Direct confocal imaging of fracture precursors in casein gel*

Gels due to their heterogeneous nature show a complex, yet characteristic delayed yielding behavior under the application of sub-critical stress^[1][2]. 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^[3]. 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 which is based on the principle of measuring the deflection of a cantilever. This setup is integrated into a confocal microscope to have simultaneous shear experiments and 3D image acquisition. Presently, the setup has a precision of 0.1mPa (on a 1cm²). area) which is at least two orders of magnitude higher than a conventional rheometer and one order higher than the best shear cell^[4]. Using image correlation, we are able to find the precursors and analyze their statistics.

[1] M. Leocmach, et al Phys.Rev. Lett., 2014, 113, 038303.
[2] S. Aime, et al, PNAS, 2018, 115, 3587–3592.
[3] L. Cipelletti et al, arXiv:1909.11961[cond-mat.soft].
[4] Lin, N. Y. C., et al, Rev. Sci. Instrum. 85.3 (2014), p. 033905.