Heterogeneous yielding dynamics in a colloidal gel

Abstract

Attractive colloidal gels display a solid-to-fluid transition as shear stresses above the yield stress are applied. This shear-induced transition is involved in virtually any application of colloidal gels. It is also crucial for controlling material properties. Still, in spite of its ubiquity, the yielding transition is far from understood, mainly because rheological measurements are spatially averaged over the whole sample. Here, the instrumentation of creep and oscillatory shear experiments with high-frequency ultrasound opens new routes to observing the local dynamics of opaque attractive colloidal gels. The transition proceeds from the cell walls and heterogeneously fluidizes the whole sample with a characteristic time whose variations with applied stress suggest the existence of an energy barrier linked to the gelation process. The present results provide new test grounds for computer simulations and theoretical calculations in the attempt to better understand the yielding transition. The versatility of the technique should also allow extensive mesoscopic studies of rupture mechanisms in soft solids ranging from crystals to glassy materials.

Shear banding in large amplitude oscillatory shear (LAOStrain and LAOStress) of polymers and wormlike micelles

“…This criterion was also predicted on the basis of fluid-universal analytical calculations in [47], and has been confirmed in numerical simulations of polymeric fluids [48] and SGMs [3]. It is consistent with experimental observations in polymers [14,33,34,41,[44][45][46]57], carbopol microgels [43], and carbon black suspensions [42].…”

“…In view of this, a natural question to ask is whether shear banding might also arise in these time-dependent flows and, if so, under what conditions. Over the past decade, a body of experimental data has accumulated to indicate that it does indeed occur: In shear startup [6,7,16,[32][33][34], following a step strain (in practice a rapid strain ramp) [35][36][37][38][39][40][41], and following a step stress [14,33,34,[41][42][43][44][45][46].…”

Homogeneous percolation versus arrested phase separation in attractively-driven nanoemulsion colloidal gels

“…For f < 0.23, the stress sweeps are qualitatively similar to hard particle gels under dilute conditions. 18,19 Specically, at sufficiently high stress amplitude, s 0 , the material exhibits a single, relatively smooth yielding transition from a linear solid (where G 0 and G 00 are independent of s 0 ) to a shear-thinning liquid with power-law scaling of G 0 and G 00 with increasing stress at high amplitudes ( Fig. 3, solid lines).…”

“…17 Whereas dilute gels typically exhibit a distinct yield stress (or strain) in nonlinear rheological measurements 18 corresponding to a single microstructural process, 19 more concentrated systems exhibit complicated yielding in which the transition from solid-like behaviour to ow is marked by a two-step or broadened transition. [20][21][22][23] Such two-step yielding has been primarily observed in systems with moderate volume fraction, and has been hypothesized to occur due to a combination of processes, including rupture of interparticle bonds and glassy dynamics of attractive clusters.…”

Criteria for Shear Banding in Time-Dependent Flows of Complex Fluids

“…However most practical flows involve a strong time-dependence, whether perpetually or during a startup process in which a steady flow is established from an initial rest-state. Data in polymers [8][9][10][11][12][13][14][15], surfactants [16][17][18], soft glasses [19][20][21][22], and simulations [23][24][25][26][27][28][29][30] reveals that shear bands often also arise during these time-dependent flows, and can be sufficiently long lived to represent the ultimate flow response of the material for practical purposes, even if the constitutive curve is monotonic, dΣ/dγ > 0.In view of these widespread observations, crucially lacking is any known criterion for the onset of banding in time-dependent flows. This Letter provides such criteria, with the same fluid-universal status as the criterion given above in steady state: independent of the internal constitutive properties of the particular fluid in question, and depending only on the shape of the experimentally measured rheological response function.…”