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Elsen C.,University of Kaiserslautern | Rapedius K.,University of Kaiserslautern | Rapedius K.,Free University of Colombia | Witthaut D.,MPI for Dynamics and Self Organization | Korsch H.J.,University of Kaiserslautern
Journal of Physics B: Atomic, Molecular and Optical Physics | Year: 2011

The resonance spectrum of a tilted periodic quantum system for a bichromatic periodic potential is investigated. For such a bichromatic Wannier-Stark system, exceptional points, degeneracies of the spectrum, can be localized in parameter space by means of an efficient method for computing resonances. Berry phases and Petermann factors are analysed. Finally, the influence of a nonlinearity of the Gross-Pitaevskii type on the resonance crossing scenario is briefly discussed. © 2011 IOP Publishing Ltd.


Farrell F.D.C.,University of Edinburgh | Hallatschek O.,MPI for Dynamics and Self Organization | Hallatschek O.,University of California at Berkeley | Marenduzzo D.,University of Edinburgh | Waclaw B.,University of Edinburgh
Physical Review Letters | Year: 2013

We study colonies of nonmotile, rod-shaped bacteria growing on solid substrates. In our model, bacteria interact purely mechanically, by pushing each other away as they grow, and consume a diffusing nutrient. We show that mechanical interactions control the velocity and shape of the advancing front, which leads to features that cannot be captured by established Fisher-Kolmogorov models. In particular, we find that the velocity depends on the elastic modulus of bacteria or their stickiness to the surface. Interestingly, we predict that the radius of an incompressible, strictly two-dimensional colony cannot grow linearly in time, unless it develops branches. Importantly, mechanical interactions can also account for the nonequilibrium transition between circular and branching colonies, often observed in the lab. © 2013 American Physical Society.


Sedlmair J.,University of Gottingen | Gleber S.-C.,University of Gottingen | Ozturk S.,MPI for Dynamics and Self Organization | Pfohl T.,MPI for Dynamics and Self Organization | And 4 more authors.
AIP Conference Proceedings | Year: 2010

The aim of this study is the comparison of samples at dry and aqueous ambient conditions. Especially in biological and environmental sciences, x-ray spectromicroscopy is a powerful tool for investigating key questions, e.g., the study of structures in soil samples or cells showing dimensions on the nanoscale, concurrently gaining insight into chemical interactions. The spectral range around the C K-edge, in the so-called water window (285 eV-523 eV), is especially well suited for the samples mentioned above, since it allows for working under aqueous and natural conditions. Exemplarily we present x-ray spectromicroscopy experiments of three different samples of high carbon content to demonstrate the importance of a sample environment as close to natural conditions as possible. © 2011 American Institute of Physics.


Thutupalli S.,MPI for Dynamics and Self Organization | Fleury J.-B.,Saarland University | Steinberger A.,MPI for Dynamics and Self Organization | Steinberger A.,CNRS Physics Laboratory | And 3 more authors.
Chemical Communications | Year: 2013

Droplet interface bilayers are a convenient tool to produce and explore lipid membrane properties. We discuss why their formation time in microfluidics can be three to six orders of magnitude faster compared to conventional bulk settings. © 2013 The Royal Society of Chemistry.


Dootz R.,MPI for Dynamics and Self Organization | Toma A.C.,University of Basel | Pfohl T.,MPI for Dynamics and Self Organization | Pfohl T.,University of Basel
Soft Matter | Year: 2011

DNA interactions with polycations are not only important for our understanding of chromatin compaction but also for characterizing DNA-binding proteins involved in transcription, replication and repair. DNA is known to form several types of liquid-crystalline phases depending, among other factors, on the polycation structure and charge density. Theoretical studies and simulations have predicted the wrapping of DNA around spherical positively charged polycations. As a potential mimic of the histone octamer or other DNA wrapping proteins, poly(amido amine) generation 6 (PAMAM6) dendrimers have been chosen for our study. The self-assembly of DNA induced by PAMAM6 has been investigated using small angle X-ray scattering (SAXS) in order to reveal the assemblies' structure dependence on the pH of the environment and dendrimer concentrations. We demonstrate that at pH 8.5 dense phases are formed and characterized by a 2D-columnar hexagonal lattice which is transformed into a 3D hexagonal lattice with increasing dendrimer concentration (charge ratio N/P). Moreover, a systematic analysis of the scattering data collected from diluted samples at pH 8.5 and 5.5 has led us to propose a wrapping scenario of DNA around PAMAM6 at pH 5.5. © 2011 The Royal Society of Chemistry.

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