Elastic recoil and inertia-driven dynamics upon shear cessation of conductive carbon black gels: a rheo-XPCS study

Carbon black (CB) consists of electrically conductive, rigid fractal aggregates. Depending on the solvent quality and CB concentration, physical agglomeration leads to stress-bearing and electrically percolated networks that constitute gels. These CB gels exhibit yield-stress fluid behavior in the low shear-rate regime, which transitions to viscoelastic fluid behavior at higher shear rates due to the physical dissociation of the network structure. Upon shear cessation, the network undergoes reconstruction within a short time scale, followed by long-time structural evolution. Simultaneous electrical measurements indicate the structural evolution of the network during shear start-up and cessation. In this study, we combine rheo-electric and time-resolved X-ray photon correlation spectroscopy (XPCS) measurements of CB gels under shear cessation tests in solvents of different polarity: polar glycerol and non-polar polydimethylsiloxane (PDMS). Using creep-recovery mode, the shear-rate response exhibits oscillations that persist even 1000 s after shear cessation, arising from elastic recoil and residual inertia. The oscillation envelope decays as a power law, consistent with microscopic velocity fluctuations observed in rheo-XPCS. We discuss the effect of solvent quality on the kinetics of network reconstruction, where CB in PDMS shows a faster increase in relaxation time and decay of normalized electrical resistance than in glycerol.