Fundamental Nonequilibrium Phenomena

Stochastic Thermodynamics

The extraction of useable work from heat has fueled the industrial revolution of the 18th century, the scientific basis of which is provided by classical thermodynamics. Although thermodynamics can be justified by statistical arguments, it is still concerned with average values due to the vast number of degrees of freedom that comprise a macroscopic body. Quite in contrast, machines on the microscale are faced with fluctuations that are large. Our very lifes depend on such machines (e.g., proteins) to work properly. Stochastic thermodynamics is a generalization of thermodynamic notions such as work and heat to include fluctuations. In particular, the probability distributions of these quantities are not arbitrary but respect certain symmetries collectively called fluctuation theorems.
For more information, please contact  Thomas Speck.

Glassy dynamics: Dynamical facilitation theory

Most liquids (including water) can be supercooled below their melting temperature and stay in the liquid state. The reason is the presence of a free energy barrier that the system has to overcome in order to crystallize. But some liquids never crystallize and at some temperature fall out of equilibrium. They form what we call a glass, a substance that macroscopically appears as a rigid solid but which microscopically is still disordered like the liquid. A comprehensive theory describing this state of matter is still missing and is one of the major challenges in condensed matter science.
For more information, please contact  Thomas Speck.

Collective Behavior of Self-propelled Particles

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.
For more information, please contact  Thomas Speck  and  Peter Virnau.

Directed Assembly of Soft Matter

Imagine your book case could construct itself without you moving a single finger. What might sound like science fiction, happens in fact every day in nature on a microscopic scale, for example when cell membranes are formed or you wash your dishes. Designing and engineering such self-assembling materials is one of the major challenges in soft matter, and holds immense promise for the large-scale fabrication of novel nanomaterials and pharmaceutics. In our group, we study the fundamental principles of these intricate systems using advanced theoretical and computational methods. In particular, we focus on the role of external fields on self-assembly, and the possibility to guide the building blocks into well-specified structures. For more information, contact Arash Nikoubashman.

Simulation of Nucleation and "Critical Clusters"

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.