Structurally determined Brownian dynamics in ordered colloidal suspensions: Self-diffusion in fluid, supercooled, and crystalline phases (original) (raw)
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Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1996
An important role of electrical double layers in the structural, rheological, and diffusional properties of colloidal suspensions, especially in the deionized state, is discussed. Formation of giant colloidal single crystals, which are brilliantly colored and very beautiful, is due to the electrostatic intersphere repulsion and to the highly expanded electrical double layers surrounding colloidal spheres. The phase diagram, rigidity and viscosity of the colloidal crystals are nicely explained with the contribution of the electrical double layers. The translational and rotational diffusion coefficients of colloidal particles are quite sensitive to the ionic concentration of the suspension, which is also explained by the thinning of the electrical double layers with increasing ionic concentration. Furthermore, diffusive modes in the colloidal crystals and liquids analyzed by dynamic light scattering measurements are consistent with the important contribution of the electrical double layers.
Solidification of colloidal suspensions
We present a mathematical model of the unidirectional solidification of a suspension of hard-sphere colloids. Similarity solutions are obtained for the volume fraction and temperature profiles ahead of a planar solidification front. The highly nonlinear functional dependence of the diffusion coefficient on the volume fraction gives rise to a range of behaviours. For small particles, Brownian diffusion dominates and the system behaviour is reminiscent of binary-alloy solidification. Constitutional supercooling occurs at the interface under certain conditions, leading potentially to an instability in the shape of the interface. For larger particles, Brownian diffusion is weak and the particles form a porous layer above the interface. In this case constitutional supercooling reaches a maximum near the surface of the layer, and the porous medium itself is potentially unstable. In stable systems there exists the possibility of secondary nucleation of ice.
Amorphous and crystalline states of ultrasoft colloids: a molecular dynamics study
Rheologica Acta, 2007
In this work, we study the temperature-induced development of "dynamically arrested" states in dense suspensions of "soft colloids" (multi-arm star polymers and/or block-copolymers micelles) by means of molecular dynamics (MD) simulations. Temperature increase in marginal solvents results in "soft sphere" swelling, dynamical arrest, and eventually crystallization. However, two distinct "dynamically arrested" states were found, one almost amorphous ("glassy") and one with a considerable degree of crystallinity, yet lower than that of the fully equilibrated crystal. It is remarkable that even that latter state permitted self-diffusion in the timescale of the simulations, an effect that underlies the importance of the "ultra-soft" nature of inter-particle potential. The "number of connections" criterion for crystallinity proved to be very successful in identifying the ultimate thermodynamic trend from the very early stages of the α-relaxation.
Rapid colloidal solidifications under local nonequilibrium diffusion conditions
Partition coefficient for rapid solidification of colloidal suspensions has been calculated under local nonequilibrium diffusion conditions typically used when processing advanced materials. It has been demonstrated that the local nonequilibrium diffusion effects stabilize the planar solid liquid interface and lead to an abrupt transition to diffusionless solidification with complete particle trapping. The effective diffusion coefficient, which depends on interface velocity and particle size, has been introduced. It explains the strong dependences of the partition coefficient and the velocity leading to absolute stability of a planar solid-liquid interface on particle size.
Physical Review E, 2013
A two-dimensional crystal of repulsive dipolar particles is studied in the vicinity of its melting transition by using Brownian dynamics computer simulation, dynamical density functional theory and phase-field crystal modelling. A vacancy is created by taking out a particle from an equilibrated crystal and the relaxation dynamics of the vacancy is followed by monitoring the time-dependent one-particle density. We find that the vacancy is quickly filled up by diffusive hopping of neighbouring particles towards the vacancy center. We examine the temperature dependence of the diffusion constant and find that it decreases with decreasing temperature in the simulations. This trend is reproduced by the dynamical density functional theory. Conversely, the phase field crystal calculations predict the opposite trend. Therefore, the phase-field model needs a temperature-dependent expression for the mobility to predict trends correctly.
Interactions, structural ordering and phase transitions in colloidal dispersions
Advances in Colloid and Interface Science, 1998
The structural ordering in colloidal dispersions is found to be very similar to that of atomic systems, such as crystalline solids, atomic liquids and even glasses. A number of intrinsic as well as extrinsic parameters influence the stability of colloids and induce transitions between different phases. It is the richness of the phase behavior that makes the colloids interesting also from the fundamental point of view. This article reviews the recent advances in the area of ordering and phase transitions brought about by parameters such as particle volume fraction, surface charge density, polydispersity, added electrolyte and external fields, such as shear, electric, magnetic and laser optical fields. Some of the recent experimental techniques that provide insight into the ordering phenomena are also covered. Microscopic investigations of suspensions under confined geometries and their implications on current understanding of the effective interparticle interaction are discussed. Finally, recent efforts in the direction of epitaxial growth of ordered structures on specially designed templates and their applications in synthesizing advanced materials are also briefly reviewed.
Ordering and single-file diffusion in colloidal systems
Chemical Physics, 2010
The structural properties and the single-file diffusion in one-dimensional interacting colloidal systems are studied by means of Brownian dynamics simulations. We consider three types of particle interactions, namely, Weeks-Chandler-Andersen, screened Coulomb, and superparamagnetic potentials. We find that, regardless of the interaction potential, at low densities particles are distributed in a typical fluidlike structure and at higher densities or potential strengths become spatially correlated at long-distances. Particularly, our findings demonstrate that one-dimensional systems, with particles interacting repulsively, show common structural and dynamical behaviors at the boundary in which the degree of ordering changes dramatically; the main peak of the static structure factor becomes highly narrow with a height of Sc~7, whereas the reduced mobility factor F, which is associated with the single-file diffusion at long-times or long wavelengths, reaches values F*~0.1. These features are analyzed and discussed in the context of a local order-disorder transition.
Role of diffusion in crystallization of hard-sphere colloids
Physical Review E, 2021
Vital for a variety of industries, colloids also serve as an excellent model to probe phase transitions at the individual particle level. Despite extensive studies, origins of the glass transition in hard-sphere colloids discovered about 30 y ago remain elusive. Results of our numerical simulations and asymptotic analysis suggest that cessation of long-time particle diffusivity does not suppress crystallization of a metastable liquid-phase hard-sphere colloid. Once a crystallite forms, its growth is then controlled by the particle diffusion in the depletion zone surrounding the crystallite. Using simulations, we evaluate the solid-liquid interface mobility from data on colloidal crystallization in terrestrial and microgravity experiments and demonstrate that there is no drastic difference between the respective mobility values. The insight into the effect of vanishing particle mobility and particle sedimentation on crystallization of colloids will help engineer colloidal materials with controllable structure.
Structural and dynamical analysis of monodisperse and polydisperse colloidal systems
The Journal of Chemical Physics, 2010
We present a semigrand ensemble Monte Carlo and Brownian dynamics simulation study of structural and dynamical properties of polydisperse soft spheres interacting via purely repulsive powerlaw potentials with a varying degree of "softness." Comparisons focus on crystal and amorphous phases at their coexistence points. It is shown through detailed structural analysis that as potential interactions soften, the "quality of crystallinity" of both monodisperse and polydisperse systems deteriorates. In general, polydisperse crystalline phases are characterized by a more ordered structure than the corresponding monodisperse ones (i.e., for the same potential softness). This counter-intuitive feature originates partly from the fact that particles of different sizes may be accommodated more flexibly in a crystal structure and from the reality that coexistence (osmotic) pressure is substantially higher for polydisperse systems. These trends diminish for softer potentials. Potential softness eventually produces substitutionally disordered crystals. However, substitutional order is apparent for the hard-spherelike interactions. Diffusionwise, crystals appear quite robust with a slight difference in the vibrational amplitudes of small and large particles. This difference, again, diminishes with potential softness. Overcrowding in amorphous polydisperse suspensions causes "delayed" diffusion at intermediate times.