(379b) Particle Sedimentation in Yield Stress Fluids With Different Microstructures

Yield stress fluids are widely used in practical applications and deeply studied by the soft matter community.  Conceptually, a yield stress fluid is easy to describe:  A solid that can be transformed into a liquid by applying a minimum stress on it and will re-solidify once the applied stress is removed.  Yield stress is useful for stably suspending particles against sedimentation if the fluid yield stress is greater than the particles’ stress.  Current methods to predict particle sedimentation are based on the calculation of a single dimensionless yield stress parameter that compares this stress ratio.  However, wide variations have been observed in the critical yield stress parameter.  We begin to explain these variations by examining the fluid microstructure.  Based on the microstructure at the origin of the yield stress, we propose a distinction between yield stress provided through colloidal glasses and yield stress provided through colloidal gels.  We compare the sedimentation behavior of a single sphere in commonly-studied Carbopol, a colloidal glass, and in microfibrous cellulose, a colloidal gel, and show that fluids with the same yield stress can differ widely in their ability to suspend a particle.  Therefore, in addition to yield stress, fluid microstructure must be characterized in order to better predict sedimentation in these fluids.