Shear-induced memory effect in boehmite gels

Aqueous suspensions of colloidal boehmite particles are used for the production of catalyst supports. The synthesis process involves an acidification step, turning the suspension into a colloidal gel due to the emergence of attractive forces between the particles. The suspension processability requires to maintain the sample’s viscosity within a specific range, which is achieved by applying a mechanical shear.
To understand the mechanisms by which the shear modifies the gel's structure after flow cessation, we study the rheological properties of a boehmite gel depending on the value of the preshear rate. We have identified a critical shear rate that separates two distinct behaviors. Soft solids obtained following a low shear intensity show a large elastic modulus with a glass-like viscoelastic spectrum and a low yield strain. On the contrary, a high pre-shear intensity leads to the formation of soft gels characterized by a three times lower elastic modulus and a higher yield strain. Such striking differences in the same sample's rheological properties indicate that two different microstructures are selected during the pre-shear step.
Complementary experiments coupling rheology to ultrasonic imaging allow us to associate each regime with distinctive flow behaviors during pre-shear.