Dokument: Colloidal Spheres under Shear: The Interplay between Macroscopic Deformation and Microscopic Properties

Titel:	Colloidal Spheres under Shear: The Interplay between Macroscopic Deformation and Microscopic Properties
URL für Lesezeichen:	https://docserv.uni-duesseldorf.de/servlets/DocumentServlet?id=52143
URN (NBN):	urn:nbn:de:hbz:061-20200130-102806-4
Kollektion:	Dissertationen
Sprache:	Englisch
Dokumententyp:	Wissenschaftliche Abschlussarbeiten » Dissertation
Medientyp:	Text
Autor:	Maßhoff, Philipp [Autor]
Dateien:	[Dateien anzeigen] Adobe PDF [Details] 112,27 MB in einer Datei [ZIP-Datei erzeugen] Dateien vom 29.01.2020 / geändert 29.01.2020
Beitragende:	Prof. Dr. Egelhaaf, Stefan U. [Betreuer/Doktorvater] Prof. Dr. Voigtmann Thomas [Gutachter]
Stichwörter:	colloids, hard sphere, shear, rheology, confocal microscopy, particle tracking, LAOS
Dewey Dezimal-Klassifikation:	500 Naturwissenschaften und Mathematik » 530 Physik
Beschreibung:	Pantha rhei (Heraclitus 535 - 475 BC) – “everything flows” So too do colloidal spheres. But how do they flow? This work investigates their behavior under shear intending to establish a link between the macroscopic (i.e., bulk) rheological response and the microscopic (i.e., single-particle level) structure and dynamics. Suspensions of colloidal spheres, that is, micrometer size particles dispersed in a liquid are a fascinating model system on their own, but can also mimic paints, pastes, or blood cells in our bodies. In this work, I study single and multi-component systems of colloidal hard spheres. Shear, in the form of large-amplitude oscillatory shear (LAOS) experiments and start-up tests, is imposed by a rheometer or a shear cell. At the same time, thousands of small fluorescent spheres are imaged with high-resolution confocal microscopy. The particle motions are either analyzed with image velocimetry or tracked using customized computer algorithms. Samples in the vicinity of the colloidal glass transition are known to yield at strain amplitudes of roughly 12%. This study confirms that their macroscopic elastic and viscous response is microscopically related to reversible and irreversible particle motions, respectively. Besides, it is found that some particles occasionally display different behavior than expected. For instance, transient nonlinear velocity profiles are observed during start-up shear at low shear rates. On a single-particle level, this is expressed in time-dependent dynamical quantities, such as local nonaffine motions and mean squared displacements. This work suggests that there is a link between the microscopic properties and the mesoscopic velocity profiles. Caution is required here since the phenomena are history-dependent, and not directly visible in the macroscopic rheological response. Another observation is that, given a sufficiently small polydispersity and a suitable external shear protocol, one can switch from amorphous to crystalline states of matter. For instance, binary mixtures can crystallize under oscillatory shear unless the size difference between particle species exceeds about 15%. Altogether, the combination of confocal microscopy and rheology is found to be advantageous in revealing the links between microscopic and macroscopic properties. As implied in the very last section, the techniques and methods developed in this thesis can be readily applied to other soft systems.
Lizenz:	Urheberrechtsschutz
Fachbereich / Einrichtung:	Mathematisch- Naturwissenschaftliche Fakultät » WE Physik » Physik der kondensierten Materie
Dokument erstellt am:	30.01.2020
Dateien geändert am:	30.01.2020
Promotionsantrag am:	28.11.2019
Datum der Promotion:	28.01.2020