Dense suspension rheology and flow phenomena

Rheology of concentrated flows of colloidal and non-colloidal suspensions is probed using simulation and experimental techniques. When subjected to shear, a concentrated suspension exhibits a rich variety of behaviors. The striking behaviors vary from shear induced ordering to shear thickening and hydrodynamic instabilities depending on the particle concentration and flow conditions. A range of these behaviors is studied in terms of microscopic and macroscopic response of the system.;The computational studies use Accelerated Stokesian Dynamics (ASD) simulation technique to investigate colloidal suspension flow at low particle Reynolds number. The first study explores shear-induced ordering in colloidal suspensions. The simulations are performed for particle volume fractions 0.47 ≤ &phis; ≤ 0.57 at Peclet numbers of 1 ≤ Pe = 6pieta g&d2; a3/kT ≤ 10 4 where eta is the suspending fluid viscosity, g&d2; is the imposed shear rate, a is the sphere radius and kT is the thermal energy. At Pe ≥ 10, when particle volume fraction is above &phis; ≈ 0.50, the suspensions undergo ordering over extended periods at the onset of flow, with remarkable reduction in the shear viscosity and self-diffusivity. The thixotropic response is a result of microstructural ordering, which is characterized by the real space pair distribution function and its Fourier transform, the static structure factor; both show that the particles tend to flow in chains with hexagonal packing in the plane normal to the flow. An order parameter is formulated to quantitatively describe the strength of this hexagonal packing.;The second computational study compares the microstructural anisotropy observed in sheared suspensions simulated by the ASD technique (&phis; < 0.50) to that observed in an experimental study of pressure-driven flow of Brownian suspensions through a micro-channel. In the experiments, three-dimensional particle locations are obtained via confocal microscopy. The features of the pair distribution function obtained experimentally show excellent qualitative match with that obtained from the simulations.;The third simulation study probes shear thickening (or jamming) in Brownian suspensions based on a 'motion correlation' approach. The correlations for the velocity-gradient (y) direction velocities of the particle pairs are studied for ASD simulated suspensions with 0.05 ≤ &phis; ≤ 0.47 at various Peclet numbers. The pair motion correlations show strong dependence on &phis; and Pe, and this novel approach captures the long-range structures at microscopic level which could be associated with the shear thickening phenomenon.;The experimental work investigates gravity-driven flow of concentrated suspensions (&phis; > 0.50) of non-Brownian spherical particles through a channel contraction at low Reynolds number. The abrupt change in the flow area at the contraction forms distinct shear-rate regions having different fluid pressures, which are related to the concept of particle pressure. A model involving particle pressure variation coupled to a Darcy-like behavior for the fluid captures the phenomenon of 'self-filtration', in which the effluent material has lower solid fraction than the input suspension.;In the above experimental set-up, when an external load is added on the suspension, the flow transforms from the smooth motion to a periodically alternating fast and slow motion for &phis; ≈ &phis;c ≥ 0.55. This remarkable alternating motion is suggestive of conversion from a liquid suspension into a thickened 'solid-like' system. The periodic flow behavior is found to be robust, occurring for a range of imposed driving pressure level, particle size and viscosity of solvent. The 'self-filtration' is found to be retained in the periodic flow conditions. The coupling of periodic flow behavior to the system's pressure response is investigated.