title: 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.
authors.affiliation: Department of Physics, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Rogers
authors.affiliation: Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: Kui
authors.initials: K
authors.lastname: Chen
authors.affiliation: Department of Physics, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5.
authors.forename: Lukasz
authors.initials: L
authors.lastname: Andrzejewski
authors.affiliation: X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: Suresh
authors.initials: S
authors.lastname: Narayanan
authors.affiliation: Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Tallahassee, Florida 32312, USA.
authors.forename: Subramanian
authors.initials: S
authors.lastname: Ramakrishnan
authors.affiliation: Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: Robert L
authors.initials: RL
authors.lastname: Leheny
authors.affiliation: Department of Physics, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5.
authors.forename: James L
authors.initials: JL
authors.lastname: Harden