Coexistence diameter in two-dimensional colloid-polymer mixtures

We demonstrate that the law of the rectilinear coexistence diameter in two-dimensional (2D) mixtures of non-spherical colloids and non-adsorbing polymers is violated. Upon approach of the critical point, the diameter shows logarithmic singular behavior governed by a term t ln(t), with t the relative distance from the critical point. No sign of a term t^2b could be detected, with b the critical exponent of the order parameter, indicating a very weak or absent Yang-Yang anomaly. Our analysis thus reveals that non-spherical particle shape alone is not sufficient for the formation of a pronounced Yang-Yang anomaly in the critical behavior of fluids.Comment: 4 pages, 4 figures, to appear in Phys. Rev. E (R

Reentrant glass transition in a colloid-polymer mixture with depletion attractions

Performing light scattering experiments we show that introducing short-ranged attraction to a colloidal suspension of nearly hard spheres by addition of free polymer produces new glass transition phenomena. We observe a dramatic acceleration of the density fluctuations amounting to the melting of a colloidal glass. Increasing the strength of the attractions the system freezes into another nonergodic state sharing some qualitative features with gel states occurring at lower colloid packing fractions. This reentrant glass transition is in qualitative agreement with recent theoretical predictions.Comment: 14 pages, 3 figure

Accurate description of bulk and interfacial properties in colloid-polymer mixtures

Large-scale Monte Carlo simulations of a phase-separating colloid-polymer mixture are performed and compared to recent experiments. The approach is based on effective interaction potentials in which the central monomers of self-avoiding polymer chains are used as effective coordinates. By incorporating polymer nonideality together with soft colloid-polymer repulsion, the predicted binodal is in excellent agreement with recent experiments. In addition, the interfacial tension as well as the capillary length are in quantitative agreement with experimental results obtained at a number of points in the phase-coexistence region, without the use of any fit parametersComment: 4 pages, 4 figure

Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers

An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and non-adsorbing polymers, that takes polymer non-ideality into account through a repulsive stepfunction pair potential between polymers, is studied with grand canonical Monte Carlo simulations and density functional theory. Simulation results validate previous theoretical findings for the shift of the bulk fluid demixing binodal upon increasing strength of polymer-polymer repulsion, promoting the tendency to mix. For increasing strength of the polymer-polymer repulsion, simulation and theory consistently predict the interfacial tension of the free colloidal liquid-gas interface to decrease significantly for fixed colloid density difference in the coexisting phases, and to increase for fixed polymer reservoir packing fraction.Comment: 10 pages, 4 figure

Description of the fluctuating colloid-polymer interface

To describe the full spectrum of surface fluctuations of the interface between phase-separated colloid-polymer mixtures from low scattering vector q (classical capillary wave theory) to high q (bulk-like fluctuations), one must take account of the interface's bending rigidity. We find that the bending rigidity is negative and that on approach to the critical point it vanishes proportionally to the interfacial tension. Both features are in agreement with Monte Carlo simulations.Comment: 5 pages, 3 figures, 1 table. Accepted for publication in Phys. Rev. Let

Colloidal stabilization via nanoparticle haloing

We present a detailed numerical study of effective interactions between micron-sized silica spheres, induced by highly charged zirconia nanoparticles. It is demonstrated that the effective interactions are consistent with a recently discovered mechanism for colloidal stabilization. In accordance with the experimental observations, small nanoparticle concentrations induce an effective repulsion that counteracts the intrinsic van der Waals attraction between the colloids and thus stabilizes the suspension. At higher nanoparticle concentrations an attractive potential is recovered, resulting in reentrant gelation. Monte Carlo simulations of this highly size-asymmetric mixture are made possible by means of a geometric cluster Monte Carlo algorithm. A comparison is made to results obtained from the Ornstein-Zernike equations with the hypernetted-chain closure

Wall-Fluid and Liquid-Gas Interfaces of Model Colloid-Polymer Mixtures by Simulation and Theory

We perform a study of the interfacial properties of a model suspension of hard sphere colloids with diameter $\sigma_c$ and non-adsorbing ideal polymer coils with diameter $\sigma_p$. For the mixture in contact with a planar hard wall, we obtain from simulations the wall-fluid interfacial free energy, $\gamma_{wf}$, for size ratios $q=\sigma_p/\sigma_c=0.6$ and 1, using thermodynamic integration, and study the (excess) adsorption of colloids, $\Gamma_c$, and of polymers, $\Gamma_p$, at the hard wall. The interfacial tension of the free liquid-gas interface, $\gamma_{lg}$, is obtained following three different routes in simulations: i) from studying the system size dependence of the interfacial width according to the predictions of capillary wave theory, ii) from the probability distribution of the colloid density at coexistence in the grand canonical ensemble, and iii) for statepoints where the colloidal liquid wets the wall completely, from Young's equation relating $\gamma_{lg}$ to the difference of wall-liquid and wall-gas interfacial tensions, $\gamma_{wl}-\gamma_{wg}$. In addition, we calculate $\gamma_{wf}, \Gamma_c$, and $\Gamma_p$ using density functional theory and a scaled particle theory based on free volume theory. Good agreement is found between the simulation results and those from density functional theory, while the results from scaled particle theory quantitatively deviate but reproduce some essential features. Simulation results for $\gamma_{lg}$ obtained from the three different routes are all in good agreement. Density functional theory predicts $\gamma_{lg}$ with good accuracy for high polymer reservoir packing fractions, but yields deviations from the simulation results close to the critical point.Comment: 23 pages, 10 figures, REVTEX. Fig 5a changed. Final versio

Critical phenomena in colloid-polymer mixtures: interfacial tension, order parameter, susceptibility and coexistence diameter

The critical behavior of a model colloid-polymer mixture, the so-called AO model, is studied using computer simulations and finite size scaling techniques. Investigated are the interfacial tension, the order parameter, the susceptibility and the coexistence diameter. Our results clearly show that the interfacial tension vanishes at the critical point with exponent 2\nu ~ 1.26. This is in good agreement with the 3D Ising exponent. Also calculated are critical amplitude ratios, which are shown to be compatible with the corresponding 3D Ising values. We additionally identify a number of subtleties that are encountered when finite size scaling is applied to the AO model. In particular, we find that the finite size extrapolation of the interfacial tension is most consistent when logarithmic size dependences are ignored. This finding is in agreement with the work of Berg et al.[Phys. Rev. B, V47 P497 (1993)]Comment: 13 pages, 16 figure

Critical behavior in colloid-polymer mixtures: theory and simulation

We extensively investigated the critical behavior of mixtures of colloids and polymers via the two-component Asakura-Oosawa model and its reduction to a one-component colloidal fluid using accurate theoretical and simulation techniques. In particular the theoretical approach, hierarchical reference theory [Adv. Phys. 44, 211 (1995)], incorporates realistically the effects of long-range fluctuations on phase separation giving exponents which differ strongly from their mean-field values, and are in good agreement with those of the three-dimensional Ising model. Computer simulations combined with finite-size scaling analysis confirm the Ising universality and the accuracy of the theory, although some discrepancy in the location of the critical point between one-component and full-mixture description remains. To assess the limit of the pair-interaction description, we compare one-component and two-component results.Comment: 15 pages, 10 figures. Submitted to Phys. Rev.

Phase behavior and structure of model colloid-polymer mixtures confined between two parallel planar walls

Using Gibbs ensemble Monte Carlo simulations and density functional theory we investigate the fluid-fluid demixing transition in inhomogeneous colloid-polymer mixtures confined between two parallel plates with separation distances between one and ten colloid diameters covering the complete range from quasi two-dimensional to bulk-like behavior. We use the Asakura-Oosawa-Vrij model in which colloid-colloid and colloid-polymer interactions are hard-sphere like, whilst the pair potential between polymers vanishes. Two different types of confinement induced by a pair of parallel walls are considered, namely either through two hard walls or through two semi-permeable walls that repel colloids but allow polymers to freely penetrate. For hard (semi-permeable) walls we find that the capillary binodal is shifted towards higher (lower) polymer fugacities and lower (higher) colloid fugacities as compared to the bulk binodal; this implies capillary condensation (evaporation) of the colloidal liquid phase in the slit. A macroscopic treatment is provided by a novel symmetric Kelvin equation for general binary mixtures, based on the proximity in chemical potentials of statepoints at capillary coexistence and the reference bulk coexistence. Results for capillary binodals compare well with those obtained from the classic version of the Kelvin equation due to Evans and Marini Bettolo Marconi [J. Chem. Phys. 86, 7138 (1987)], and are quantitatively accurate away from the fluid-fluid critical point, even at small wall separations. For hard walls the density profiles of polymers and colloids inside the slit display oscillations due to packing effects for all statepoints. For semi-permeable walls either similar structuring or flat profiles are found, depending on the statepoint considered.Comment: 15 pages, 13 figure