(98bd) A Simple Shear Cell for the Direct Visualization of Step-Stress Deformation Without Creep Ringing in Soft Materials

The application of external stresses to a viscoelastic material can induce complex rheological responses such as creep, shear-induced strengthening, and delayed yielding.  Understanding the underlying physics of these phenomena requires the ability to characterize how the material’s microstructure evolves during the deformation. Here, We introduce a custom-built stress-controlled shear cell coupled to a fast-scanning confocal microscope for direct visualization of constant-stress shear deformation in soft materials and complex fluids. 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 model and experimentally 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. Further, our step-stress experiments on viscoelastic materials are devoid of creep ringing, which is an advantageous aspect of the viscous coupling between the torque generator and the viscoelastic sample.  We specifically demonstrate this aspect of our shear cell both theoretically and experimentally with a rubber-like crosslinked polymer whose viscoelasticity is well approximated by the Kelvin-Voigt model.