Colloidal Suspensions

Magnetic resonance measurements of migration and irreversible dynamics in the capillary shear flow of a Brownian suspension were reported. The results demonstrate the presence of phenomena typically associated with concentrated non-colloidal systems and indicate the role of many body hydrodynamics in dilute Brownian suspension transport. The application of concepts from chaos theory and nonequilibrium statistical mechanics was demonstrated. The shear induced migration of colloidal particles in channel flow, which has a varying shear rate, has been investigated using Magnetic Resonance Microscopy. Magnetic Resonance methods have the ability to measure spatially resolved velocity and probability distributions of displacement, even within a multi-phase colloidal system. For a dilute suspension of ~2.5 µm Brownian model hard spheres under shear flow in a 1 mm diameter glass capillary, particle migration inwards to the capillary center was found using spectrally resolved Pulsed Gradient Spin Echo techniques. While particle migration has been measured and is expected in concentrated and noncolloidal suspensions, it has not been unequivocally detected in dilute Brownian suspensions, where two particle collisions are assumed the dominant interaction mechanism, and is therefore not considered in the modeling of flow systems.

The flow and distribution of Newtonian, polymeric and colloid suspension fluids at low Reynolds numbers in bifurcations has importance in a wide range of disciplines, including microvascular physiology and microfluidic devices. A bifurcation consisting of circular capillaries laser etched in a hard polymer with inlet diameter 2.50±0.01 mm, bifurcating to a small diameter outlet of 0.76±0.01 mm, and a large diameter outlet of 1.25±0.01 mm is examined. Four distinct fluids (water, 0.25% (w/w) xanthan gum, 8% and 22% (v/v) polydisperse hard-sphere colloidal suspensions) were flowed. Velocity in all three spatial directions was examined to determine the impact of secondary flows and characterize the transport in the bifurcation. The velocity data provides direct measurement of the volumetric distribution of the flow between the two channels as a function of flow rate. Water and the 8% colloidal suspension show a constant distribution (i.e. same proportion of flow into each channel) with increasing flow rate, the xanthan gum shows an increase in fluid going into the larger outlet with higher flow rate and the 22% colloidal suspension shows a decrease in fluid entering the larger channel with higher flow rate. For the colloidal particle flow the distribution of colloid particles down the capillary is also determined by examining the spectrally resolved propagator for the oil inside the core-shell particles in a direction perpendicular to the axial flow. Using dynamic Magnetic Resonance Microscopy the potential for using magnetic resonance for 'particle counting' in a microscale bifurcation is thus demonstrated.