Timescales in creep and yielding of attractive gels

Abstract

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 σ, 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 γ dot above(t) characterized by two well-defined and separated timescales τc and τf. First γ dot above 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 τc 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 σc, i.e. τc ∼ (σ - σc)(-β), with β = 2-3 depending on the gel concentration. The end of this first regime is marked by a jump of the shear rate by several orders of magnitude, while the gel slowly slides as a solid block experiencing strong wall slip at both walls, despite rough boundary conditions. Finally, a second sudden increase of the shear rate is concomitant with the full fluidization of the material which ends up being homogeneously sheared. The corresponding fluidization time τf robustly follows an exponential decay with the applied shear stress, i.e. τf = τ0 exp(-σ/σ0), as already reported for smooth boundary conditions. Varying the gel concentration C in a systematic fashion shows that the parameter σ0 and the yield stress σc exhibit similar power-law dependences with C. Finally, we highlight a few features that are common to attractive colloidal gels and to solid materials by discussing our results in the framework of theoretical approaches of solid rupture (kinetic, fiber bundle, and transient network models).

Triggers and signatures of shear banding in steady and time-dependent flows

“…It has been seen experimentally in polymers [27,29,36], wormlike micelles [25,26,35], carbopol gel [37,81], carbon black [38,82], and a colloidal glass [83]. Particle-based simulations of molecular glasses have captured it [84].…”

Wall slip across the jamming transition of soft thermoresponsive particles

“…Such apparent discontinuity in velocity at a boundary reflects the existence of a thin and highly sheared region adjacent to the wall where the viscosity is much smaller than in the bulk material [2]. Although historically described as a mere artifact, wall slip appears as an intrinsic feature of complex fluids that has been reported in polymeric fluids [3][4][5] as well as in soft glassy materials such as colloidal suspensions [6,7], foams [8][9][10], emulsions [11][12][13], microgels [14,15], attractive gels [16], etc. Moreover, wall slip is often non-trivially coupled to the bulk dynamics of complex fluids, especially during the yielding transition where it may even govern the nature of the subsequent steady-state flow [17][18][19].…”

Avalanche-like fluidization of a non-Brownian particle gel

“…To conclude this subsection, complex cycles of hysteresis such as the one reported in Fig. 2 have also been reported for numerous other attractive gels including carbon black gels 45 and clay suspensions. 34,78 Although significant progress has been made in extracting quantitative information from hysteresis loops, 79 we rather chose to focus our study on shear startup The present experiments are all thus performed on a sample prepared in a solid-like state, so as to investigate the shearinduced fluidization scenario of the non-Brownian particle gel.…”

“…Yet the roughness and/or chemical nature of the walls are known to have a crucial influence not only on rheological measurements but also on the local flow both close to the walls and in the bulk. 33,34,45,86,87 Therefore the influence of boundary conditions on the complex fluidization scenario reported here appears as the next key question to address in future work. As a first step, we report preliminary tests on the role of boundary conditions in the above results in the ESI.…”

“…On the one hand, experiments performed under constant [42][43][44][45][46] and oscillatory 43,47,48 stress reveal that the fluidization process of attractive gels, initially at rest, may take up to tens of hours. Experiments coupled to velocimetry have revealed that such a process is mostly activated, as evidenced by the Arrhenius-like dependence of the fluidization time with the applied shear stress, 43,45 and that it is strongly heterogeneous in both the vorticity and the flow directions. 48 On the other hand, attractive gels…”