Search

We study experimentally and theoretically the hydrodynamic coupling between Brownian colloidal particles diffusing along a linear channel. The quasi-one-dimensional confinement, unlike other constrained geometries, leads to a sharply screened interaction. Consequently, particles move in concert only when their mutual distance is smaller than the channel width, and two-body interactions remain dominant up to high particle densities. The coupling in a cylindrical channel is predicted to reverse sign at a certain distance, yet this unusual effect is too small to be currently detectable.

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. [ABSTRACT FROM AUTHOR]

We report the results of digital video microscopy studies of the large particle displacements in a quasi-two-dimensional binary mixture of large (L) and small (S) colloid particles with diameter ratio σL/σS=4.65, as a function of the large and small colloid particle densities. As in the case of the one-component quasi-two-dimensional colloid system, the binary mixtures exhibit structural and dynamical heterogeneity. The distribution of large particle displacements over the time scale examined provides evidence for (at least) two different mechanisms of motion, one associated with particles in locally ordered regions and the other associated with particles in locally disordered regions. When ρL*=NπσL2/4A≤0.35, the addition of small colloid particles leads to a monotonic decrease in the large particle diffusion coefficient with increasing small particle volume fraction. When ρL*>=0.35 the addition of small colloid particles to a dense system of large colloid particles at first leads to an increase in the large particle diffusion coefficient, which is then followed by the expected decrease of the large particle diffusion coefficient with increasing small colloid particle volume fraction. The mode coupling theory of the ideal glass transition in three-dimensional systems makes a qualitative prediction that agrees with the initial increase in the large particle diffusion coefficient with increasing small particle density. Nevertheless, because the structural and dynamical heterogeneities of the quasi-two-dimensional colloid liquid occur within the field of equilibrium states, and the fluctuations generate locally ordered domains rather than just disordered regions of higher and lower density, it is suggested that mode coupling theory does not account for all classes of relevant fluctuations in a quasi-two-dimensional liquid. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the results of a study of crystallization in quasi-two-dimensional binary mixtures of large and small colloids. The experiments sample the parameter spaces of colloid particle diameter ratio, large particle density, and small particle packing fraction. The depletion potential between the large particles, induced by the presence of the small particles in the system, affects the density at which the large particles undergo a liquid-to-solid freezing transition. For systems with a large to small particle diameter ratio of 4.6, the addition of small particles increases the large particle liquidus transition density, a seemingly counterintuitive result given that the depletion potential is purely attractive when the small particle packing fraction is low. When the large to small particle diameter ratio is 8.8, the same trend in the large particle liquidus transition density is seen, but to a lesser extent. The other system properties for the system with diameter ratio 8.8 show the same trends as for the system with diameter ratio 4.6. Liquid-liquid phase separation is observed for binary mixtures with diameter ratios of 20 and 40. Although the particles used in our experiment can be well modeled as hard spheres, our results cannot be readily explained by extant descriptions of the depletion interaction developed for three-dimensional binary hard sphere mixtures. Inversion of the pair correlation functions obtained from our measurements yields a depletion interaction that is much stronger than predicted for the same densities and diameter ratio in a three-dimensional hard sphere mixture. Our results imply that the depletion interaction is strongly dependent on the degree of confinement of the system. [ABSTRACT FROM AUTHOR]

We report the results of studies, using digital video microscopy, of the spatial distribution of particles in a quasi-one-dimensional colloidal liquid. The system studied consists of a water suspension of silica particles with diameter σ=1.58 μm confined in a 3 μm×3 μm×2 mm silicone elastomer channel. The measured spatial distributions have been characterized by the nearest-neighbor distribution function and the pair correlation function, and the pair correlation function has been inverted, using the hypernetted chain approximation, to yield the effective colloid–colloid interaction. The effective colloid–colloid interaction is found to have an attractive potential well with depth of ∼0.3k[sub B]T at a colloid-colloid separation Rapprox. 1.18σ. Molecular-dynamics simulations of the nearest neighbor and pair correlation functions carried out using the empirically determined colloid–colloid interaction are in very good agreement with the experimental data. The experimental data obtained rule out any important charge–charge contribution to the measured colloid–colloid interaction. It is suggested that because the host liquid does not wet the channel wall, and the colloid particles are tightly confined in the long narrow channel, capillary forces determine the effective colloid–colloid interaction. This suggestion is consistent with the determination (also reported) that the effective colloid–colloid interaction in a quasi-two-dimensional suspension of the same particles is different from that in the quasi-one dimensional suspension. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper reports the results of experimental studies of the dynamics of particles in a dense quasi-two-dimensional colloidal liquid. We find that at high density, near close packing but still in the liquid phase, the spatial configurations of the particles in the colloidal liquid consist of small ordered domains separated by disordered boundaries. There are frequent exchanges of particles between the ordered and disordered domains, so the lifetime of a particular ordered domain is short and the state of the system is ergodic. The motion of a particle in an ordered domain is constrained but fully two dimensional. The motion of a particle in a disordered boundary has considerable one-dimensional file-server character. By virtue of exchanges of particles between the ordered and disordered domains, the time dependence of the particle displacement has mixed character. We find that the particle dynamics in the dense quasi-two-dimensional colloidal liquid can be characterized with three simultaneous competing relaxation processes, each of which contributes to the mean square displacement of a particle, W(t). For an interval that is shorter than the time between collisions, W(t) is linear in time; this behavior results from particle displacements within a cage of fluctuating neighbors. At somewhat longer time W(t) has a sublinear dependence on time. We associate this temporal behavior with a file-server-like contribution to W(t), arising from correlated motion in the disordered boundary regions. At very long time we find that there are contributions to W(t) from infrequent large displacements, of the order of a particle diameter in length. The large displacements are associated with density fluctuations that fluidize the surroundings of a particle. These large displacements are not ballistic; rather the particle diffuses rapidly enough to generate a large displacement per unit time. The common occurrence of structural heterogeneity in simulation studies of syste... [ABSTRACT FROM AUTHOR]