Negative Thixotropy

AQUEOUS solutions of ammonium oleate are strongly elastic. If given a rotary motion and then allowed to come to rest, the rotation is in part retraced before they do so. In 1926, Hatschek and Jane¹ examined these solutions in a Couette-type viscometer. They found, inter alia, that when the outer cylinder was rotated at a constant angular velocity, the torque on the inner one, after a short period of normal magnitude, increased to an irregularly fluctuating value many times greater. After a period of rest, the behaviour was repeated. It appeared that a gelatinous structure was actually built up by the process of shear, and dissipated again at rest. The subsequent discovery² that the anomalous increase of torque was associated with the onset of turbulence distracted attention³ from this first conclusion. It seems probable, however, that the turbulence itself, also anomalous and of abnormal appearance², was another result of the structure built up rather than the direct cause of the increased torque.