(102d) Transient Shear Induced Dynamic Heterogeneities in Dilute Colloidal Gels

Colloidal gels are often subject to transient non-linear shear deformation during steps involved in their manufacturing, processing and handling. The implications of such processes on the deformation of the microstructure, and the intricately linked rheological response, are poorly defined. Here, we report the shear-induced dynamic and structural evolution of dilute, depletion induced colloidal gels suspended in a density- and refractive index- matched solvent mixture. The study is carried out in a custom built cone-and-plate shear cell designed to directly assemble onto the stage of a fast scanning confocal microscope, facilitating real time observation of the gel network's response. The shear-induced evolution of the gel network displays three distinct regimes with clearcut transition points when the structure is anchored to the stationary plate: (i) an initial period of structural reconfiguration to an anisotropic network, (ii) an intermediate regime in which the microstructure is predominantly immobilized, albeit with intermittent, sporadic rearrangements, and (iii) a final regime characterized by disconnected clusters that are advected along the flow direction in a plug-flow like manner. Remarkable heterogeneities are observed in the dynamics of the gel network within the intermediate phase. Quantitative analysis of these transient phenomena are presented and discussed in light of the non-linear rheological response of the gels.