Dense suspension rheology beyond flow curves*

Dense suspensions often undergo dramatic shear thickening, where the apparent viscosity can increase by an order of magnitude or more upon increasing stress or shear rate.  Rheological measurements of average flow curves (suspension viscosity as a function of applied stress or strain rate)  can be quantitatively described by mean field models attributing shear thickening to  a  transition from primarily hydrodynamic interactions at low stress to primarily frictional interactions at high stress, or more generally to a stress-induced increase in  constraints on relative particle motion. Our measurements of surface stresses with high spatial and temporal resolution, however, show a complexity that is not evident in flow curves and is not captured in mean field models [1-5].  We have observed similar behavior in Brownian monodisperse colloids [1,2], non-Brownian irregular particles (Cornstarch) [3], and colloidal rods [4].  We observe substantial concentration fluctuations associated with stress fluctuations [3-5],  including directly observable fluid migration[3]. In sheared colloidal rod suspensions, we find that  stress fluctuations are associated  with fluctuations in rod orientation and a dynamic order-disorder transition [4].  These results highlight the importance of moving beyond measurements of average properties and mean field models in order to develop a mechanistic understanding of shear thickening.

1. Rathee, V., Blair, D. L. & Urbach, J. S. Localized stress fluctuations drive shear thickening in dense suspensions. PNAS 114, 8740–8745 (2017).

2. Rathee, V., Blair, D. L. & Urbach, J. S. Localized transient jamming in discontinuous shear thickening. J Rheol 64, 299–308 (2020).

3. Rathee, V., Miller, J., Blair, D. L. & Urbach, J. S. Structure of propagating high-stress fronts in a shear-thickening suspension. Proc National Acad Sci 119, e2203795119 (2022).

4. Rathee, V. et al. Role of particle orientational order during shear thickening in suspensions of colloidal rods. Phys Rev E 101, 040601 (2020).

5. Miller, J. M., Blair, D. L. & Urbach, J. S. Order and density fluctuations near the boundary in sheared dense suspensions. Frontiers in Physics 10, 991540 (2022).