We investigated the effects of particle shape on shear thickening in densely packed suspensions. Collaborating with Joseph DeSimone's group at UNC (link) and Liquidia Technologies, NC (link), rods of different aspect ratios and unique non-convex hooked rods were fabricated in gram quantities using the PRINT® (Particle Replication in Non-wetting Templates) process. Having such large quantities of well-controlled particles made it possible to use a rheometer for measuring viscosity curves and normal stresses for a wide range of packing fractions. Suspensions of each shape exhibit qualitatively similar Discontinuous Shear Thickening. The logarithmic slope of the stress/shear-rate relation increases dramatically with packing fraction and diverges at a critical packing fraction \phi_c which depends on particle shape. The packing fraction dependence of the viscosity curves for different convex shapes can be collapsed when the packing fraction is normalized by \phi_c. Intriguingly, viscosity curves for non-convex particles do not collapse on the same set as convex particles, showing strong shear thickening over a wider range of packing fraction. The value of \phi_c is found to coincide with the onset of a yield stress at the jamming transition, suggesting the jamming transition also controls shear thickening. The yield stress is found to correspond with trapped air in the suspensions, and the scale of the stress can be attributed to interfacial tension forces which dramatically increase above \phi_c due to the geometric constraints of jamming. The relationship between shear and normal stresses is found to be linear in both the shear thickening and jammed regimes, indicating that the shear stresses come from friction. In the limit of zero shear rate, normal stresses pull the rheometer plates together due to the surface tension of the liquid below \phi_c, but push the rheometer plates apart due to jamming above \phi_c.

1. •Eric Brown, Hanjun Zhang, Nicole A. Forman, Benjamin W. Maynor , Douglas E. Betts, Joseph M. DeSimone, and Heinrich M. Jaeger, ““Shear thickening and jamming in densely packed suspensions of different particle shapes”, submitted to Phys. Rev. E (2011), arXiv pdf