The Effects of Inter-particle Attractions on Colloidal Sedimentation

We use a mesoscopic simulation technique to study the effect of short-ranged inter-particle attraction on the steady-state sedimentation of colloidal suspensions. Attractions increase the average sedimentation velocity $v_s$ compared to the pure hard-sphere case, and for strong enough attractions, a non-monotonic dependence on the packing fraction $\phi$ with a maximum velocity at intermediate $\phi$ is observed. Attractions also strongly enhance hydrodynamic velocity fluctuations, which show a pronounced maximum size as a function of $\phi$. These results are linked to a complex interplay between hydrodynamics and the formation and break-up of transient many-particle clusters.Comment: 4 pages 4 figure

How Peclet number affects microstructure and transient cluster aggregation in sedimenting colloidal suspensions

We study how varying the P \'eclet number (Pe) affects the steady state sedimentation of colloidal particles that interact through short-ranged attractions. By employing a hybrid molecular dynamics simulation method we demonstrate that the average sedimentation velocity changes from a non- monotonic dependence on packing fraction {\phi} at low Pe numbers, to a monotonic decrease with {\phi} at higher Pe numbers. At low Pe number the pair correlation functions are close to their equilibrium values, but as the Pe number increases, important deviations from equilibrium forms are observed. Although the attractive forces we employ are not strong enough to form permanent clusters, they do induce transient clusters whose behaviour is also affected by Pe number. In particular, clusters are more likely to fragment and less likely to aggregate at larger Pe numbers, and the probability of finding larger clusters decreases with increasing Pe number. Interestingly, the life-time of the clusters is more or less independent of Pe number in the range we study. Instead, the change in cluster distribution occurs because larger clusters are less likely to form with increasing Pe number. These results illustrate some of the subtleties that occur in the crossover from equilibrium like to purely non-equilibrium behaviour as the balance between convective and thermal forces changes.Comment: 8 page

Crowding of Polymer Coils and Demixing in Nanoparticle-Polymer Mixtures

The Asakura-Oosawa-Vrij (AOV) model of colloid-polymer mixtures idealizes nonadsorbing polymers as effective spheres that are fixed in size and impenetrable to hard particles. Real polymer coils, however, are intrinsically polydisperse in size (radius of gyration) and may be penetrated by smaller particles. Crowding by nanoparticles can affect the size distribution of polymer coils, thereby modifying effective depletion interactions and thermodynamic stability. To analyse the influence of crowding on polymer conformations and demixing phase behaviour, we adapt the AOV model to mixtures of nanoparticles and ideal, penetrable polymer coils that can vary in size. We perform Gibbs ensemble Monte Carlo simulations, including trial nanoparticle-polymer overlaps and variations in radius of gyration. Results are compared with predictions of free-volume theory. Simulation and theory consistently predict that ideal polymers are compressed by nanoparticles and that compressibility and penetrability stabilise nanoparticle-polymer mixtures.Comment: 18 pages, 4 figure

The Asakura-Oosawa model in the protein limit: the role of many-body interactions

We study the Asakura-Oosawa model in the "protein limit", where the penetrable sphere radius $R_{AO}$ is much greater than the hard sphere radius $R_c$. The phase behaviour and structure calculated with a full many-body treatment show important qualitative differences when compared to a description based on pair potentials alone. The overall effect of the many-body interactions is repulsive.Comment: 9 pages and 11 figures, submitted to J. Phys.: Condensed Matter, special issue "Effective many-body interactions and correlations in soft matter

Effective interaction in asymmetric charged binary mixtures: The non-monotonic behaviour with the colloidal charge

In this work we study the effective force between charged spherical colloids induced by the presence of smaller charged spheres using Monte Carlo simulations. The analysis is performed for two size ratios, q = R s/R b, two screened direct repulsions, $\kappa$, and two small particle packing fractions, $\phi_{s}$ . We specially focus on the effect of the charge of the big colloids (Zb), and observe that the repulsion between big particles shows a non-monotonic behaviour: for sufficiently small charge, we find an anomalous regime where the total repulsion weakens by increasing the big colloid charge. For larger charges, the system recovers the usual behaviour and the big-big interaction becomes more repulsive increasing Zb. This effect is linked to the existence of strong attractive depletion interactions caused by the small-big electrostatic repulsion. We have also calculated the effective force using the Ornstein-Zernike equation with the HNC closure. In general, this theory agrees with the simulation results, and is able to capture this non-monotonic behaviour

Density profiles and solvation forces for a Yukawa fluid in a slit pore.

The effect of varying wall-particle and particle-particle interactions on the density profiles near a single wall and the solvation forces between two walls immersed in a fluid of particles is investigated by grand canonical Monte Carlo simulations. Attractive and repulsive particle-particle and particle-wall interactions are modeled by a versatile hard-core Yukawa form. These simulation results are compared to theoretical calculations using the hypernetted chain integral equation technique, as well as with fundamental measure density functional theory (DFT), where particle-particle interactions are either treated as a first order perturbation using the radial distribution function or else with a DFT based on the direct-correlation function. All three theoretical approaches reproduce the main trends fairly well, but exhibit inconsistent accuracy, particularly for attractive particle-particle interactions. We show that the wall-particle and particle-particle attractions can couple together to induce a nonlinear enhancement of the adsorption and a related "repulsion through attraction" effect for the effective wall-wall forces. We also investigate the phenomenon of bridging, where an attractive wall-particle interaction induces strongly attractive solvation forces

Brownian dynamics simulation of monolayer formation by deposition of colloidal particles: A kinetic study at high bulk particle concentration

Brownian dynamics simulations (BDS) of sedimentation and irreversible adsorption of colloidal particles on a planar surface were carried out at bulk particle volume fractions (φ) in the range 0.05 to 0.25. The sedimentation and adsorption of colloidal particles were simulated as a non-sequential process that allows simultaneous settling and adsorption of particles. A kinetic model for the formation of particle monolayers based on the available surface fraction (θ A ) is proposed to predict simulation results. The simulations show a value of 0.625 for the maximum fractional surface coverage (θ ∞) and a monolayer structure insensitive to φ. However, the kinetic order of the monolayer formation process has a strong dependence with φ, changing from a value close to a unit, at low φ, to a value around two at high φ. This change in the kinetic reaction order is associated to differences of particle adsorption mechanism on the surface. At low φ values, the monolayer formation is achieved by independent adsorption of single particles and the reaction order is close to 1. At high φ values, the simultaneous adsorption of two particles on the surface leads to an increase of the reaction order to values close to 2

Structure of charged colloid-polymer mixtures

We use a light scattering technique to investigate the effect of adding non-adsorbing charged polymers to a very dilute electrostatically stabilized colloidal suspension at low electrolyte concentration. The experimental results show that, as the polymer concentration increases, the main peak of the colloid-colloid structure factor moves to higher q-values, which cannot be only due to the screening of the direct colloid-colloid electrostatic repulsion. We show that the colloid-polymer electrostatic repulsions lead to enhanced depletion forces that have a strong influence on the colloid structure, even for diluted suspensions. The experimental results are interpreted using the off-lattice Polymer Reference Interaction Site Model (PRISM), and very good agreement is found for all polymer concentrations