Wall Effects in Shear-Flowing Suspensions

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In many industrial processes the flow behaviour of liquids plays an important role. The macroscopic flow properties as volume flux and pressure loss are determined by the local structure of the flowing substance and production efficiency, optimization processes, security, ecological aspects or quality management might be affected extremely. Within the production of many materials suspensions are involved. The filler material affects the properties of the material that for instance may be paints and varnish, ceramics, pastes and slurries, paper, certain foodstuffs or cosmetics. Also non-industrial fields as medical science (blood circuits) and phenomena in nature like dunes formed by sedimentation are connected with the streaming of a particle-laden fluid. In the shear flow of a suspension hydrodynamic particle-particle interactions lead to stochastic particle movements and the so-called hydrodynamic diffusion takes place. A demixing might be generated if a systematic particle migration orthogonal to the flow direction is induced by gradients of the interaction frequency and of the resistance force to transverse particle movement. These phenomena were investigated experimentally and analyzed theoretically (for instance by Happel and Brenner ¹²). A gradient of resistance also affects the particle drift. Such a gradient originates in the vicinity of solid boundary walls or a viscosity gradient. Near the wall the resistance coefficient regarding particle movements orthogonal to the wall is increased. An enhanced relative viscosity coming along for instance with an increased particle concentration also leads to an increased resistance coefficient. A laminar tube flow of a suspension at steady state develops a boundary layer at the wall with a low particle concentration leading to the phenomenon of pseudo wall slip resulting in a decreased pressure difference related to a constant particle flux. Due to inhomogenity the local shear rates, velocities and particle concentrations vary causing differing residence times in the tube and different degrees of mechanical strain.