in Photocatalysis Suspension System for Hydrogen Production

Physicochemical Properties of Milk Protein Concentrate Extrudates Generated Using Supercritical Carbon Dioxide

This study elucidated the comparative effects of steam extrusion (SX, 115 °C) and supercritical fluid extrusion (SCFX, 85 °C) on the physicochemical properties of extruded milk protein concentrate. SCFX with SC–CO2 injection at 2.5 (SC-2.5) and 5 (SC-5) wt % feed moisture temporarily lowered the pH of melt to 4.90 and 4.75, respectively, while the pH of 6.2 was maintained during SX. Acidic conditions during SCFX prevented temperature and shear-induced protein aggregation of calcium bridging and disulfide type. Compared to unextruded samples, the free sulfhydryl content was reduced by 12.6% in SC-5 and 75.9% in SX extrudates, indicating decreased sulfhydryl interactions in SCFX extrudates. The water-soluble protein fractions in SC-5 exhibited mean particle size and zeta potential of 166 nm and −28 mV, respectively, compared to 566 nm and −12.8 mV for SX. Thus, temporarily induced acidity and lower operating temperature of SCFX prevented protein aggregation and contributed to improving the physicochemical properties of milk protein extrudates.

Study of the Shear-Thinning Effect between Polymer Nanoparticle Surfaces during Shear-Induced Aggregation

In this research, we studied the impact of material fusion as an adhesion mechanism on the size and structure of fractal aggregates formed during shear aggregation of fully destabilized polymer nanoparticles (NPs). The nanoparticles have a core–shell structure, where the core is composed of poly(methyl methacrylate) (PMMA) and the shell consists of a combination of PMMA and polybutyl acrylate (PBA). Due to significantly different glass transition temperatures (Tg’s) of these polymers, the core acts as a hard sphere, while the presence of PBA in the shell gives the surface a soft character. By varying the system temperature, material fusion is induced between the particles in contact. The strength of the formed physical bond is tested under various shear rate conditions. It was found that the increase in temperature leads to an increase in aggregate size, caused by an increase in adhesion between NP surfaces. This phenomenon occurs due to a material softening of the polymer shell triggered by the increase in temperature, resulting in the formation of a viscous sticky surface. Additionally, it was observed that at temperatures above the Tg of the polymer composing the shell, the increase in the shear rate causes a reduction of the interparticle contact strength suggesting a shear-thinning effect during contact. The interplay between these two contradicting mechanisms determines the final mechanical properties of produced material.

Effects of pressurized thermal processing on native proteins of raw skim milk and its concentrate

Heating, pressurization, and shearing can modify native milk proteins. The effects of pressurized heating (0.5 vs. 10 MPa at 75 or 95°C) with shearing (1,000 s-1) on proteins of raw bovine skim milk (SM, ∼9% total solids) and concentrated raw skim milk (CSM, ∼22% total solids) was investigated. The effects of evaporative concentration at 55°C and pressurized shearing (10 MPa, 1,000 s-1) at 20°C were also examined. Evaporative concentration of SM resulted in destabilization of casein micelles and dissociation of αS1- and β-casein, rendering CSM prone to further reactions. Treatment at 10 MPa and 1,000 s-1 at 20°C caused substantial dissociation of αS1- and β-casein in SM and CSM, with some dissociated caseins forming shear-induced soluble aggregates in CSM. The pressure applied at 10 MPa induced compression of the micelles and their dissociation in SM and CSM at 75 or 95°C, resulting in reduction of the micelle size. However, 10 MPa did not alter the mineral balance or whey proteins denaturation largely, except by reduction of some β-sheets and α-helices, due to heat-induced conformational changes at 75 and 95°C.

Shear-Induced Amyloid Aggregation in the Brain: V. Are Alzheimer's and Other Amyloid Diseases Initiated in the Lower Brain and Brainstem by Cerebrospinal Fluid Flow Stresses?

Amyloid-β (Aβ) and tau oligomers have been identified as neurotoxic agents responsible for causing Alzheimer’s disease (AD). Clinical trials using Aβ and tau as targets have failed, giving rise to calls for new research approaches to combat AD. This paper provides such an approach. Most basic AD research has involved quiescent Aβ and tau solutions. However, studies involving laminar and extensional flow of proteins have demonstrated that mechanical agitation of proteins induces or accelerates protein aggregation. Recent MRI brain studies have revealed high energy, chaotic motion of cerebrospinal fluid (CSF) in lower brain and brainstem regions. These and studies showing CSF flow within the brain have shown that there are two energetic hot spots. These are within the third and fourth brain ventricles and in the neighborhood of the circle of Willis blood vessel region. These two regions are also the same locations as those of the earliest Aβ and tau AD pathology. In this paper, it is proposed that cardiac systolic pulse waves that emanate from the major brain arteries in the lower brain and brainstem regions and whose pulse waves drive CSF flows within the brain are responsible for initiating AD and possibly other amyloid diseases. It is further proposed that the triggering of these diseases comes about because of the strengthening of systolic pulses due to major artery hardening that generates intense CSF extensional flow stress. Such stress provides the activation energy needed to induce conformational changes of both Aβ and tau within the lower brain and brainstem region, producing unique neurotoxic oligomer molecule conformations that induce AD.

Purification of Cas9-RNA complexes by ultrafiltration.

The discovery of CRISPR-Cas9 has revolutionized molecular biology, greatly accelerating the introduction of genetic modifications into organisms and facilitating the development of novel therapeutics and diagnostics. For many applications, guide RNA and Cas9 protein are expressed, combined, and purified to produce a ribonucleic enzyme complex that is then added into a diagnostic device or delivered into cells. The objective of this work was to develop an ultrafiltration process for the selective purification of Cas9 ribonucleoprotein by removal of excess guide RNA. A His-tagged Streptococcus pyogenes Cas9 protein was produced in Escherichia coli, purified by metal affinity chromatography, and complexed with a 40 kDa (124 nucleotide) single guide RNA. Ultrafiltration experiments were first performed on solutions containing either guide RNA or Cas9 protein to identify the effect of filtration conditions and membrane pore size on the selectivity. Shear-induced aggregation of the Cas9 led to significant fouling under some conditions. A diafiltration process was then developed using a Biomax® 300 kDa polyethersulfone membrane to selectively remove excess guide RNA from a solution containing Cas9-bound guide RNA and free guide RNA. These results demonstrate the potential of using ultrafiltration for the removal of excess RNA during the production of functional ribonucleoprotein complexes.

New Generalized Viscosity Model for Non-Colloidal Suspensions and Emulsions

The viscous behavior of solids-in-liquid suspensions and liquid-in-liquid emulsions of non-Brownian solid particles and liquid droplets dispersed in Newtonian liquids is thoroughly discussed and reviewed. The full concentration range of the dispersed particles/droplets is covered, that is, 0<ϕ<ϕm, where ϕ is the volume fraction of inclusions (particles or droplets) and ϕm is the maximum packing volume fraction of inclusions. The existing viscosity models for suspensions and emulsions are evaluated using a large pool of experimental viscosity data on suspensions and emulsions. A new generalized model for the viscosity of suspensions and emulsions is proposed and evaluated. The model takes into consideration the influence of shear-induced aggregation of particles and droplets. It also includes the effect of the droplet-to-matrix viscosity ratio λ on the viscosity of emulsions. In the limit of high ratio of droplet viscosity to matrix viscosity (λ→∞), the model reduces to the suspension viscosity model. The proposed model uncovers some important and novel characteristics of suspension systems rarely discussed heretofore in the literature. The model is validated using twenty sets of experimental viscosity data on solids-in-liquid suspensions and twenty-three sets of experimental viscosity data on liquid-in-liquid emulsions.

Shear-Induced Heteroaggregation of Oppositely Charged Colloidal Particles.

This paper investigates numerically the shear-induced aggregation of mixed populations of colloidal particles leading to the formation of clusters. Suspensions with different amounts of positively and negatively charged colloidal particles are simulated. To resolve the aggregation kinetics and structural properties of the formed clusters, we resort to a mixed deterministic-stochastic simulation method. The method is built on a combination of a Monte Carlo algorithm to sample a statistically expected sequence of encounter events between the suspended particles and a discrete element method built in the framework of Stokesian dynamics to simulate the encounters in a fully predictive manner. Results reveal a strong influence of the composition of the population on both the aggregation kinetics and the aggregate structure. In particular, we observe a size-stabilization phenomenon taking place in the suspension when the relative concentration of the majority particles lies in the range 80–85%; i.e., starting from primary particles, after a short growth period, we observed a cessation of aggregation. Inspection of the aggregate morphology shows that the formed clusters are composed of few minority particles placed in the inner region, while the aggregate surface is covered by majority particles, acting to provide a shielding effect against further growth.

Shear-induced aggregation of amyloid β (1–40) in a parallel plate geometry

Protein aggregation is induced by various environmental or external factors and associated with various neurodegenerative diseases. Among various external factors, shear stress is inevitable for both in vivo and in vitro applications of proteins. In this study, Aβ (1–40) peptide, a derivative of the amyloid precursor protein, was subjected to constant (300, 500, 700 s−1) and varying (ramp) shear in a parallel plate geometry to explore the implications of shear in terms of macro (viscosity) and micro (secondary structure, morphology) characteristics. Aβ (1–40) solution followed a shear thickening flow behaviour with performance index value ‘n’ of 2.12. The fibrillation process resulting from the shear force was evaluated in terms of dissipation energy, which was found to exceed the free energy of unfolding. This resulted in the formation of β-sheet rich structures, which were confirmed by CD and FTIR analyses and enhanced Th-T fluorescence. The apparent rate of aggregation () was found to increase with the shear rate, and inversely related to the solution viscosity. The maximum value was 0.21 ± 0.3 min−1 at 700 s−1. The molecular weights of aggregates were determined using gel filtration, which were proportionally related to the solution viscosity. The average molecular weights were estimated to be 70, 62 and 52 KDa for samples sheared at 300, 500 and 700 s−1, respectively. The present study has deciphered the interplay of viscosity, a fluid property, with the aggregation process and its corresponding change in the secondary structures of the peptide. These findings provide useful insights for understanding various proteopathies under shear force. Communicated by Ramaswamy H. Sarma

Temperature modulated polymer nanoparticle bonding: A numerical and experimental study

In this research, we investigated the impact of nanoparticle adhesive properties on the size of micro-clusters formed during shear-induced aggregation at different temperatures. To precisely control particle adhesion, we used nanoparticles with a core-shell structure, where the core is composed of polymethyl methacrylate and the shell is composed of a combination of polymethyl methacrylate and polybutylacrylate. Due to significantly different glass transition temperature (Tg) of these polymers, the core act as a hard-sphere, while the presence of polybutylacrylate in the shell, with a glass transition temperature of 50 °C, gives the surface mechanical softness upon increasing temperature. We observed that the size of the aggregates grow significantly when the temperature rises above Tg, indicating an increase of adhesive force between the nanoparticles. Under these conditions, the surface of the nanoparticles exhibits a transition from plastic to viscous behavior that allows core-shell nanoparticles to bond physically upon contact in a controlled coalescence effect. To further investigate the micro-mechanical behavior of the micro-clusters during aggregation, a numerical study of a simple shear flow setup using CFD-DEM with a customized particle interaction model was carried out. This model has the capability to describe non-contact as well as contact forces present in colloidal systems. Depending on the system temperature, the model can simulate either elastic, elastic-plastic or viscoplastic deformation between the interacting nanoparticles. Using this feature, it is demonstrated that it is possible to reproduce the experimentally observed growth in aggregates with temperature rise by simulating an increase in adhesion using primary particle mechanical parameters. Furthermore, these results clearly demonstrate the direct relation between surface properties of the nanoparticles with the macroscopic behavior of the colloidal system.

Application of population balance-based thixotropic model to human blood

Modeling blood rheology remains challenging in part because of its multiphase, aggregating colloidal nature that gives rise to complex viscoplastic and time-dependent (thixotropic) behavior. Here, we demonstrate that a multiscale approach incorporating a direct coupling of coarse-graining particle-level modeling to the macroscopic phenomenological modeling can provide new insights and a promising methodology. Specifically, a general population balance-based, multiscale, thixotropic modeling approach, first proposed by Mwasame et al., AIChE J. 63 (2017) 517–531, is applied to account for the rouleaux-induced thixotropy in human blood in shear flow. Population balances offer a compelling alternative to previously proposed structure-based heuristic kinetics models of aggregating colloidal suspensions as they use a statistical approach to describe the aggregate size distribution with well-defined processes for either shear-induced or Brownian aggregation and breakup under shear flow. When applied to human blood, the population balance approach offers a first attempt to model the size evolution of predominantly coin-stack like rouleaux structures of the red blood cells that are the primary source behind the observed yield stress and thixotropy of blood at low shear rates. This microscopic information, suitably coarse-grained, is then introduced into a semi-phenomenological macroscopic model that expresses the total stress in terms of an elastic and viscous contribution. Shear thinning introduced due to the red blood cell deformation at high shear rates is accounted for by following Horner et al., J. Rheol. 62 (2018) 577–591. An advantage of this modeling approach is that the parameters have specific physical meaning that allows for independent estimates and/or evaluations through appropriately designed independent experiments. Conversely, parameters with specific microscopic interpretations, such as the fractal dimension of the aggregates, df, are obtained fits of macroscopic shear experiments. Fitting and predictions use steady shear, and unidirectional large amplitude oscillatory shear (UD-LAOS) experiments on whole blood samples of two healthy donors, as reported in Horner et al. We obtain values for df in the range of 1.5 ± 0.2 which is consistent with the rod-like shape of rouleaux structures reported in the literature. Furthermore, the shear predictions compare favorably against the experiments. While this approach is not as accurate as the fits of prior structure kinetics modeling of Horner et al., these promising results provide a pathway for model improvement by including independently verified physical properties of blood. This work demonstrates a new particle-level approach for describing and predicting the non-Newtonian, thixotropic rheology of human blood.

Morphology of Shear-Induced Colloidal Aggregates in Porous Media: Consequences for Transport, Deposition, and Re-entrainment.

Colloid deposition in granular media is relevant to numerous environmental problems. Classic filtration models assume a homogeneous pore space and largely ignore colloid aggregation. However, substantial evidence exists on the ubiquity of aggregation within porous media, suggesting that deposition is enhanced by it. This work studies the deposition process in relation to aggregate size and structure. We demonstrate that aggregation is induced at typical groundwater velocities by comparing the repulsive DLVO force between particle pairs to the hydrodynamic shear force opposing it. Column experiments imaged with high-resolution X-ray computed tomography are used to measure aggregate structure and describe their morphology probability distribution and spatial distribution. Aggregate volume and surface area are found to be power-law distributed, while Feret diameter is exponentially distributed with some flow rate dependencies caused by erosion and restructuring by the fluid shear. Furthermore, size and shape of aggregates are heterogeneous in depth, where a small number of large aggregates control the concentration versus depth profile shape. The range of aggregate fractal dimensions found (2.22-2.42) implies a high potential for restructuring or breaking during transport. Shear-induced aggregation is not currently considered in macroscopic models for particle filtration, yet is critical to consider in the processes that control deposition.

Antithrombotic Effects of Paeoniflorin from Paeonia suffruticosa by Selective Inhibition on Shear Stress-Induced Platelet Aggregation.

Antiplatelet agents are important in the pharmacotherapeutic regime for many cardiovascular diseases, including thrombotic disorders. However, bleeding, the most serious adverse effect associated with current antiplatelet therapy, has led to many efforts to discover novel anti-platelet drugs without bleeding issues. Of note, shear stress-induced platelet aggregation (SIPA) is a promising target to overcome bleeding since SIPA happens only in pathological conditions. Accordingly, this study was carried out to discover antiplatelet agents selectively targeting SIPA. By screening various herbal extracts, Paeonia suffruticosa and its major bioactive constituent, paeoniflorin, were identified to have significant inhibitory effects against shear-induced aggregation in human platelets. The effects of paeoniflorin on intraplatelet calcium levels, platelet degranulation, and integrin activation in high shear stress conditions were evaluated by a range of in vitro experiments using human platelets. The inhibitory effect of paeoniflorin was determined to be highly selective against SIPA, through modulating von Willebrand Factor (vWF)-platelet glycoprotein Ib (GP Ib) interaction. The effects of paeoniflorin on platelet functions under high shear stress were confirmed in the ex vivo SIPA models in rats, showing the good accordance with the anti-SIPA effects on human platelets. Treatment with paeoniflorin significantly prevented arterial thrombosis in vivo from the dose of 10 mg/kg without prolonging bleeding time or blood clotting time in rats. Collectively, our results demonstrated that paeoniflorin can be a novel anti-platelet agent selectively targeting SIPA with an improved safety profile.

Shear-induced amyloid formation of IDPs in the brain.

The IDP amyloid β-protein (Aβ) has been both the prime causative suspect and drug development target in the fight against Alzheimer's disease (AD). Unfortunately, all clinical trials against Aβ based on this assumption have failed. This review proposes that the Aβ IDP conformation ensembles in the research laboratory may be somewhat different than those of Aβ found within the human brain. It is argued that the multiple quiescent Aβ molecular conformations may be the incorrect drug targets. Instead there may be much more complex ensembles found in the flowing liquids within the brain. The highly flexible Aβ is quite sensitive to shear-induced aggregation. Different shear types and shear energies must be generated by Aβ-containing fluids flowing through different constricted brain spaces at different velocities. A model is proposed in which Aβ subjected to shear-induced aggregation produces toxic Aβ oligomers that are different from those produced in the laboratory. It is proposed that amyloid researchers seeking Alzheimer drug candidates should perform experiments under shear conditions that attempt to mimic those found in the brain. Because Aβ experiments in devices with narrow bore capillaries are rather limited, it is imperative that further experiments of this type be carried out using spinal tap needles and microliter syringes. It is believed that analytical errors may be generated by plating out of amyloids on the inner wall surfaces of these capillary devices. It is suggested that shear-induced aggregation due to flow in confined brain fluid flow pathways may be responsible for many related amyloid diseases and brain trauma events.

Hydrodynamic dispersion and aggregation induced by shear in non-Brownian magnetic suspensions

In this paper, we examine a two-particle problem in order to study transport phenomena on magnetic suspensions such as the shear-induced hydrodynamic diffusion and the shear-induced aggregation in the regime of non-Brownian particles and creeping flow. New results are presented for the shear-induced hydrodynamic diffusivities and the rate of particle doublet formation, resulting from the diffusive and aggregative irreversible trajectories produced by particle magnetic interactions. The numerical computation of the particle shear-induced diffusivities and the rates of particle aggregation are performed by using a Monte Carlo integration scheme for different values of the relevant magnetic parameter α. This parameter represents the non-dimensional strength of the dipole-dipole magnetic interactions between the particles. For small values of α, the shear-induced self-diffusivity is remarkably described by a slight adaptation of the asymptotic theory for rough spherical particles interacting hydrodynamically [F. R. Cunha and E. J. Hinch, “Shear-induced dispersion in a dilute suspension of rough spheres,” J. Fluid Mech. 309, 211 (1996)]. We just replace the roughness parameter ϵ with α5/4, giving for the self-shear-induced diffusivity for small values of α, 0.156γ̇a2ϕα0.547log(1/α5/4)+1.347−0.701, where γ̇ is the applied shear rate, a is the radius of the magnetic spheres, and ϕ is the particle volume fraction. In addition, a root-square law dependence is obtained for the rate of particle aggregation as (1.830/π)ϕ1N1γ̇α1/2, where N1 and ϕ1 are, respectively, the number and the volume fraction of the isolated particles in the suspension. A comparison shows that the root-square law prediction is in excellent agreement with the results of the numerical simulations for all values of the parameter α investigated.

Shear-induced aggregation or disaggregation in edible oils: Models, computer simulation, and USAXS measurements

The effects of shear upon the aggregation of solid objects formed from solid triacylglycerols (TAGs) immersed in liquid TAG oils were modeled using Dissipative Particle Dynamics (DPD) and the predictions compared to experimental data using Ultra-Small Angle X-ray Scattering (USAXS). The solid components were represented by spheres interacting via attractive van der Waals forces and short range repulsive forces. A velocity was applied to the liquid particles nearest to the boundary, and Lees-Edwards boundary conditions were used to transmit this motion to non-boundary layers via dissipative interactions. The shear was created through the dissipative forces acting between liquid particles. Translational diffusion was simulated, and the Stokes-Einstein equation was used to relate DPD length and time scales to SI units for comparison with USAXS results. The SI values depended on how large the spherical particles were (250 nm vs. 25 nm). Aggregation was studied by (a) computing the Structure Function and (b) quantifying the number of pairs of solid spheres formed. Solid aggregation was found to be enhanced by low shear rates. As the shear rate was increased, a transition shear region was manifested in which aggregation was inhibited and shear banding was observed. Aggregation was inhibited, and eventually eliminated, by further increases in the shear rate. The magnitude of the transition region shear, γ̇t, depended on the size of the solid particles, which was confirmed experimentally.