Timescales in creep and yielding of attractive gels
Résumé
The stress-induced yielding scenario of colloidal gels is investigated under rough boundary conditions by means of rheometry coupled with local velocity measurements. Under an applied shear stress s, the fluidization of gels made of attractive carbon black particles dispersed in a mineral oil is shown to involve a previously unreported shear rate response g_ (t) characterized by two well-defined and separated timescales sc and sf. First g_ decreases as a weak power law strongly reminiscent of the primary creep observed in numerous crystalline and amorphous solids, coined the "Andrade creep". We show that the bulk deformation remains homogeneous at the micron scale, which demonstrates that whether plastic events take place or whether any shear transformation zone exists such phenomena occur at a smaller scale. As a key result of this paper, the duration sc of this creep regime decreases as a power law of the viscous stress, defined as the difference between the applied stress and the yield stress sc, i.e. sc (s sc) b, with b ¼ 2-3 depending on the gel concentration.
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