A two-axes shear cell for rheo-optics

Abstract: We develop and test a rheo-optical platform based on a two-axes, parallel plates shear cell coupled to an optical microscope and a photon correlation imaging setup, for simultaneous investigation of the rheological response and the microscopic structure and dynamics of soft materials under shear. Each plate of the shear cell is driven by an air bearing linear stage, actuated by a voice coil motor. A servo control loop reading the plate displacement through a contactless linear encoder enables both strain-controlled and stress-controlled rheology. Simultaneous actuation of both linear stages enables both parallel and orthogonal superposition rheology. We validate the performance of our device in both oscillatory and transient rheological tests on a microgel soft glass, and we demonstrate its potential through orthogonal superposition rheology experiments. During steady-state flow, we reconstruct the strain field across the gap by tracking the motion of tracer particles, to check for slip or shear banding instabilities. At the same time, we measure the microscopic dynamics, both affine and non-affine, resolving them in space and time using photon correlation imaging.