Rheopexy in highly concentrated emulsions

This work describes a detailed study of the rheopectic effect in the flow of highly concentrated emulsions at low stresses. Experiments with the shear rate sweep demonstrated that the upward and downward branches of the flow curves coincide above some specific shear rate value. The upward experiments show the existence of the Newtonian part on the flow curve in the low-shear-rate domain, while the effect of yielding is observed in the downward curve. Restoration of the initial structure (and properties) after cessation of loading occurs very quickly. This allows associating the rheopexy with elastic deformations and the relaxation process (observed in frequency dependence of dynamic moduli) with characteristic times of about 0.03 s. Transient processes proceed in the range of shear deformation of the order of several units. Some quantitative measures of the rheopectic behavior are proposed and discussed. Normal stresses are constant in the low shear stress domain, but decrease sharply above the range of rheopectic behavior. The mechanisms of the observed effects are discussed. Temperature is not an important factor in the rheopectic behavior studied in this work.