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 (q2D) 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 (SD) 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 form 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 non-zero 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.

Local-heterogeneous responses and transient dynamics of cage breaking and formation in colloidal fluids

“…(1), the summand term refers to the configuration of the first neighboring cage, and n j is the corresponding number of the first neighboring particles around particle j. Usually, n j is chosen either as a constant of value six, i.e., six nearest neighbors, 26,32 or as a variable dependent on local coordination around particle j (first nearest neighbors) defined by using a Voronoi construction. 31,33,38 The value ψ j 6 = 1 implies perfect hexatic order solely with six nearest neighboring particles, whereas ψ j 6 1 implies a region deviating from hexatic order.…”

“…26. The sample consists of an aqueous suspension of uniform size silica spheres (diameter d = 2.56 ± 0.04 μm) and the suspension is confined between two hard walls with height (separation) of 3.2 μm, i.e., slightly larger than the particle diameter.…”

“…The so-called bond-orientational order parameter, [28][29][30] ψ 6 , has often been used in quasi-two-dimensional (q2D) colloidal fluids, 26,[31][32][33] in order to quantify how nearby particles are closely packed to a reference particle, i.e., in the form of a hexagonal configuration. Dynamics of ψ 6 was studied in granular liquids near the glass transition to probe the dynamical heterogeneity and slow dynamics.…”

“…This enables investigating the microscopic origin of complex fluid behavior, such as microviscosities in the shear thickening and shear thinning regimes as a function of the speed of the probe particle. 24,25 By applying a force to the probe particle, one can explore the instantaneous and average structural responses 26 and the spatial reorganization during and after the perturbation. 27 However, the interplay between DH and the spatio-temporal behavior of the caging of particles by their neighbors is still poorly understood.…”

“…26 We first introduce a modified bond-orientational order parameter, denoted byψ 6 , to properly quantify the time evolution of bond-orientational ordering. This overcomes some drawbacks that exist in the original definition of ψ 6 .…”