Colloidal and other Model Systems: Selected Projects
Dynamical collective behavior observed in, e.g., schools of fish and flocks of birds can often be described with simple models of so-called self-propelled particles. Even complex behavior can be reproduced by simple rules that are followed by all individuals (e.g., follow your neighbors but do not bump into them). On the microscale, both bacteria and colloidal particles have emerged as model systems to study a wealth of different phenomena such as swirling, swarming, and turbulence.
We are especially interested in the novel collective properties of colloidal Janus particles that are propelled by diffusio-phoresis or similar means, which have been realized experimentally very recently. Depending on the interplay of volume exclusion, hydrodynamic alignment of orientations, and attractive forces, several phenomena like living crystals and phase separation are observed.
Metastable phases decay by rare statistical fluctuations, termed "nucleation", where a "critical cluster" (i.e., a nanoscopic aggregate of the new phase having the minimal size to be able to grow) forms. Such critical clusters may form in the bulk ("homogeneous nucleation"), which facilitate this cluster formation by reducing the free energy barrier that needs to be overcome. Specialized computer simulation methods are developed to estimate the surface excess free energies, that control these processes, and related quantities (such as contact angles, line tension, Tolman length). This research is carried out in the framework of the priority program SPP 1296 "Heterogeneous Nucleation and Structure Formation" of the German Research Foundation (DFG), and we collaborate with J. Horbach (DLR Cologne) and S. Egelhaaf and H. Löwen (University of Düsseldorf), where complementary studies are done, as well as with S. K. Das (Bangalore) and S. Puri (New Delhi, India). For more information, please contact Kurt Binder and/or Peter Virnau.
In the framework of the transregional collaborative research center TR6 "Physics of colloidal dispersions in external fields" we study colloid-polymer mixtures confined in either slit pores or cylindrical pores, focusing on the interplay between finite size and surface effects on the possible phase separation between polymer-rich and colloid-rich phases, and on the structure of such systems under steady-state shear. In a variety of closely related subprojects, also the effects of confinement on one- or two-component colloidal crystals are studied, and the mechanical distortion of their struture caused by lateral compression, and the response of a dense colloidal dispersion is studied when one colloidal particle is dropped through the system ("microrheology"). Finally, also phase separation in mixtures of rodlike and spherical colloids is studied. For this research, we collaborate with J. Horbach (DLR Cologne), H. Löwen and S. Egelhaaf (University of Düsseldorf), P. Nielaba and G. Maret (University of Konstanz) and T. Schilling (University of Luxembourg). For more information, contact Kurt Binder and/or Peter Virnau.