  We investigated the effects of particle shape on shear thickening in densely
packed suspensions. Rods of different aspect ratios and non-convex hooked rods
were fabricated. Viscosity curves and normal stresses were measured using a
rheometer for a wide range of packing fractions for each shape. 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.
