Measurements of the viscosities of suspensions of oriented rods using falling ball rheometry

Using falling ball rheometry, we measured the viscosities of suspensions of oriented rods ( average aspect ratio = 19.83 ). The rods were neutrally buoyant in the suspending fluid and were sufficiently large so that Brownian and colloidal forces were negligible. The rods were oriented hydrodynamically by passing a fixture through the suspension which produced a local flow-that tended to align the particles along the axis of the glass column containing the suspension. Balls were dropped along the axis of the cylinder and the average terminal velocity was measured using an eddy current detector. We find that the apparent viscosity for a suspension of rods that are approximately aligned is less than that for a suspension having the same volume fraction of rods that are randomly oriented. For example, at a volume fraction of 0.05, the relative viscosity of an oriented suspension is 1.52, whereas a randomly oriented suspension has a relative viscosity of 2.37. In addition, we find that the viscosity measured for the aligned rods closely correlates with the viscosities of suspensions of short fibers (having similar aspect ratios and concentrations) measured in shearing flows.