  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
[Sudreau et al., J. Rheol. 66, 91-104 (2022), and Phys. Rev. Material 6,
L042601 (2022)] has revealed such \textit{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 $\dot\gamma_{\rm p}$
larger than a critical value $\dot\gamma_{\rm c}$ are insensitive to shear
history, gels formed after application of $\dot\gamma_{\rm p}<\dot\gamma_{\rm
c}$ 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 $\dot\gamma_{\rm p} <\dot\gamma_{\rm c}$ show
that scattering patterns develop an anisotropic shape that persists upon flow
cessation, whereas gels exposed to $\dot\gamma_{\rm p}>\dot\gamma_{\rm c}$
display isotropic scattering patterns upon flow cessation. Moreover, as the
shear rate applied prior to flow cessation is decreased below $\dot\gamma_{\rm
c}$, the level of anisotropy frozen in the sample microstructure grows
similarly to the viscoelastic properties, thus providing a direct link between
mechanical reinforcement and flow-induced microstructural anisotropy.
