Influence of a depletion interaction on dynamical heterogeneity in a dense quasi-two-dimensional colloid liquid

Dynamical heterogeneities as fingerprints of a backbone structure in Potts models

We investigate slow nonequilibrium dynamical processes in a two-dimensional q-state Potts model with both ferromagnetic and ±J couplings. Dynamical properties are characterized by means of the mean-flipping time distribution. This quantity is known for clearly unveiling dynamical heterogeneities. Using a two-times protocol we characterize the different time scales observed and relate them to growth processes occurring in the system. In particular we target the possible relation between the different time scales and the spatial heterogeneities originated in the ground-state topology, which are associated to the presence of a backbone structure. We perform numerical simulations using an approach based on graphis processing units (GPUs) which permits us to reach large system sizes. We present evidence supporting both the idea of a growing process in the preasymptotic regime of the glassy phases and the existence of a backbone structure behind this process.

The quasi-one-dimensional colloid fluid revisited

We report the results of studies of the pair correlation function and equation of state of a quasi-one-dimensional colloid suspension, focusing attention on the behavior in the density range near close packing. Our data show that, despite deviations from true one-dimensional geometry, the colloid fluid is well described as a hard rod Tonks fluid. In our experimental realization, the colloid suspension does not wet the confining walls, one consequence of which is a surface tension induced weak attractive interaction between the particles. The reality of this interaction is confirmed after correction of the raw experimental data for overlap of the optical images of particles that are nearly in contact and by an alternative particle location algorithm based on edge location.

Diffusive and structural properties in a binary mixture of charged particles

Diffusive and structural properties in the binary mixture of charged particles are investigated based on the molecular dynamical simulation. It is shown that the particles with higher charge enhance or weaken the diffusion of the whole system depending on the coupling strength, and regimes of subdiffusion as well as normal diffusion are identified. The nonmonotonous diffusion, i.e., the diffusion at first is increased and then is decreased with increasing the higher charged particles, is strongest close to the phase from liquidlike state to solidlike state. Stronger coupling and lower concentration of higher charged particles are in favor of the formation of ordered shells centered at the higher charged particles.

Pair diffusion in quasi-one- and quasi-two-dimensional binary colloid suspensions

The authors report the results of measurements of the center of mass and relative pair diffusion coefficients in quasi-one-dimensional (q1D) and quasi-two-dimensional (q2D) binary colloid suspensions. The new results extend the findings of similar studies of one-component quasi-one-dimensional and quasi-two-dimensional colloid suspensions. Our principal new finding is that the presence of the smaller diameter component can destroy the oscillatory structure of the separation dependence of the q2D relative pair diffusion coefficient of the large particles even though the oscillatory character of the large particle equilibrium pair correlation function remains prominent, and that no such effect occurs with the q1D suspension. An interpretation of these results is proposed.

Three-particle correlation functions of quasi-two-dimensional one-component and binary colloid suspensions

We report the results of experimental determinations of the triplet correlation functions of quasi-two-dimensional one-component and binary colloid suspensions in which the colloid-colloid interaction is short ranged. The suspensions studied range in density from modestly dilute to solid. The triplet correlation function of the one-component colloid system reveals extensive ordering deep in the liquid phase. At the same density the ordering of the larger diameter component in a binary colloid system is greatly diminished by a very small amount of the smaller diameter component. The possible utilization of information contained in the triplet correlation function in the theory of melting of a quasi-two-dimensional system is briefly discussed.

Like-charge attraction in confinement: myth or truth?

It is general wisdom that like-charged colloidal particles repel each other when suspended in liquids. This is in perfect agreement with mean field theories being developed more than 60 years ago. Accordingly, it was a big surprise when several groups independently reported long-ranged attractive components in the pair potential () of equally charged colloids. This so-called (LCA) was only observed in thin sample cells while the pair-interaction in unconfined suspensions has been experimentally confirmed to be entirely repulsive. Despite considerable experimental and theoretical efforts, LCA remains one of the most challenging mysteries in colloidal science. We experimentally reinvestigate the pair-potential () of charged colloidal particles with digital video microscopy and demonstrate that optical distortions in the particle's images lead to slightly erroneous particle positions. If not properly taken into account, this artefact pretends a minimum in () which was in the past misleadingly interpreted as LCA. After correcting optical distortions we obtain entirely repulsive pair interactions which show good agreement with linearized mean field theories.

Anomalous behavior of the depletion potential in quasi-two-dimensional binary mixtures

We report an experimental determination of the depletion interaction between a pair of large colloid particles present in a binary colloid mixture that has a high density of large particles and is tightly confined between two parallel plates, as a function of the small colloid particle density. The bare interaction between the large particles in the one component large colloid suspension, and the effective potential between the large particles in the binary colloid suspension represented as a pseudo-one-component fluid, were obtained by inverting the Ornstein-Zernike equation with the hypernetted chain closure. The depletion interaction is defined by subtracting the bare potential from the effective potential at fixed large colloid density. We find that the depletion potential in the quasi-two-dimensional (Q2D) system is purely attractive and short ranged as described by Asakura-Oosawa model. However, the depth of the depletion potential is found to be almost an order of magnitude larger than the counterpart depletion potential predicted for the same density and diameter ratio in a three-dimensional system. Although it is expected that the confining walls in the Q2D geometry enhance the excluded volume effects that generate entropic attraction, the observed enhancement is much larger than predicted for a Q2D binary mixture of hard spheres. We speculate that this anomalously strong confinement-induced depletion potential is a signature of characteristics of the real confined binary colloid mixture that are not included in any extant theory of the depletion interaction, specifically the omission of the role of the solvent in those theories. One such characteristic could be differential wall or particle wetting that generates a wall induced one-particle effective potential that confines the centers of the small particles to lie closer to the midplane between the walls than expected from the wall separation and the direct particle-wall interaction, thereby enhancing the depletion interaction.