The dynamics of colloidal particles suspended in a second-order fluid

Abstract

The transient flow response of colloidal particles in sheared suspensions has been examined with linear dichroism measurements. Data are reported for dilute suspensions of spheroidal haematite (α-Fe₂O₃) particles in both a Newtonian and a non-Newtonian Boger fluid consisting of 1000 ppm poly-isobutylene in low-molecular-weight polybutene. Haematite samples with average axis ratios of 2.66, 3.61 and 5.82 were used in the studies. Flow reversal and flow start-up experiments were conducted for shear rates up to 10 s^–1 using both Couette and parallel-plate flow devices. The experimental data for suspensions in the non-Newtonian fluid are in qualitative agreement with predictions from the theory for particle motion in a second-order fluid. The particles experienced orbit drift and tended toward alignment along the vorticity axis of the flow. Also the period of rotation of the particles relative to that in the Newtonian suspending fluid increased with increasing shear rate. The predictions do not explicitly account for particle geometry, so experimental trends with axis ratio cannot be compared directly with the theory. In addition, Brownian motion is not considered by the theory, and we expect that the presence of Brownian motion in our suspensions strongly contributed to the steady-state distribution of particle orientation.