Yielding and rheopexy of aqueous xanthan gum solutions

Xanthan gum (XG) is widely used in cosmetic and pharmaceutic products (creams, pastes) and in oil industry (drilling fluids) as a stabilizing and/or thickening agent. In literature, its rheological behavior is mainly presented as that of a shear-thinning or a yield stress fluid. Here, in order to clarify this rheological behavior, we study in detail the flow characteristics during continued flow under given conditions (i.e., controlled stress) for a mass concentration ranging from 0.2 to 5%. We are thus able to identify the apparent flow curve of the material after a short flow duration and the flow curve in steady state (i.e., after a long flow duration). The validity of this flow curve, determined from standard rheometry, is confirmed by magnetic resonance velocimetry. These materials start to exhibit a yield stress behavior beyond some critical xanthan or salt concentration. In that case, a significant increase (by a factor up to 5) of the apparent viscosity is observed during flow in some range of stresses, before reaching a steady state. This original rheopectic effect might be due, after some time of flow associated with deformation and reconfiguration of the XG molecules, to the progressive formation of intermolecular links such as hydrogen bonds and/or intermolecular association due to acetate residues.