Project

Discipline

crystal nucleation; phase transitions; microscopy; colloids; microgels; scattering; hard spheres

Lead
Die Entstehung von Kristallen in Flüssigkeiten oder Gasen (homogene Kristallbildung) oder auf Substraten (heterogene Kristallbildung) wird schon seit Jahrzehnten untersucht. Unter vielen thermodynamischen Bedingungen ist die Kristallbildung unterdrückt, da die Bildung kleiner Kristallkeime energetisch ungünstig ist. Aufgrund der Inkompatibilität des flüssigen oder gasförmigen mit dem kristallinen Zustand, muss eine Energiebarriere überwunden werden, um Kristallkeime zu bilden, die zu grossen Kristalliten weiter wachsen können.Die initiale Bildung der kleinsten Kristallkeime ist allerdings noch immer nicht gut verstanden, was sich in grossen Unterschieden zwischen gemessenen und berechneten Kristall-Bildungsraten widerspiegelt. Deshalb sind direkte Untersuchungen der Bildung der kleinsten Kristallkeime von grossem Interesse, um einen detaillierten Einblick in den Bildungsprozess der Kristallkeime zu erhalten.
Lay summary
In diesem Projekt wird die heterogene Kristallbildung in kolloidalen Suspensionen (Teilchen im Nano- oder Mikrometerbereich in Lösung) auf gekrümmten Oberflächen untersucht. Kolloidale Suspensionen sind experimentell besonders zugänglich und erlauben es, die Bildung kleinster Kristallkeime direkt zu verfolgen. Die Kristallbildung auf Oberflächen ist für Anwendungen besonders interessant, da sie eine Kontrolle über den Kristallisationsprozess ermöglichen. Der Einfluss der Krümmung des Substrats wird systematisch untersucht, um einerseits den Übergang von einer Behinderung der Kristallisation bei starker Krümmung zu einer Bevorzugung der Kristallisation auf flachen Substraten zu studieren und andererseits um den Einfluss von Oberflächen mit positiver und negativer Krümmung als Funktion des Krümmungsgrades zu verglichen. Die Untersuchungen werden mit kugelförmigen und Torus-förmigen Substraten durchgeführt. Für mögliche Anwendungen sollen Substrate mit besonders guten Eigenschaften für die Kontrolle der Kristallbildung identifiziert werden.

Direct link to Lay Summary

Last update: 23.06.2014

Active participation

Self-organised

Communication	Title	Media	Place	Year

Title	Year

Number	Title	Start	Funding scheme

Although the 1st order phase transition of crystal nucleation has been studied for decades, the understanding of both homogenous and heterogenous crystal nucleation are still far from being understood in detail. This is highlighted by the large differences between experimental and theoretical results for the nucleation density rate in many materials. It is generally accepted that the free energy difference between fluid and crystal, the free energy barrier for nucleation, is the key for a detailed understanding of crystal nucleation. However, the properties of small crystal nuclei forming in the supercooled fluid are not well understood and, therefore, important information about the details of the free energy barrier is missing. This is the case for both homogenous and heterogenous nucleation. While homogenous nucleation is important for the fundamental understanding of crystallization, heterogenous nucleation is of higher relevance for applications. Therefore, an improved understanding of heterogenous nucleation has the potential to lead to novel ways for the manufacturing of crystalline materials.We propose a systematic study of heterogenous crystal nucleation on curved seeds using colloidal model systems. Colloids are experimentally much more accessible than atomic or molecular materials, as their size is >10^3 times larger and the typical time scale for structural changes is >10^9 times slower. This gives the opportunity to study crystal nucleation in colloids with single particle resolution and with a time resolution allowing to follow the formation of crystal nuclei, which is hard to achieve with atomic or molecular materials. Experiments have shown that strongly curved seeds can suppress crystal nucleation, while flat seeds can give rise to spontaneous crystallization, an effect that is not captured by Classical Nucleation Theory. We plan to study crystal nucleation on seeds with controlled curvature and topology. First, convex and concave spherical seeds with positive Gaussian curvature will be studied and, secondly, toroidal seeds will be investigated that allow to study the effect of negative Gaussian curvatures and saddle points.The key parameters for crystal nucleation will be estimated using both scattering and real-space imaging techniques. This will allow a detailed characterization and comparison of heterogenous and homogenous nucleation as a function of the properties of seeds and colloidal suspensions.We plan to use hard sphere-like and soft colloidal model systems. Soft colloids (microgels) will allow to study crystal nucleation in a system being a model for materials with many potential applications. Hard spheres are an important general model system for fluids and materials with important steric interaction. Therefore, insight into the behavior of hard spheres is relevant for many materials also beyond colloids.The phase behavior of microgel suspensions is already being studied in our SNF-project 200021_132128. A one-year continuation of this project is part of this proposal, as it has an important overlap with the study of crystal nucleation in microgels.Therefore, we request funding for two PhD students:- Three years for student A working on the investigation of heterogenous crystal nucleation. He/she will work for half a year in Prof. A. Fernandez-Nieves' laboratory at GaTech, Atlanta USA, and for 2.5 years at PSI.- A one-year extension for Andrea Scotti, the PhD student of the running project 200021_132128. He will work for three months at GaTech, Atlanta USA, and for nine months at PSI. The extension is planned to overlap with the first year of student A.The extension for Andrea Scotti will allow to investigate the consequences of the responsiveness of soft microgel particles by real-space imaging and to study homogenous crystal nucleation in microgel suspensions. The latter has been found to be an interesting point during Andrea Scotti's running project, as two crystal structures appear to compete during crystal nucleation. As both students will work with microgels, project 200021_132128 and the extension for Andrea Scotti will be a platform for knowledge transfer and a quick start of student A's work on heterogenous nucleation. The students will profit from each other.