Echoes in x-ray speckles track nanometer-scale plastic events in colloidal gels under shear

Abstract

We report x-ray photon correlation spectroscopy experiments on a concentrated nanocolloidal gel subject to in situ oscillatory shear strain. The strain causes periodic echoes in the speckle pattern that lead to peaks in the intensity autocorrelation function. Above a threshold strain that is near the first yield point of the gel, the peak amplitude decays exponentially with the number of shear cycles, signaling irreversible particle rearrangements. The wave-vector dependence of the decay rate reveals a power-law distribution in the size of regions undergoing shear-induced rearrangement. The gel also displays strain softening well below the threshold, indicating a range of strains at which the rheology is nonlinear but the microscopic deformations are reversible.

Reversibility and criticality in amorphous solids

“…There is increasing evidence to suggest that the microscopic mechanism of plastic deformation is a local rearrangement of particles involving a change of nearest neighbours (so called shear transformation zones), which results in an Eshelby-like elastic field 2 3 4 5 . However, it was recently shown that these rearrangements can be repetitive or irreversible depending on the strain amplitude 6 7 8 9 10 11 12 13 14 15 16 17 18 (a similar phenomenon has been observed in the shearing of colloidal suspensions, granular systems, dislocations and super-conducting vortices 19 20 21 22 23 24 25 26 27 28 and in the compaction of granular matter 29 ).…”

Rheological characterization of triglyceride shortenings

“…At sufficiently high strains (γ 0 > 0.01-0.02), crystal clusters begin to break up and/or become aligned by the shear flow as stress decays (I 3 / 1 keeps increasing or drops). The prevalence of a double-arc shape in I 3 / 1 in some samples might be attributed to two separate yielding processes, as observed in some concentrated colloidal gel or shear instabilities (Wilhelm et al 1999;Rogers et al 2014). To describe the evolution of I 3/1 , we used a descriptive function proposed by Wilhelm et al (2000).…”

Macroscopic yielding in jammed solids is accompanied by a nonequilibrium first-order transition in particle trajectories

“…Whereas γ × is frequently quoted as "the" yielding point in the literature, a stress overshoot also serves to identify yielding in shear-start experiments [25]. A stress overshoot is reported in some oscillatory shear experiments [19], but is absent in others [12,13]. A possible explanation is that experiments are typically performed at somewhat larger frequencies, where additional contributions to the shear stress (lubrication forces, hydrodynamic effects) might hide this behaviour.…”