The origin of hierarchical viscoelastic response in soft particulate gels.*

Soft particulate gels consist of a small amount of solid material (colloidal particles or small aggregates) embedded in a fluid. Due to attractive inter-particle interactions, the solid component self-assembles into an open, porous network which controls the overall mechanical response of the material.  We have used  large scale simulations with Optimally Windowed Chirp (OWCh) signals in a 3D gel model to investigate the microscopic origin of the linear viscoelastic response of soft particulate gels. The main ingredients of the model are short-ranged attractive interactions and bending stiffness for the inter-particle bonds. Using the OWCh protocol we have analyzed the key features of the frequency-dependent dynamic modulus G*(ω) and their dependence on the gel connectivity and on the preparation protocol. Our analysis indicates that the viscoelastic spectrum of a wide range of gels, with different microstructures, is controlled by an underlying fractal characteristic of the gel network, i.e., its initial rigid backbone, and by the associated hierarchy of time scales. We discuss the microscopic structural and dynamical origin of these hierarchical processes and the emerging scaling behaviors.