Shear-induced Stiffening in Boehmite Gels: A Rheo-X-ray-scattering Study

Boehmite, an aluminum oxide hydroxide gamma-AlO(OH), is broadly used in the form of particulate dispersions in industrial applications, e.g., for the fabrication of ceramics and catalyst supports or as a binder for extrusion processes. Under acidic conditions, colloidal boehmite dispersions at rest form gels, i.e., space-spanning percolated networks that behave as soft solids at rest, and yet yield and flow like liquids under large enough deformations. Like many other colloidal gels, the solid-like properties of boehmite gels at rest are very sensitive to their previous mechanical history. Our recent work [J. Rheol. 66, 91 (2022); Phys. Rev. Mater. 6, L042601 (2022)] has revealed such memory effects, where the shear experienced prior to flow cessation drives the elasticity of boehmite gels: while gels formed following application of a shear rate <(gamma)over dot>(p) larger than a critical value <(gamma)over dot>(c) are insensitive to shear history, gels formed after application of <(gamma)over dot>(p) < <(gamma)over dot>(c) display reinforced viscoelastic properties and non-negligible residual stresses. Here, we provide a microstructural scenario for these striking observations by coupling rheometry and small-angle X-ray scattering. Time-resolved measurements for <(gamma)over dot>(p) < <(gamma)over dot>(c) show that scattering patterns develop an anisotropic shape that persists upon flow cessation, whereas gels exposed to <(gamma)over dot>(p) > <(gamma)over dot>(c) display isotropic scattering patterns upon flow cessation. Moreover, as the shear rate applied prior to flow cessation is decreased below <(gamma)over dot>(c), the level of anisotropy frozen in the sample microstructure grows similarly to the viscoelastic properties, thus providing a direct link between mechanical stiffening and flow-induced microstructural anisotropy.