This talk was part of the Thematic Programme on "Non-equilibrium Processes in Physics and Biology" held at the ESI August 19 -- October 11, 2024.

Due to their heterogeneous nature, gels show a complex, yet characteristic delayed yielding behavior under the application of sub-critical stress. This makes us interested in understanding what are the signs or precursors existing within the material before the final catastrophic failure. Scattering studies hint toward microscopic changes well before failure [S. Aime et al., PNAS, 115, 14 (2018)]. What we are interested in is the direct real-space observation of the gel microstructure under shear. For this, we have designed our own setup ICAMM [A. Singh at al., Meas. Sci. Technol., 32 125603 (2021)], which can be integrated with a confocal microscope and has precision in stress application and measurement up to 6 mPa on an area as wide as the field of view of the microscope. This setup enables simultaneous shear experiments and 3D image acquisition of the whole sheared volume. Using a 3D implementation of a dense optical flow algorithm on the confocal images of the sheared gel, we are able to obtain the local strain rate field and thus, detect and have statistics on structural precursors (strand breaking events).