title: Structural responses of quasi-two-dimensional colloidal fluids to excitations elicited by nonequilibrium perturbations.
abstract: We investigate the response of a dense monodisperse quasi-two-dimensional colloid suspension when a particle is dragged by a constant velocity optical trap. Consistent with microrheological studies of other geometries, the perturbation induces a leading density wave and trailing wake. We also use a hybrid version of Stokesian dynamics simulations to parse direct colloid-colloid and hydrodynamic interactions. We go on to analyze the underlying individual particle-particle collisions in the experimental images. The displacements of particles occur in chains reminiscent of stress propagation in sheared granular materials. From these data, we can reconstruct steady-state dipolar-like flow patterns that were predicted for dilute suspensions and previously observed in granular analogs to our system. The decay of this field differs, however, from point Stokeslet calculations, indicating that the nonzero size of the colloids is important. Moreover, there is a pronounced angular dependence that corresponds to the surrounding colloid structure, which develops in response to the perturbation. Put together, our results show that the response of the complex fluid is highly anisotropic owing to the fact that the effects of the perturbation propagate through the structured medium via chains of colloid-colloid collisions.
authors.affiliation: Department of Chemistry and James Franck Institute, The University of Chicago, 929 E. 57th St., Chicago, Illinois 60637, USA.
authors.forename: Jelena
authors.initials: J
authors.lastname: Pesic
authors.forename: Joseph Zsolt
authors.initials: JZ
authors.lastname: Terdik
authors.forename: Xinliang
authors.initials: X
authors.lastname: Xu
authors.forename: Ye
authors.initials: Y
authors.lastname: Tian
authors.forename: Alejandro
authors.initials: A
authors.lastname: Lopez
authors.forename: Stuart A
authors.initials: SA
authors.lastname: Rice
authors.forename: Aaron R
authors.initials: AR
authors.lastname: Dinner
authors.forename: Norbert F
authors.initials: NF
authors.lastname: Scherer