Rhéo-épaississement discontinu : le comportement spectaculaire de la maïséna
à 11 h en salle C. Brot
We study the emergence of discontinuous shear-thickening (DST) in cornstarch suspension, by combining macroscopic rheometry with local Magnetic Resonance Imaging measurements. It follows that as a function of the applied stress the suspension is first solid (yield stress), then liquid, and then solid again when it shear thickens. For the onset of thickening, we find that the smaller the gap of the shear cell, the lower the shear rate at which thickening occurs. Shear thickening can then be interpreted as a consequence of dilatancy : the system under flow wants to dilate but instead undergoes a jamming transition because it is confined, as confirmed by measurement of the dilation of the suspension as a function of the shear rate. In the shear-thickened state, the observed macroscopic DST occurs only when the flow separates into a low-density flowing and a high-density jammed region and the local rheology in the flowing region, is not DST but, strikingly, is most often shear-thinning. Our data are not consistent with recent theoretical suggestions based on the presumed existence of s-shaped flow curves. Instead, they support that, in cornstarch, DST should be attributed to the existence of a shear jamming limit at volume fractions quite significantly below random close packing.
Fluides & Matériaux Complexes
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