Macroscopic Discontinuous Shear Thickening versus Local Shear Jamming in Cornstarch

We study the emergence of discontinuous shear thickening (DST) in cornstarch by combining macroscopic rheometry with local magnetic resonance imaging measurements. We bring evidence that macroscopic DST is observed only when the flow separates into a low-density flowing and a high-density jammed region. In the shear-thickened steady state, the local rheology in the flowing region is not DST but, strikingly, is often shear thinning. Our data thus show that the stress jump measured during DST, in cornstarch, does not capture a secondary, high-viscosity branch of the local steady rheology but results from the existence of a shear jamming limit at volume fractions quite significantly below random close packing.

Figure 1

Macroscopic rheometry data. (a) Torque $T$ vs rotation rate $\mathrm{\Omega }$ during a logarithmic ramp at various packing fractions $\varphi$. (b) Apparent viscosity vs apparent shear rate extracted from (a). (c) Critical shear rates ${\dot{\gamma }}_{\text{CST}}$ and ${\dot{\gamma }}_{\text{DST}}$ vs $\varphi$. (d) Reversibility test at $\varphi =43.9\%$: succession of up, down, and up ramps.

Figure 2

Steady MRI data for a ${\varphi }_0=43.9\%$ cornstarch suspension and different rotational velocities $\mathrm{\Omega }$. (a) Velocity profiles. (b) Density profiles; solid lines indicate ${\varphi }_{\text{RLP}}$ and ${\varphi }_{\text{RCP}}$.

Figure 3

(a) Comparison of local rheometry data obtained from MRI measurements (open symbols) and near the inner cylinder in macroscopic rheometry (filled symbols) in homogeneous conditions. Upper data: $\varphi =43.9\%$; lower data: $\varphi \approx 33.5\%$. (b) From the velocity profiles [Fig. 2]: fraction of the gap which is jammed vs $\mathrm{\Omega }$.

Figure 4

Phase diagram: scatter plot of the local $\mathbf{(}\dot{\gamma }(r,\mathrm{\Omega }),\varphi (r,\mathrm{\Omega })\mathbf{)}$ values compared with the ${\dot{\gamma }}_{\text{CST}}(\varphi )$ (red) and ${\dot{\gamma }}_{\text{DST}}(\varphi )$ (blue) lines obtained from macroscopic rheometry [Fig. 1].