  Shear history plays an important role in determining the linear and nonlinear
rheological response of colloidal gels and can be used for tuning their
structure and flow properties. Increasing colloidal particle aspect ratio
lowers the critical volume fraction for gelation due to an increase of the
particle excluded volume. Using a combination of rheology and confocal
microscopy we investigate the effect of steady and oscillatory pre-shear
history on the structure and rheology of colloidal gels formed by silica
spheres and rods of length L and diameter D (L/D = 10) dispersed in 11 M CsCl
solution. We use a non-dimensional Mason number, Mn (= F_visc./F_attr.) to
compare the effect of steady and oscillatory pre-shear on gel viscoelasticity.
We show that after pre-shearing at intermediate Mn, attractive sphere gel
exhibits strengthening whereas attractive rod gel exhibits weakening.
Rheo-imaging of gels of attractive rods shows that at intermediate Mn,
oscillatory pre-shear induces large compact rod clusters in the gel
microstructure, compared to steady pre-shear. Our study highlights the impact
of particle shape on gel structuring under flow and viscoelasticity after shear
cessation.
