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We utilize a custom-built shear cell mounted on a confocal microscope to directly visualize and quantify the microdynamic mechanisms that mediate the rheology of a nearly jammed colloidal suspension under constant-stress deformation, with and without attractive interparticle interactions. The application of external stresses systematically increases particle mobility, as well as the ease by which the colloids can escape from topological cages formed by their nearest neighbors. We quantify the characteristic size and timescale of microstructural rearrangements within the suspension and show that these relaxation events become less spatiotemporally heterogeneous as the applied stress is increased. When interparticle attraction is introduced, the colloids tend to move more congruently under low stresses and the characteristic size of dynamically cooperative clusters increases. However, particle displacements become decorrelated under large external loads, with an abrupt transition occurring at the yield stress. In contrast, the repulsive system shows a more gradual transition from creep deformation to flow, with a nonmonotonic dependence of the particle displacement correlation function on the applied stress. Our results contribute to a better understanding of the jamming phase diagram in disordered colloidal materials, and the connection between the microstructure and nonlinear rheology of colloidal gels and glasses.

We introduce a custom-built stress-controlled shear cell coupled to a confocal microscope for direct visualization of constant-stress shear deformation in soft materials. The torque generator is a cylindrical Taylor–Couette system with a Newtonian fluid between a rotating inner bob and a free-to-move outer cup. A spindle/cone assembly is coaxially coupled to the cup and transfers the torque exerted by the fluid to the sample of interest in a cone-and-plate geometry. We demonstrate the performance of our device in both steady-state and transient experiments with different viscoelastic materials. Our apparatus can conduct unidirectional constant-stress experiments as accurately as most commercial rheometers, with the capability to directly visualize the flow field using tracer particles. Further, our step-stress experiments on viscoelastic materials are devoid of creep ringing, which is an advantageous aspect of our torque generation mechanism. We believe that the device presented here could serve as a powerful and cost-effective tool to investigate the microstructural determinants of nonlinear rheology in complex fluids.

We experimentally characterize the microstructure and rheology of a carefully designed mixture of immiscible fluids and near-neutral-wetting colloidal particles. Particle bridging across two fluid interfaces provides a route to highly stable gel-like emulsions at volume fractions of the dispersed phase well below the random close-packing limit for spheres. We investigate the microstructural origins of this behavior by confocal microscopy and reveal a percolating network of colloidal particles that serves as a cohesive scaffold, bridging together droplets of the dispersed phase. Remarkably, the mixture's salient rheological characteristics are governed predominantly by the solids loading and can be tailored irrespective of the droplet volume fraction. The identification of this rheological hallmark could provide a means toward the improved design of modern products that utilize solid-stabilized interfaces.

We investigate the nonlinear rheology of dilute, depletion-induced colloidal gels and report that these systems yield via a two-step process. We propose the two yield points to be associated with interparticle bond rotation and bond breakage, respectively. These distinct yielding mechanisms lead to remarkable creep profiles at intermediate values of the applied stress, highlighted by an anisotropic shear-induced strengthening and flow arrest at very large accumulated strains (γ∼ 80). The possible microstructural origins of this behavior are discussed.