Shear-thickening in presence of adhesive contact forces: the singularity of cornstarch

A number of dense particle suspensions experience a dramatic increase in viscosity with the shear stress, up to a solid-like response. This shear-thickening process is understood as a transition under flow of the nature of the contacts – from lubricated to frictional – between initially repellent particles. Most systems are now assumed to fit in with this scenario, which is questionable.

To better understand the mechanism of shear thickening of cornstarch solutions, we study the structure of the suspensions under flow, with a focus on the high shear region.

 Using in-house sensors, we probe at the millimeter-scale, in a new way, the normal stresses that develop. The nature of this signal changes with the constraining conditions: at low weight fractions and large gap widths he signal is consistent with a stress wave moving 1 to 1.5 times faster than the geometry. At high weight fractions  and small gap, the signal arises from the passage of a solid aggregate rolling between the plates. This transition is explained by taking into account the presence of adhesive forces between the particles at high shera stress. The presence these forces, combined with the existence of a yield stress (showing that the particles are also in contact at low stress) clearly indicates that cornstarch particles are in contact all along the flow curve.

From the previous discussion, we see that cornstarch suspensions stand out of the classical shear-thickening frame. We suggest here that cornstarch particles are very weakly attractive in suspensions in CsCl brine (to account for the yield stress), and strongly adhesive once the normal stresses pushes them into contact. More precisely, the initial contact between particles induces a small yield stress. When subjected to stress, the fragile bonds break and the suspension turns to a liquid, exhibiting a strongly shear thinning behavior. Beyond a critical stress, the particle pressure pushes the particles in contact and involves an important indentation. The polysaccharide chains surrounding the starch granules become entangled. An adhesive force coming from hydrogen bonds is set up. In this regime, there is a competition between the time for the polysaccharide to disentangle and the contact time ($propto1/dotgamma$) between the particles. When the first is larger than the latter, the system thickens.