Studies of Suspension Behavior: I. Instabilities of Non-Brownian Suspensions. II. Microrheology of Colloidal Suspensions

colloids;instability;microrheology;suspensions

Carpen, Ileana Cristina ; Brady, John F.

University:California Institute of Technology

Department:Chemistry and Chemical Engineering

关键词: colloids;    instability;    microrheology;    suspensions;   

Others  :  https://thesis.library.caltech.edu/2388/1/thesis.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Complex fluids are present in a multitude of forms: polymers, foods, paints, inks, biological materials, pharmaceuticals, cosmetics, etc.Many of these are suspensions, which have a particulate phase suspended in a solvent phase.This multiphase character gives a rich variety of behaviors, making suspensions interesting and useful materials but difficult to process.We investigate two different aspects of suspension behavior: instabilities in suspension flows and the use of microrheology in colloidal suspensions.

We look at two different mechanisms that generate instabilities and pattern formation in suspension flows.In the first, a jump in normal stresses at the interface between two fluids may lead to growing perturbations of the interface that ultimately give rise to migration of the particle phase into enriched regions.Fluids with a negative second normal stress difference, such as suspensions, can be unstable with respect to transverse or spanwise perturbations. The mechanism appears to be generic, although the details will depend on the specific system. The second mechanism may affect suspensions whose particle phase is not density-matched to the fluid.In this case, a flow can be unstable to spanwise perturbations of the particle phase when the shearing motion generates a density profile that increases with height.This is a Rayleigh--Taylor-like instability, due to having heavier material over light.As with the first instability, this mechanism may play an important role in pattern formation in multiphase flows.

The second aspect of suspension behavior we examine is the application of microrheology to colloidal suspensions.Microrheology has great promise for the study of soft, heterogeneous materials, but is not as well understood as traditional rheology. Most methods use tracer particles to investigate a medium, sometimes passively---tracking random motion (well established but restricted to the linear viscoelastic regime)---and more rarely actively---applying an external force to drive the tracers and access the medium's nonlinear response. Active microrheology is not well understood, and we study it by simulating a prototypical example, the motion of a particle due to an imposed force through a colloidal suspension. The deformation of the microstructure results in resistance to the tracer’s motion.This system displays 'force-thinning', analogous to the 'shear-thinning' in a macrorheologically sheared suspension, but the comparison is not exact, and care needs to be taken in the use and application of microrheological results. Comparable length scales between the measurement device (the tracer) and the medium lead to interesting effects and distinctions between types of microrheological methods.