Colloidal microstructure effects on particle sedimentation in yield stress fluids

Yield stress fluids are widely used in industry, deeply studied as an example of soft matter, and easy to conceptually describe: A solid-like material that can be yielded and made to flow by applying a minimum stress but will re-solidify once the applied stress is removed. Similarly, a particle will be stably suspended against sedimentation by a yield stress fluid if the stress it exerts on the fluid does not exceed the yield stress. In this article, we examine the current approach to predicting particle suspension in a yield stress fluid. We focus on a key cause of variability in both the fluid yield stress and propagation of particle stress: The fluid microstructure. We measure the prevention of particle sedimentation by examples of the two key microstructures used to create a yield stress suspension: A colloidal glass, Carbopol, which forms high volume fraction elastic structures by crowding, and a colloidal gel, microfibrous cellulose or MFC, which forms a sparser low volume fraction elastic network by inter-particle attachments. Comparing the sedimentation behavior of a single sphere in Carbopol and in MFC indicates that fluids with the same yield stress value can differ by a factor of 6 in their stability against particle sedimentation as a result of microstructure differences. Such suspensions cannot be characterized by yield stress alone, so the different fluids' yielding, and possibly recovery, from applied stress must also be studied. The work points to methods of improved design of microstructured fluids in a range of formulated product applications and also links shared goals of the rheology and microrheology communities.