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Stuttgart Mühlhausen, Germany

Gambassi A.,International School for Advanced Studies | Dietrich S.,Max Planck Institute for Intelligent Systems (Stuttgart) | Dietrich S.,Institute For Theoretische Und Angewandte Physik
Soft Matter | Year: 2011

Among the various kinds of effective forces in soft matter, the spatial range and the direction of the so-called critical Casimir force - which is generated by the enhanced thermal fluctuations close to a continuous phase transition - can be controlled and reversibly modified to an uncommonly large extent. In particular, minute temperature changes of the fluid solvent which provides the nearcritical thermal fluctuations lead to a significant change of the range and strength of the effective interaction among the solute particles. This feature allows one to control, e.g., the aggregation of colloidal dispersions or the spatial distribution of colloids in the presence of chemically or topographically patterned substrates. The spatial direction of the effective force acting on a solute particle depends only on the surface properties of the immersed particles and can be spatially modulated by suitably patterned surfaces. These critical Casimir forces are largely independent of the specific materials properties of both the solvent and the confining surfaces. This characteristic universality of critical phenomena allows systematic and quantitative theoretical studies of the critical Casimir forces in terms of suitable representative and simplified models. Here we highlight recent theoretical and experimental advances concerning critical Casimir forces with a particular emphasis on the numerous possibilities of controlling these forces by patterned substrates. © 2011 The Royal Society of Chemistry.

Bier M.,University of Stuttgart | Van Roij R.,Institute For Theoretische Und Angewandte Physik | Van Roij R.,University Utrecht | Dijkstra M.,University Utrecht
Journal of Chemical Physics | Year: 2010

Phase diagrams of binary mixtures of oppositely charged colloids are calculated theoretically. The proposed mean-field-like formalism interpolates between the limits of a hard-sphere system at high temperatures and the colloidal crystals which minimize Madelung-like energy sums at low temperatures. Comparison with computer simulations of an equimolar mixture of oppositely charged, equally sized spheres indicate semiquantitative accuracy of the proposed formalism. We calculate global phase diagrams of binary mixtures of equally sized spheres with opposite charges and equal charge magnitude in terms of temperature, pressure, and composition. The influence of the screening of the Coulomb interaction upon the topology of the phase diagram is discussed. Insight into the topology of the global phase diagram as a function of the system parameters leads to predictions on the preparation conditions for specific binary colloidal crystals. © 2010 American Institute of Physics.

Kondrat S.,University of Stuttgart | Kondrat S.,Institute For Theoretische Und Angewandte Physik | Bier M.,University of Stuttgart | Bier M.,Institute For Theoretische Und Angewandte Physik | And 2 more authors.
Journal of Chemical Physics | Year: 2010

Bulk properties of ionic liquid crystals are investigated using density functional theory. The liquid crystal molecules are represented by ellipsoidal particles with charges located in their center or at their tails. Attractive interactions are taken into account in terms of the Gay-Berne pair potential. Rich phase diagrams involving vapor, isotropic and nematic liquid, as well as smectic phases are found. The dependence of the phase behavior on various parameters such as the length of the particles and the location of charges on the particles is studied. © 2010 American Institute of Physics.

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