Spatial stress correlation in strong colloidal gel systems*

Colloidal gel systems exhibit increasingly slow relaxation and ultra-long-ranged spatial correlations of the dynamics similar to other jammed materials. These cooperative dynamics point to the presence of long-ranged stress correlation in these systems, which remain largely uninvestigated in the literature. In this work, we systematically investigate the nature of stress correlations in soft colloidal gel materials in the limit of moderate to high packing fractions and strong attraction. In this regime, centrosymmetric potential description for particle interaction fails as strong attraction can lead to frictional contacts, as shown explicitly in previous experiments. Accordingly, we model the system similarly to the cohesive granular media with Langevin dynamics to incorporate the effects of rolling and sliding resistant contacts and thermal fluctuations. We show that the spatial stress correlations are long ranged with very slow spatial decay close to the gel point. Similarly to previous studies on the frictional granular matter, the full stress autocorrelation matrix is dictated by the pressure and torque autocorrelations due to mechanical balance and material isotropy constraints. Surprisingly, it is observed that the gel materials do not behave as a normal elastic solid close to the gel point as assumed loosely in the literature because the real-space pressure fluctuations decay slower than normal. Furthermore, we link the abnormal pressure fluctuations to the non-hyperuniform behavior of the system (granular matter and gel) with respect to the local packing fraction fluctuations, thus relating the deviations from the normal elastic behavior across various jammed systems under a common framework.