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ABSTRACT: We report Small-Angle X-ray Scattering (SAXS) measurements of the columnar phase of hard colloidal gibbsite platelets. We have been able to create large oriented domains of the columnar phase both perpendicular and parallel to the sample wall, varying the volume fraction of platelets and adding non-adsorbing polymer to the dispersion. In conjunction with the increased resolution of the SAXS setup, this allowed a detailed analysis of the columnar phase, providing unambiguous evidence for the hexagonal nature of the phase.
The European Physical Journal E 04/2012; 16(3):253-258. · 1.94 Impact Factor
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ABSTRACT: Free-volume theory for understanding depletion phenomena in mixtures of two species is generally derived using scaled-particle theory for those specific entities. Here we first give a general scaled-particle method for convex bodies in terms of the characteristic geometrical measures of the depletion agent, i.e., its volume, surface area, and integrated mean curvature, in mixtures with hard spheres. Second, we show that similar results can be derived from fundamental-measure theory. This different approach allows us to get a deep insight into the meaning of the various contributions to the theory from a geometrical point of view. From these two methods we arrive at a generalized "recipe" to free-volume theory. This recipe can be based on a desired equation of state for any convex shape of the depletion agents and is also valid for (polydisperse) mixtures of those. This is illustrated by mixtures of spheres with ellipsoids, spheres with several geometries as models for disklike mesogens, e.g., gibbsite, as well as depletion of spheres due to bar-shaped colloids, e.g., goethite.
The Journal of Chemical Physics 07/2005; 122(21):214502. · 3.33 Impact Factor
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ABSTRACT: The behavior of mixtures of silica spheres and smaller silica-coated gibbsite plates is studied by means of confocal microscopy. Addition of plates slows down the crystallization of the settling spheres. Liquidlike microphases of the plates are found in the sediments of the spheres. It is argued that this is due to simultaneous sedimentation of the plates and spheres as well as depletion interaction between both species. Typical length scales in the sediments, derived from Fourier transforms of the confocal images, suggest there are still specific interactions present.
Physical Review E 05/2005; 71(4 Pt 1):041406. · 2.26 Impact Factor
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ABSTRACT: The crystallization of sedimentating silica spheres in the presence of silica-coated boehmite rods in low-salt dimethylformamide is studied by means of confocal scanning laser microscopy. As expected, addition of rods gives rise to a net attraction due to the depletion effect. Upon increasing rod volume fractions, below a predicted equilibrium binodal, crystalline ordering of the spheres takes place faster but gives cause for more grain boundaries. Addition of rods at volume fractions in the theoretically predicted two-phase region gives rise to aggregation and glasslike sediments. We explain these results on the basis of the different gravitational lengths and sedimentation rate of both species: higher rod concentrations drive the system quicker into the two-phase region of the predicted phase-diagram.
10/2004;
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ABSTRACT: Phase diagrams of mixtures of colloidal hard spheres with hard discs are calculated by means of the free-volume theory. The free-volume fraction available to the discs is determined from scaled-particle theory. The calculations show that depletion induced phase separation should occur at low disc concentrations in systems now experimentally available. The gas-liquid equilibrium of the spheres becomes stable at comparable size ratios as with bimodal mixtures of spheres or mixtures of rods and spheres. Introducing finite thickness of the platelets gives rise to a significant lowering of the fluid branch of the binodal.
The Journal of Chemical Physics 03/2004; 120(5):2470-4. · 3.33 Impact Factor
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ABSTRACT: The depletion potential between two large hard spheres due to the presence of hard disks has been derived up to first order in the number density of disks by Piech and Walz [J. Colloid Interface Sci. 232, 86 (2000)] using the Derjaguin approximation. Using the generalized Gibbs equation, we compare this depletion potential to the exact solution up to first order in density. The Derjaguin approximation turns out to be surprisingly accurate; for aspect ratios smaller than 0.25 the error is less than 1%.
Physical Review E 09/2003; 68(2 Pt 1):021404. · 2.26 Impact Factor
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ABSTRACT: Two different levels of the free-volume approximation for describing one-dimensional mixtures of hard rods of two different lengths are analysed and tested against the exact solution. The mean-field level free-volume approach, where only the statistically pre-averaged free volume of the larger rods is accounted for, yields a spurious phase transition. Taking correlations into account not only gives better quantitative results, it also shows qualitative right physics, i.e. no phase transition.
Physica A: Statistical Mechanics and its Applications.
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