  We investigated the transient relaxation of a Discontinuous Shear Thickening
suspension of cornstarch in water. We performed 2 types of relaxation
experiments starting from a steady shear in a parallel plate rheometer,
followed by either stopping the top plate rotation and measuring the transient
torque relaxation, or removing the torque on the plate and measuring the
transient tool rotation. We found that at low weight fraction
$\phi_{eff}<58.8\pm0.4\%$, the suspensions exhibited a relaxation behavior
consistent with a generalized Newtonian fluid. However, for larger weight
fraction $58.8\% < \phi_{eff} < 61.0\%$, near the liquid-solid transition
$\phi_c=61.0\pm0.7\%$, we found relaxation behaviors different from the
generalized Newtonian model. The relaxation time in this range scales with the
inverse of the critical shear rate at the onset of shear thickening. In this
range the relaxation time was the same in both stress and rate controlled
experiments, rather than the viscosity calculated from the relaxation time
which is expected to be intrinsic material parameter in the generalized
Newtonian model. The discrepancy between the measured relaxation times and the
generalized Newtonian prediction was found to be up to $10^4$, and
extrapolations diverge in the limit of $\phi_c$ as the generalized Newtonian
prediction approaches 0. At the highest weight fractions, the relaxation time
scales were measured to be on the order of $\sim 1$ s. The fact that this
timescale is resolvable by the naked eye may be important to understanding some
of the dynamic phenomenon commonly observed in these systems. We also showed
that using the critical shear rate $\dot\gamma_c$ at the onset of shear
thickening to characterize the effective weight fraction can more precisely
characterize material properties near the critical point $\phi_c$, allowing us
to resolve this transition so close to $\phi_c$.

Fluid Dynamics;Soft Condensed Matter