Article

# Sedimentation and multi-phase equilibria in mixtures of platelets and ideal polymer

EPL (Europhysics Letters) (Impact Factor: 2.27). 01/2007; 66(1):125. DOI: 10.1209/epl/i2003-10140-1

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**ABSTRACT:**Studies of temperature-dependent phase behaviors of discotic colloids are found infrequently in the literature. We establish here the isotropic-to-nematic (I-N) transition phase diagram of charged platelets in the temperature-versus-volume fraction plane. We discover that the N phase can be melted by increasing temperature, and that coexistent samples are more sensitive to polydispersity at higher temperature and higher concentration.Physical Review E 08/2014; 90(2-1):020504. · 2.33 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Recent work on mineral reactions and microstructures in metamorphic rocks has focused on forward modelling of phase equilibria and on their description through chemical potential relationships which control mass transfer in rocks. The available thermodynamic databases and computer programs for phase equilibria modelling have significantly improved the quantification and understanding of geodynamic processes. Therefore, our current methodological framework seems to be satisfactory. However, the quantification approaches in petrology focus on chemical processes with oversimplified mechanics. A review of the recent literature shows that mechanical effects in rocks may result in the development of pressure variations even on a hand specimen or grain scale. Such variations are critical for interpreting microstructural and mineral composition observations in rocks. Mechanical effects may influence element transport and mineral assemblage in rocks. Considering the interplay of mechanical properties and metamorphic reactions is therefore crucial for a correct interpretation of microstructural observations in metamorphic rocks as well as for quantification of processes. In this contribution, arguments against pressure variations are inspected and disproved. The published quantification procedure for systems with grain-scale pressure variations is reviewed. We demonstrate the equivalence of using Gibbs and Helmholtz energy in an isobaric system and go on to suggest that Gibbs free energy is more convenient for systems with pressure variations. Furthermore, we outline the implications of the new quantification approach for phase equilibria modeling as well as diffusion modeling. The appropriate modification of a macroscopic flux for a system with a pressure variation is derived and a consequence of using mass or molar units in diffusional fluxes is discussed. The impact of ignoring grain-scale pressure variations on geodynamic modeling and our understanding of the processes in the Earth’s interior is assessed. We show that if a pressure variation is overlooked, the error in depth estimates from crustal metamorphic rocks could be as large as the thickness of the crust.Lithos 01/2015; 36. · 3.65 Impact Factor -
##### Article: Sedimentation dynamics and equilibrium profiles in multicomponent mixtures of colloidal particles

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**ABSTRACT:**In this paper we give a general theoretical framework that describes the sedimentation of multicomponent mixtures of particles with sizes ranging from molecules to macroscopic bodies. Both equilibrium sedimentation profiles and the dynamic process of settling, or its converse, creaming, are modeled. Equilibrium profiles are found to be in perfect agreement with experiments. Our model reconciles two apparently contradicting points of view about buoyancy, thereby resolving a long-lived paradox about the correct choice of the buoyant density. On the one hand, the buoyancy force follows necessarily from the suspension density, as it relates to the hydrostatic pressure gradient. On the other hand, sedimentation profiles of colloidal suspensions can be calculated directly using the fluid density as apparent buoyant density in colloidal systems in sedimentation-diffusion equilibrium (SDE) as a result of balancing gravitational and thermodynamic forces. Surprisingly, this balance also holds in multicomponent mixtures. This analysis resolves the ongoing debate of the correct choice of buoyant density (fluid or suspension): both approaches can be used in their own domain. We present calculations of equilibrium sedimentation profiles and dynamic sedimentation that show the consequences of these insights. In bidisperse mixtures of colloids, particles with a lower mass density than the homogeneous suspension will first cream and then settle, whereas particles with a suspension-matched mass density form transient, bimodal particle distributions during sedimentation, which disappear when equilibrium is reached. In all these cases, the centers of the distributions of the particles with the lowest mass density of the two, regardless of their actual mass, will be located in equilibrium above the so-called isopycnic point, a natural consequence of their hard-sphere interactions. We include these interactions using the Boublik-Mansoori-Carnahan-Starling-Leland (BMCSL) equation of state. Finally, we demonstrate that our model is not limited to hard spheres, by extending it to charged spherical particles, and to dumbbells, trimers and short chains of connected beads.Journal of Physics Condensed Matter 01/2014; 26(7):075101. · 2.22 Impact Factor

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