Shear dynamics of aqueous suspensions of cellulose whiskeys

The rheology of rigid rod cellulose whisker suspensions has been investigat ed. The isotropic-at-rest suspension flow curves show two plateaus, one at low shear rates and another at high shear rates, that reflect the flow of i sotropic (at low shear rates) or oriented (at high shear rates) suspensions . From the low shear rate viscosity plateau vs concentration, we see that t he system is in the semidilute region above 0.02 wt %, in agreement with th e theoretical predictions. From the same curve, we can estimate a maximum p acking concentration of whiskers rods that corresponds to the experimentall y measured isotropic-to-anisotropic transition. The high shear rate plateau viscosity data show that the. suspension is still in the dilute state abov e 0.6 wt %. The critical concentration at the dilute-semidilute transition is thus strongly dependent on the state of order, which suggests that care has to be taken when measuring parameters extracted from flowing solutions or suspensions, as for example occurring with Ubbelhode viscometry. Above a certain critical concentration, the suspensions become anisotropic at rest . The viscosity vs concentration curve has a maximum that vanishes at high shear rates as for liquid crystalline polymer solutions. Rheological and rh eo-optical observations show fast inception and relaxation of both the rheo logical functions and the texture, in complete contrast with liquid crystal line polymer solutions.