[CCoE Notice] Dissertation Defense: Confined Flow of Attractive Colloidal Suspensions

[Grayson, Audrey A]

Rahul Pandey- PhD Defense

DATE: Friday, April 17, 2015

TIME: 12:30 pm

PLACE: Civil Engineering Conference Room-N137

DISSERTATION CHAIR: Dr. Jacinta Conrad

________________________________
TITLE:
Confined Flow of Attractive Colloidal Suspensions

Abstract
Attractive particulate fluids flowing through complex confined geometries are frequently used in technological applications, such as drilling and fracturing fluids in exploration and production services or colloidal inks in inkjet printing and three-dimensional printing (or rapid prototyping). While the flow properties of hard-sphere suspensions in micro-scale geometries have been studied extensively, the effects of interparticle attractions and particle size dispersity on the confined flow properties of particulate suspensions are not well understood. We used confocal microscopy, particle tracking, and bulk rheology to study the confined structure, dynamics, and flow properties of colloid-polymer mixtures, which serve as simple models of attractive particulate suspensions. We employed poly(methyl-methacrylate) spheres that were suspended in a refractive-index and density- matched solvent, and induced a controlled short-range depletion attraction between particles by adding non-absorbing linear polystyrene.
First, we investigated the effects of particle size dispersity on confinement-induced solidification of colloid-polymer mixtures. We formulated mixtures of polymer and bidispersed colloids with particle size ratio aS/aL ≈ 0.49 at a constant total volume fraction fT and measured the dynamics of the large particles as a function of the volume fraction of large particles. The dynamics of large particles became slower as the volume fraction of large particles r=fL/fT was decreased or the confinement thickness was decreased, indicating increasingly solid-like behavior. The slowest dynamics appeared at minimum confinement thickness and minimum volume fraction of large particles.
Second, we investigated the effects of variation in particle size dispersity r on the rheology and microstructure of mixtures of polymer and bidispersed colloids. Significant changes in rheology and microstructure were observed only at high concentration of large particles. By contrast, dense suspensions fT = 0.40 were strong gels at all concentration of large particles and exhibited only modest rheological and microstructural changes as volume percent of large particles was varied. These results suggested that the effect of particle size dispersity on the dynamics and flow properties of colloid-polymer mixtures are most pronounced near the gelation boundary.
Third, we investigated the effects of variation in interparticle attractions on the microchannel flow of colloid-polymer mixtures. In suspensions with weak interparticle attractions, the number density of particles increased downstream in the channel due to shear-induced migration and consolidation by compression. In suspensions with stronger interparticle attractions, an interconnected network of particles suppressed these mechanisms and prevented the increase in density downstream.
Finally, we quantified the effects of particle size dispersity on the confined flow properties of attractive bidispersed suspensions. As the concentration of large particles was decreased the flow became increasingly non Newtonian at all flow rates. The shear induced migration of large particles increased with increase in flow rate for monodispersed suspension of large particle and decreased when the concentration of large particles was decreased at all flow rates. These results show that particle size dispersity can be tuned to control and optimize the segregation due to shear migration of a species of particles during microchannel flow.
Together, our results indicate that the confined structure, dynamics, and flow properties of attractive colloidal suspensions can be controllably tuned from fluid-like to solid-like by varying the interparticle attractions and the particle size dispersity. Our results establish a platform for development of fundamental understanding for the rational design of suspensions with phase behavior and flow properties for technological applications.
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