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

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The dynamics of colloidal particles at infinite dilution, under the influence of periodic external potentials, is studied here via experiments and numerical simulations for two representative potentials. From the experimental side, we analyzed the motion of a colloidal tracer in a one-dimensional array of fringes produced by the interference of two coherent laser beams, providing in this way an harmonic potential. The numerical analysis has been performed via Brownian dynamics (BD) simulations. The BD simulations correctly reproduced the experimental position- and time-dependent density of probability of the colloidal tracer in the short-times regime. The long-time diffusion coefficient has been obtained from the corresponding numerical mean square displacement (MSD). Similarly, a simulation of a random walker in a one dimensional array of adjacent cages with a probability of escaping from one cage to the next cage is one of the most simple models of a periodic potential, displaying two diffusive regimes separated by a dynamical caging period. The main result of this study is the observation that, in both potentials, it is seen that the critical time t*, defined as the specific time at which a change of curvature in the MSD is observed, remains approximately constant as a function of the height barrier U0 of the harmonic potential or the associated escape probability of the random walker. In order to understand this behavior, histograms of the first passage time of the tracer have been calculated for several height barriers U0 or escape probabilities. These histograms display a maximum at the most likely first passage time t′, which is approximately independent of the height barrier U0, or the associated escape probability, and it is located very close to the critical time t*. This behavior suggests that the critical time t*, defining the crossover between short- and long-time regimes, can be identified as the most likely first passage time t′ as a first approximation.

If a colloidal particle is exposed to an external field, its Brownian motion is modified. In the case of an anisotropic particle, the external potential might not only affect its translation but also its rotation. We experimentally investigate the dynamics of a trimer, which consists of three spherical particles, within a random potential energy landscape. This energy landscape has energy values drawn from a Gamma distribution, a spatial correlation length similar to the particle size and is realized by a random light field, that is a laser speckle pattern. The particle translation and rotation are quantified by the mean squared (angular) displacement, the van Hove function and other observable quantities. The translation shows an intermediate subdiffusive regime and a long-time diffusion that slows down upon increasing the modulation of the potential. In contrast, the mean squared angular displacement exhibits only small deviations from a linear time dependence but a more detailed analysis reveals discrete angular jumps reflecting the symmetry of the trimer. A coupling between the translation and rotation is observed and found to depend on the length scale.