Understanding suspension mechanics is crucial to the design and characterization of new and developing technologies since particle-laden materials occur in a myriad of fluid systems, e.g. particle-reinforced plastics, colloidal materials for chemical and biological detection, disease identification and drug delivery, cellular suspensions, emulsions, and building materials. Quantitative understanding of common suspension flows is complicated by the coupling of the particle concentration distribution to the rheology of the material. In particular, typical suspensions are non-Newtonian as they exhibit dependence of the effective viscosity and normal stress differences on both the local concentration of particles and the shear or extension rate. Furthermore, the particle distribution in a flow typically evolves in directions both parallel and transverse to the mean flow direction due to the phenomenon of shear-induced migration. This feature of the transport process affects both the dispersion of a plug of particles, and the average speed of motion of the suspension. We have recently made progress towards characterizing theoretically axial dispersion in a dilute suspension, where the fluctuations responsible for the dispersion occur as a consequence of shear-induced migration/diffusion of the particles. Most significantly, for typical flow conditions, the effective axial dispersion of a bolus of particles is reduced when shear-induced diffusion effects are significant. In addition, the effective axial dispersivity scales linearly with the average flow speed and as the third power of the pipe or channel radius, both features differ from the traditional Brownian motion-driven axial dispersion. The focus of the proposal centers on three main problems: (i) the generalization of axial dispersion ideas for the case where axial spreading of a monodisperse suspension is produced by fluctuations that are driven by shear-induced migration/diffusion, (ii) analyzing the influence of shear-induced diffusion on transport and particle separation in bidisperse suspension flows, and (iii) analyzing the axial dispersion process for channels with non-uniform cross-sectional shapes as is typical of many microdevices. In each case it is necessary to couple the distribution of particles to the transport properties (effective diffusivity, effective viscosity, normal stress differences, etc.). Experiments will complement the modeling. The results will be applicable to new applications involving suspensions or detection methods whereby a bolus of specially designed particles is injected into a flow.
Many common materials are suspensions, which refers to solid particles (or other kinds of particulates) distributed in a liquid. These materials are used industrially and are in common uses in households, food products, pharmaceuticals, etc. Often it is necessary to blend additional particles into a suspension, either to modify the properties of the suspension (e.g. how easily the material flows), or to add a functionality to the mixture (e.g. adding a drug), or to add objects that may detect harmful particles in the flow (e.g. small sensors that might identify or capture pathogenic cells). In these cases it is necessary to understand quantitatively and qualitatively how this mixing takes places, and how the additive is redistributed in the suspension. Although there are sophisticated theoretical and numerical tools for such mixing of molecular solutes, there is much less known about the spreading and distribution of macroscopic particles. We will address these questions using theory, modeling and experiments, and develop quantitative guidelines for understanding these dispersive, i.e. spreading, processes.
