Statistical Physics

The law of mass action describes the speed of chemical reactions. Our model calculations demonstrate, however, that at sufficiently high reactant densities the law is violated. The deviations are attributed to many-particle effects. more

Dense liquids of infinitely thin, hard needles challenge Statistical
Mechanics: while its equilibrium properties correspond to those of an
ideal gas, the steric hindrance of the needles induces complex dynamic
behavior. An intriguing phenomenon is the enhancement of translational
diffusion over several orders of magnitude as density increases. Such a
behavior contradicts the experience that transport becomes slow in dense
complex liquids. We observed the emergence of a spacious zigzag motion of
the needle, which results in a power-law increase of the
diffusion coefficient with the mysterious exponent 0.8. more

Conformational transitions in macromolecular complexes often involve the reorientation of lever-like structures. Using a simple theoretical model, we show that the rate of such transitions is drastically enhanced if the lever is bendable, e.g. at a localized "hinge". Surprisingly, the transition is fastest with an intermediate flexibility of the hinge. In this intermediate regime, the transition rate is also least sensitive to the amount of "cargo" attached to the lever arm, which could be exploited by molecular motors. To explain this effect, we generalize the Kramers-Langer theory for multi-dimensional barrier crossing to configuration dependent mobility matrices. more

Spatial heterogeneities often give rise to intriguing slow dynamics in complex materials, manifested for example by broad frequency-dependent relaxation processes in colloidal gels which form stress-sustaining networks close to the sol-gel transition. A further prominent example are sodium silicates, where the formation of a space-filling network of channels allows for slow diffusion of sodium ions in an arrested host matrix. A minimal model that encompasses spatial disorder and slow dynamics is provided by the Lorentz model, i.e., classical point particles explore without mutual interaction a d-dimensional space in the presence of a frozen array of randomly distributed (possibly overlapping) hard spherical obstacles. more

With help of a light microscope one can see single chromosomes, the closest packed form of DNA. Not only during cell division, but throughout the whole cell cycle, DNA in eucaryotes is packed in one way or another. On the smallest scale, DNA is wrapped around histones and forms nucleosomes. The readout of information - between cell divisions - is controlled by so called transcription factors, which can bind to DNA without ATP consumption. How can that be if DNA is packed? more

Driven diffusive lattice gases have not only been frequently used to model biological transport by molecular motors, but serve also to investigate the intriguing problems of non-equilibrium physics. Two such systems are the total asymmetric exclusion process (TASEP) and the symmetric exclusion process (SEP) that can be regarded as the simplest paradigms of driven (TASEP) and diffusive (SEP) transport. more

Cars jamming in city centers or on highways belong to our every-day experience. In the present work, we theoretically explore such traffic phenomena occurring at a much smaller size, on the nanometer scale. At these tiny lengths, the fields of biological physics and information technology become increasingly intertwined. Still, different paradigms rule in these areas. Brownian motion governs biological systems, e.g., it drives molecular motors along parallel one-dimensional filaments in cells, serving as transport engines. On the other hand, quantum effects become visible in the field of electronic information processing. more

Nature as well as modern technology presents us a variety of disordered materials ranging from composites over gels to the inner structure of biological cells. We have shown that the transport properties of microscopic particles in such materials are directly connected to strong structural heterogeneities resulting from the presence of a broad range of length scales. It is known that these heterogeneities lead to a dramatic slowing down of transport processes due to a fractal dynamic behavior which has to be contrasted to normal diffusion. The latter being tightly bound to Brownian motion, is the dominant transport process in homogeneous materials and can be characterized by single length and time scales. Heterogeneous materials, however, lack such a single length scale, and the new fractal transport law depends on non-integer, i.e. fractal, powers of time and length. more

Superimposing optical interference patterns to two-dimensional colloidal systems generates versatile soft matter systems which allow to study the subtle interplay between thermal fluctuations, intermolecular interactions and external potentials on the ordering of the colloidal particles. These systems serve as model systems to study hard matter physics: the interfering laser patterns mimic the substrate potential of a crystalline surface and the assemblies of colloidal particles in the potential minima mimic molecules of various symmetries. Depending on the strength of the fluctuations, the symmetry of the laser patterns and the soft colloidal molecules novel phases emerge. more