Shear-induced migration, of particle

D. Leighton and A. Acrivos, The shear-induced migration of particles in concentrated suspensions.. Fluid Mech., 181 (1987) 415-439. 

Leighton, D. Acrivos, A. Viscous resuspension. Chem. Eng. Sci. 1986, 41 (6), 1377-1384. Leighton, D. Acrivos, A. Measurement of shear-induced self-diffusion in concentrated suspensions of spheres. J. Fluid Mech. 1987, 177, 109-131. Leighton, D. Acrivos, A. The shear-induced migration of particles in concentrated suspensions. J. Fluid Mech. 1987, 181, 415-439. Altobelli, S.A. Givler, R.C. Fukushima, E. Velocity and concentration measurements of... 

Jana, S.C. (2003) Loss of surface and volume electrical conductivities in polymer compoimds due to shear induced migration of conductive particles. Polym. Eng. Sci., 43, 570. 

A particle migration model was proposed by Gadala-Maria and Acrivos to describe experimental shear-induced migration observations. This model allows for a better understanding of the shear effects on particle diffusion for concentrated suspensions. Based on these studies, a conservation equation for the solid phase was established by Phillips, Amstrong, and Brown, which takes into account convective transport, diffusion due to particle-particle interactions, and the variation of viscosity within the suspension, namely ... 

Finally, shear viscosity is strongly affected by the clay in the blends, especially at high PEN contents. A lubricating effect rather than a filler effect reveals the possibility that the clay is not well dispersed in the polymer blend, and migration of particles in the flow to the wall region can explain the observed reduction in shear viscosity. When MMT clay is mixed with crystallizable polymers such as polyesters, some processing problems arise because the crystallization process is modifled by nucleation effects induced by the nanoparticles. Moreover, these particles also influence the kinetics of transesteriflcation between PET and PEN, besides other factors such as the reaction time and extruder processing temperature. In Reference 72, a quaternary alkyl ammonium compound (Cl8) and MAH were used to modify the surface properties of the clay... 

These surface vortices provide an efficient means for microfiuidic mixing, as shown in Fig. 10, where a dye is rapidly mixed within several seconds. The mixing can be enhanced by inducing the vortex instabilities wherein turbulent-like mixing efficiencies are observed [18]. In addition, particles dispersed in the flow are also observed to be drawn into the vortices due to positive dielectrophoresis toward a point on the interface closest to the needle where the field is most intense. Once a sufficient particle concentration is achieved within the vortex, shear-induced migration leads to crossstreamline transport such that the interior of the vortex is populated [18], as shown in Fig. 11a. 

A study using confocal Raman spectrometry was carried out to determine the concentration profile within the extrudate of rubbery particles in a polyethylene matrix during capillary flow (Chartier et al. 2010). Chartier et al. reported that the effect of the concentration of particles on the apparent viscosity of polymer melts measured using capillary flow was the opposite of that based on observations made using linear dynamic viscosity measurements (Fig. 7.41). Shear-induced migration can be detected from the concentration profile of the components of the... 

The effect of ac, rather than dc, fields on the steady shear viscosity has also been investigated. An ac field might be desirable as a means of periodically reversing, and thereby minimizing, electrically induced migration or chemical reactions. High-frequency ac fields are also required if one is to avoid a crossover to a conduction-dominated regime in which the effective polarizability is controlled by conductivities rather than polarizabilities of the particles and medium (see Section 8.2.1.1). Klass and Martinek (1967) found that at a fixed... 

Although this picture is remarkably generic, the mechanisms responsible for the formation of a particle-lean layer adjacent to the wall depend on the properties of the material under consideration. For the case of solid particle dispersions, wall depletion, particle migration, and solid-liquid separation are the most frequent sources of solvent layer lubrication. Wall depletion occurs whenever dispersions are brought into contact with smooth and solid surfaces because the suspended particles cannot penetrate rigid boundaries [147]. Particle migration is due to various forces arising from fluid inertia, fluid elasticity, and shear-induced diffusivity effects [165]. Solid-liquid separation, which frequently occurs in flocculated suspensions like... 

For l-/im particles, the inertial force is quite small relative to the drag force, and rapid formation of a particle or gel layer is predicted. However, the particle layer does not grow steadily until it plugs the channel but seems to reach a steady-state thickness. One explanation is that the high shear rate near the wall causes a tumbling motion of individual particles, which expands the layer and leads to migration away from the wall. This shear-induced dispersion and the particle movement toward regions of lower concentration can be modeled with a particle diffusivity, which is proportional to the shear rate and the square of the particle size. More work is needed to understand the effects of particle shape and surface characteristics. 

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