Yielding and bifurcated aging in nanofibrillar networks

TL;DR

This paper investigates how nanofibrillar networks recover their stress-supporting ability after yielding, revealing a bifurcation in aging behavior around a critical stress that helps better understand and locate the yield stress in disordered materials.

Contribution

It uncovers the bifurcated aging response in fibrillar networks around the yield stress, linking microstructural dynamics to macroscopic stress support recovery.

Findings

Aging response bifurcates at the critical stress $\sigma_c$.

Networks yield faster and at lower stress after initial yielding.

The bifurcation aids in localizing the yield stress in disordered materials.

Abstract

The yielding of disordered materials is a complex transition involving significant changes of the material's microstructure and dynamics. After yielding, many soft materials recover their quiescent properties over time as they age. There remains, however, a lack of understanding of the nature of this recovery. Here, we elucidate the mechanisms by which fibrillar networks restore their ability to support stress after yielding. Crucially, we observe that the aging response bifurcates around a critical stress $\sigma_\mathrm{c}$, which is equivalent to the material yield stress. After an initial yielding event, fibrillar networks subsequently yield faster and at lower magnitudes of stress. For stresses $\sigma<\sigma_\mathrm{c}$, the time to yielding increases with waiting time $t_\mathrm{w}$ and diverges once the network has restored sufficient entanglement density to support the stress. When $\sigma > \sigma_\mathrm{c}$, the yield time instead plateaus at a finite value because the developed network density is insufficient to support the applied stress. We quantitatively relate the yielding and aging behavior of the network to the competition between stress-induced disentanglement and dynamic fluctuations of the fibrils rebuilding the network. The bifurcation in the material response around $\sigma_c$ provides a new possibility to more rigorously localize the yield stress in disordered materials with time-dependent behavior.