Rheological behavior of dilute imogolite suspensions

Imogolite is one of the most important clay minerals contained in volcanic ash soil. The morphology of imogolite is very unique and comprises thin fibrous tubes with inside and outside diameters of approximately 1 and 2 nm, respectively. The chemical structure of imogolite features SiOH groups along the inner walls of the tubes and AlOH groups on the outer surfaces of the tubes. As a result of this morphology and the chemical structure, imogolite coagulates at high pH. In the present study, we focus on the detailed differences of the non-Newtonian behavior of the dilute imogolite suspension in the dispersed and coagulated states. Experiments on the viscosity as a function of volume fraction were performed using a concentric cylindrical viscometer. The applied shear rate was increased from 6.45 to 258 s−1 and then decreased to the initial value; this procedure was repeated three times. The flow curves for the dispersed imogolite suspensions showed slight hysteresis. Additionally, shear thinning, which is a decrease of the viscosity with increasing shear rate, emerged; this is thought to be due to the change of the orientation distribution of the imogolite particles. For the coagulated suspensions, the area of the hysteresis loop increased with increasing volume fraction. The flow curves tended to shift upwards and became constant as the shearing cycles increased. Moreover, shear thinning at high pH was more significant than that at low pH. The yield stress of the coagulated state was determined from the intercept of the flow curve. We propose that imogolite at high pH forms flocs that are repeatedly subjected to breakup and re-coagulation by the given shear stress; thus, a stronger structure is formed.