Compressive yield stress of depletion gels from stationary centrifugation profiles

We have investigated the stationary sedimentation profiles of colloidal gels obtained by an arrested phase-separation process driven by depletion forces, which have been compressed either by natural gravity or by a centrifugal acceleration ranging between 6g and 2300g. Our measurements show that the gel rheological properties display a drastic change when the gel particle volume fraction exceeds a value ${\varphi }_c$, which barely depends on the strength of the interparticle attractive forces that consolidate the network. In particular, the gel compressive yield stress $\Pi \left(\varphi \right)$, which increases as $\Pi \left(\varphi \right)\sim {\varphi }^{4.2}$ for $\varphi \lesssim {\varphi }_c$, displays a diverging behaviour for $\varphi >{\varphi }_c$, with an asymptotic value that is close to the random close packing value for hard spheres. The evidence we obtained suggests that ${\varphi }_c$ basically coincides with the liquid (colloid-rich) branch of the metastable coexistence curve, rather than with the lower (and ϕ-dependent) values expected for an attractive glass line penetrating inside the coexistence region. (paper)