Residual stresses and shear-induced overaging in boehmite gels

Colloidal gels respond as soft solids at rest, whereas they flow liquids under external shear. Starting from a fluidized state an applied shear rate ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{p}$, abrupt cessation triggers liquid-to-solid transition during which the stress relaxes towards so-called residual ${\ensuremath{\sigma}}_{\mathrm{res}}$ that tallies macroscopic signature of previous history. Here, we report on in boehmite, aluminum oxide, shows remarkable nonmonotonic relaxation ${\ensuremath{\sigma}}_{\mathrm{res}}({\stackrel{\ifmmode \.{}\fi{}}{\ensuremath{\gamma}}}_{p})$ characterized by dual behavior relative to critical value \.{}\fi{}}{\ensuremath{\gamma}}}_{c}$ \.{}\fi{}}{\ensuremath{\gamma}}}_{p}$. Following \.{}\fi{}}{\ensuremath{\gamma}}}_{p}>{\stackrel{\ifmmode \.{}\fi{}}{\ensuremath{\gamma}}}_{c}$, gel obtained upon is insensitive history, and negligible. However, for \.{}\fi{}}{\ensuremath{\gamma}}}_{p}<{\stackrel{\ifmmode encodes some memory increases decreasing rate, directly contributing reinforcing viscoelastic properties. Moreover, show both properties increase logarithmically with strain accumulated period preceding cessation. Such shear-induced ``overaging'' phenomenon bears great potential tuning rheological colloidal gels.