The “roll” of constraints in yielding of soft particulate gels

Soft particulate gels, encompassing a wide range of consumable, injectable, and 3D-printable applications, exhibit intriguing rheological properties due to their unique microstructure. These gels comprise a small volume fraction of particles dispersed within a fluid medium, forming an aggregated network through attractive interactions such as Van der Waals or depletion forces. The resulting complexity in their rheological behavior, including weak but finite yield stress, shear thinning, and thixotropy, makes them promising candidates for various in-situ solidifying applications from cosmetics to tissue constructs. While it is widely acknowledged that system-spanning particulate structures contribute to the observed yield stress, a comprehensive understanding of the underlying microscopic mechanisms remains elusive. In this study, we present coarse-grained simulations focusing on model depletion gels to shed light on this intriguing phenomenon. Contrary to conventional belief, our simulations reveal that the mere presence of attractive interactions and aggregate formation is not sufficient to explain the observed yield stress. Instead, we identify a crucial element of colloidal physics in the form of microscopic constraints on the relative rotational motion between attractively bonded particles. Through a detailed analysis of microstructure and particle cluster dynamics, we elucidate how these constraints lead to the emergence of yield stress in soft particulate gels. This research provides new insights into the micromechanical origins of yield stresses in soft particulate gels, paving the way for improved understanding and engineering of these versatile materials for a wide range of real-world applications.