Soft Condensed Matter
The term “colloidal matter” characterizes a class of materials consisting of large assemblies of colloidal particles. As the individual particles are substantially larger than the atomic scale but still much smaller than a macroscopic size, these materials have unusual thermodynamic, rheological and optical properties bridging the gap between the molecular and macroscopic world. This makes colloidal matter interesting, both from the fundamental and applied point of view. In addition, the size of the building blocks makes these materials amenable to relatively simple chemical modifications, allows for quantitative 3D analysis with confocal microscopy and makes it possible to manipulate the structures with external fields. Computer simulations on the same systems that are studied experimentally in real-space provide a powerful combination to increase our understanding of these (soft) condensed matter model systems. Our fundamental work focuses on using colloids as a way to extend our knowledge of condensed matter problems like, freezing/melting, the glass transition. Our interest in more applied use of soft condensed matter focuses on photonics.
Colloidal Model Systems
Using chemical synthesis techniques a variety of colloidal core-shell particles can be prepared. For fluorescence confocal microscopy the core consists of a fluorescently labeled material, while the shell consists of a non-fluorescent material like pure silica. In this case the core enables the confocal detection while the larger shell makes it possible to detect even touching spheres and to chemically modify the surface. Most of the syntheses and characterization are done at the Van ‘t Hoff Laboratory of the Debye Institute (Utrecht University).
Different core-shell morphologies give the particles different specific properties suited for various applications: optical tweezers, photonic crystals, ER fluids.
In confocal microscopy a pinhole is used in the focal plane both at illumination and at detection. In this way out of focus emitted light is effectively rejected by the detection pinhole and an increased resolution is obtained (top left). By scanning through the focal plane an image of a slice inside the sample can be taken (top right). From several slices taken at different heights a 3D-image of the sample can be reconstructed (bottom left) and particle coordinates can be obtained (right). A 3D reconstruction of a colloidal liquid-crystal interface is shown (bottom right).