Suspensions of a moderate particle volume fraction tend to demix in nonlinear shear. Although this effect has been studied extensively in simple flows to determine how suspension rheology results in migration, few studies have considered how migration occurs in more complicated flows. Migration in industrial processes has long complicated process development and has become increasingly important as researchers strive to process and analyze blood and other biological suspensions in complicated BioMEMS flows. The objectives of this research are to broaden our fundamental understanding of flowing suspensions by considering how the fundamental symmetries within flows interplay with the underlying suspension structure. In 1D, 2D and 3D microchannel flows commonly used in BioMEMS, the competition between suspension demixing via shear migration resulting from multibody hydrodynamic interactions and chaotic advection generated within microchannels designed to enhance mixing will be investigated. Direct measurement of flow and concentration profiles will be performed using high speed 3D confocal laser scanning microscopy (CLSM). This technique, coupled with a flow-stop-scan protocol, allows direct measurement of suspension structural anisotropy that generates the normal stresses that result in migration. These studies will be extended to examination of technologically relevant fluids, including polydisperse suspensions, electrostatic-stabilized suspensions, suspensions in viscoelastic media, and whole blood. Because flow, migration, and local structure is determined directly using CLSM, the effect of the addition of monosized suspension on cell migration and rouleau formation can be examined. In addition, continuum models will be used to explore the coupling between suspension migration and the underlying topology in chaotic flows.
The intellectual merits and transformative aspects of this study include the development of directly measuring suspension structure and demonstrating the coupling between chaos and segregation in flowing suspensions. The broader impacts of this research include a new paradigm of using chaotic advection as a template for self-organization in suspensions that could revolutionize suspension processing from the microscale to industrial-scale, a potential new platform for whole blood fractionation and detection, and integration of graduate, undergraduate, middle school hands-on and web-based learning of the broader field of particle technology. Specifically, an Image of the Week website that presents the results of this research and university-wide microscale and nanoscale research to a broader community will be developed. Likewise, in order to introduce the broader subject of particle technology to early scientists, collaboration with a local middle school to develop a hands-on learning module for exploring granular media will engage students from low social economic status and otherwise underrepresented minority groups within engineering and the sciences.
Suspensions of particles in viscous fluids are found in natural systems, consumer products, and utilized in many industrial processes. While we have a strong understanding how dilute suspensions behave, suspensions of high volume fraction solids are highly complex in their flow properties and rheology. Dense suspensions have long been known to have shear thinning and shear thickening viscosity with increase of shear rate and display time dependent behavior such has shear migration, or demixing, of particles where they concentrate in different regions of the flow. Research related to this grant directly visualized the 3D locations of fluorescent particles flowing in microchannels using high speed confocal microscopy. The precision of these measurements provided unprecedented experimental evidence of how microstructure relates to the rheology of these suspensions. Previous work by Brady and coworkers using Stokesian Dynamics simulations have shown that the microstructure of sheared suspensions changes dramatically when the hydrodynamic stress exceeds the stress resulting from the random, Brownian motion of the particles at equilibrium. Our experimental results enable quantitative calculation of the hydrodynamic stress resulting from the suspension microstructure and agree with previous predictions. The presence of electrostatic repulsion that gives non-hydrodynamic ‘soft’ interactions alters the microstructure generally agreeing with previous studies. These measurements go beyond what simulations currently can accurately predict by measuring microstructure near solid walls. The microstructure is significantly altered as much as 6 particle diameters away from smooth walls, an effect that is largely mediated by introducing rough walls. Because of this change in microstructure, the local suspension viscosity is also altered, potentially hindering the ability to measure viscosity in highly confining systems. Likewise, microstructure in complex flows that include a mixture of time-dependent shear and extensional profiles can be directly measured and suggest a complex interplay between the local fluid motion and the suspension structure. This analysis is useful for understanding suspensions flowing through porous materials and analytical bioMEMS devices. Likewise, with the interest of developing advanced materials with specific microstructure, these results point to new heuristics that can be used in designing processes that control microstructure by altering the flow profile. Because of the precision in these measurements, these results may serve as benchmarks for future simulation tools and understanding indirect methods of microstructure characterization such as light or neutron scattering.
