Short-time dynamics of monomers and dimers in quasi-two-dimensional colloidal mixtures

Abstract

We report on the short-time dynamics in colloidal mixtures made up of monomers and dimers highly confined between two glass-plates. At low concentrations, the experimental measurements of colloidal motion agree well with the solution of the Navier-Stokes equation at low Reynolds numbers, which takes into account the increase of the drag force on each particle due to wallparticle hydrodynamic forces. We find that the ratio of the short-time diffusion coefficients of the monomer and that of the center of mass of the dimer remains independent of both the total packing fraction and the dimer molar fraction up to concentrations near to the crystallization transition. The same physical scenario is observed for the ratio between the parallel and perpendicular components of the short-time diffusion coefficients of the dimer. This dynamical behavior is corroborated by means of Molecular Dynamics computer simulations that explicitly include the particle-particle hydrodynamic forces induced by the solvent. Thus, our results point out toward that the effects of the particle-particle hydrodynamic interactions on the diffusion coefficients are identical and, thus, factorable in both species.Introduction. Many phenomena observed in colloidal dispersions resemble those of atomic systems. However, the colloidal dynamics exhibits special features due to the solvent-mediated forces typically known as hydrodynamic interactions (HI) [1]. Contrary to direct particleparticle interactions, HI can be tuned, but never completely screened or switched off. In a simple physical picture, HI can be understood as follows. The motion of a given colloidal particle induces a flow field in the solvent which is felt by the surrounding colloids. Thus, the motion of one colloidal particle causes a solvent-mediated force on the neighboring colloidal particles.In contrast to the static counterpart, the colloidal dynamics is far from being completely understood. The reason is partially related with the fact that the colloidal dynamics extends over a wide range of temporal and length scales due to the enormous difference in size and mass between the colloids and the solvent molecules, giving rise to complex and long-ranged HI [1,2]. The latter ones lead to non-trivial coupling among colloids that extends over many mean-interparticle distances [1,2]. The understanding of HI is thus of relevance not only in physics, but also in several branches of science, such as biology, since phenomena like hydrodynamic synchronization in either biological systems (sperm, cilia, flagella) [3,4] or active fluids [5], and the dynamics of microswimmers [6,7] can only be explained in terms of hydrodynamic forces.During the last few decades, the study of the hydrodynamic coupling between two or three spherical colloids [8,9] or a spherical colloid near a wall [10][11][12] has been

On the Time Transition Between Short- and Long-Time Regimes of Colloidal Particles in External Periodic Potentials

“…Using the total mean square displacement (associated to the free diffusion in the y− and x − axis directions), the short time diffusion coefficient was found to be D 0 1.8 × 10 − 13 m 2 /s, which in turn is used in our Brownian dynamics simulations, allowing us to reproduce the observed experimental behavior as it is shown in Figure 3. The reduction of D 0 is about 35% with respect the Stokes-Einstein equation for free diffusion in the bulk, in accordance with previous results for a similar geometry confinement [45]. Some deviations from these value are expected experimentally as the separation between glass plates varies within the sample, but deviations in D 0 are also expected to be small and thus does not greatly affect the comparison between experimental and numerical results.…”

Decoupling of translational and rotational diffusion in quasi-2D colloidal fluids

“…This is reasonable, as (1) our dimers are not the perfect dumbbell shape as assumed by theory, and (2) Eqn. 1 does not take into account the hydrodynamic influence of the bottom wall, [32] which affects translational and rotational modes differently. [30,33] It is also possible that the different values of l l for dimers ostensibly made from identical particles give rise to dimers of different shapes (different amounts of overlap of the two particles) which could account for the scatter in our data.…”

Colloidal Soft Matter Physics

“…This proposal, widely accepted and used for three decades, has been put under the experimental scrutiny [147] finding that, although the idea of factoring the effects is correct, the long-time diffusion coefficient does not seem to depend, surprisingly, on the HIs in a two-dimensional system. Similarly, in a recent theoretical-experimental contribution, the dynamics of short times in mixtures of spherical and non-spherical particles has been studied [81]; the results suggest that the dynamics of short times in asymmetric mixtures can also be uncoupled and factored in terms that depend on the HIs and the direct interactions between the molecules. It is therefore interesting to understand the connection between the dynamics of short times and those of long times [148], since they determine many of the dynamical processes that give rise to the non-equilibrium states of matter.…”

“…Among them, clusters formed by assembling hard spheres represent the first step in the direction of a new colloidal model system [79]. Preliminary investigations have demonstrated the formation of colloidal clusters or aggregates and studied their translational and rotational diffusion properties [80][81][82]. On the other hand, its phase diagram has not been fully determined and the form of the depletion forces that can be induced by either polymers or other col-loidal components of different sizes has not been investigated in detail.…”