Microscopic dynamics and failure precursors of a gel under mechanical load

Abstract

Material failure is ubiquitous, with implications from geology to everyday life and material science. It often involves sudden, unpredictable events, with little or no macroscopically detectable precursors. A deeper understanding of the microscopic mechanisms eventually leading to failure is clearly required, but experiments remain scarce. Here, we show that the microscopic dynamics of a colloidal gel, a model network-forming system, exhibit dramatic changes that precede its macroscopic failure by thousands of seconds. Using an original setup coupling light scattering and rheology, we simultaneously measure the macroscopic deformation and the microscopic dynamics of the gel, while applying a constant shear stress. We show that the network failure is preceded by qualitative and quantitative changes of the dynamics, from reversible particle displacements to a burst of irreversible plastic rearrangements.

Uncovering the dynamic precursors to motor-driven contraction of active gels

“…4a and S8, left sides of plots). This behavior is reminiscent of failure precursors observed in passive colloidal gels submitted to a constant shear stress, which also manifested as enhanced microscopic dynamics before macroscopic failure [49]. By analogy, we interpret the enhanced dynamics of actin-myosin networks as a dynamic precursor to macroscopic contraction.…”

“…These rupture events likely gradually weaken the network before it ultimately collapses. Prior studies have investigated dynamic precursors to catastrophic failure events across a broad range of systems, including earthquakes [82], avalanches [83], and fracture of gels [49,84]. The dynamic precursors to motor-induced contraction which we identify here demonstrate that similar events occur in active systems that evolve by internal driving.…”

“…Once the sample has begun to macroscopically contract in stage III, the dynamics are dominated by the contractile strain rate. In this regime, the macroscopic dynamics follows the macroscopic deformation, as seen in passive gels [49,86], for which ballistic dynamics (n = −1) was reported.…”

“…Simultaneous measurements at several scattering vectors reveal that the dynamics associated to these sudden rearrangements are length-scale independent. Both precursors are measurable long before macroscopic gel contraction occurs, in striking analogy to the dynamic precursors of macroscopic failure recently unveiled in passive gels subject to a mechanical load [49]. Previous research [50][51][52] highlighted the analogy between the microscopic dynamics of biological networks and glassy systems, in the regime where the stresses acting on the network are sufficiently small to prevent macroscopic failure.…”

Maximally stiffening composites require maximally coupled rather than maximally entangled polymer species

“…Thus, we limit ourselves to speculate that there may be a connection between these sudden dynamics and mechanical failure in soft materials. Indeed, it has been observed that peculiar dynamics, such as reversible particle displacements, irreversible rearrangements and heterogeneities often precede network failure [33]. Future avenues to characterize the mechanical implications of such rearrangements are discussed below.…”