14 Externe wissenschaftliche Einrichtungen
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Item Open Access Phase behavior of colloidal suspensions with critical solvents(2013) Mohry, Thomas F.; Dietrich, Siegfried (Prof. Dr.)Colloidal suspensions, i.e., nano- to micrometer sized particles immersed in a solvent, are interesting, both for applications which are present in various kinds in our daily life as well as for exploring fundamental physics. In this work colloidal suspensions are studied the solvents of which are binary liquid mixtures which exhibit a miscibility gap. The focus is set on the thermodynamic region close to the critical point of demixing of this solvent. In that region the fluctuations of the concentration of the two components forming the binary liquid mixture are correlated on meso- to macroscopic length scales. The spectrum of these correlations and the concentration profile of the solvent particles are affected by the presence of the colloidal solute particles. Among others, these alterations result in an effective force acting between the solute particles. Close to the critical point of the solvent this effective force acquires an universal contribution which is known as the critical Casimir force. This effective force acting between surfaces confining a critical medium is studied, in particular its dependence on the bulk ordering field. In the case of a binary liquid mixture the bulk ordering field is related to the deviation of the concentration of the solvent from its critical value. A small value of the bulk ordering field can enhance the strength of the critical Casimir force up to ten times as compared with its value for zero bulk field. The critical Casimir force which acts between solute particles with the same adsorption preferences for one of the two components forming the solvent is attractive. This attraction, which can be tuned by minute temperature changes, can lead to reversible aggregation of the solute particles. These aggregation phenomena are studied theoretically, e.g., in terms of the radial distribution function or the second virial coefficient. The effective attraction due to the critical Casimir forces may also lead to a phase separation into a colloidal rich and a colloidal poor phase. Colloidal suspensions are often treated as effective one-component systems of colloidal particles between which solvent mediated effective interactions act. This effective (and often successful) description is shown to fail in general in the presence of phase-separating solvents. The thermodynamics and the phase diagram of such colloidal suspensions are discussed thoroughly and approaches for the description of the full ternary mixture are presented. The theoretical results are compared in detail with in the literature available experimental data.