Shear-Induced Yielding In Dilute Colloidal Gels

The response of colloidal gels to macroscopically imposed flow is relevant to a wide host of technologies that use these materials as feedstock, and is governed by a complex, circular interplay between the thermodynamic and hydrodynamic interparticle interactions and the applied shear stresses. In this study, quantitative confocal microscopy and rheology are combined to investigate the transient shear-induced yielding and flow in dilute, depletion-induced colloidal gels.  Over the range of depletant concentrations probed, the gels evolve via a two-step yielding process in which the secondary yielding event is encountered at very large values of the accumulated strain.  We directly probe and quantify the microstructural mechanisms that mediate this nonlinear mode of gel yielding.  In addition, the terminal microstructure of the sheared gel displays a remarkable segregation into dense clusters and large voids, reminiscent of phase-separating mixtures.  The implications of this behavior for the phase behavior and rheology of colloidal gels are discussed.