Physik - Open Access LMU - Teil 01/02

Localized wave fronts are a fundamental feature of biological systems from cell biology to ecology. Here, we study a broad class of bistable models subject to self-activation, degradation, and spatially inhomogeneous activating agents. We determine the conditions under which wave-front localization is possible and analyze the stability thereof with respect to extrinsic perturbations and internal noise. It is found that stability is enhanced upon regulating a positional signal and, surprisingly, also for a low degree of binding cooperativity. We further show a contrasting impact of self-activation to the stability of these two sources of destabilization. DOI: 10.1103/PhysRevLett.110.038102

Actively propelled particles undergoing dissipative collisions are

known to develop a state of spatially distributed coherently moving clusters.

For densities larger than a characteristic value, clusters grow in time and form

a stationary well-ordered state of coherent macroscopic motion. In this work

we address two questions. (i) What is the role of the particles’ aspect ratio in

the context of cluster formation, and does the particle shape affect the system’s

behavior on hydrodynamic scales? (ii) To what extent does particle conservation

influence pattern formation? To answer these questions we suggest a simple

kinetic model permitting us to depict some of the interaction properties between

freely moving particles and particles integrated in clusters. To this end, we

introduce two particle species: single and cluster particles. Specifically, we

account for coalescence of clusters from single particles, assembly of single

particles on existing clusters, collisions between clusters and cluster disassembly.

Coarse graining our kinetic model, (i) we demonstrate that particle shape (i.e.

aspect ratio) shifts the scale of the transition density, but does not impact the

instabilities at the ordering threshold and (ii) we show that the validity of particle

conservation determines the existence of a longitudinal instability, which tends to amplify density heterogeneities locally, and in turn triggers a wave pattern

with wave vectors parallel to the axis of macroscopic order. If the system is in

contact with a particle reservoir, this instability vanishes due to a compensation

of density heterogeneities.

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Letztlich inkarnieren sich Systeme in der physikalischen Wirklichkeit, indem sie sie komplex informieren (ordnen, ermöglichen, steuern, formieren).

Das Universum würde dann der Materialisierung und Dekodierung von autopoetischen Systemen dienen.

We demonstrate how organic solar cell efficiency can be increased by introducing a pure polymer interlayer between the PEDOT:PSS layer and the polymer: fullerene blend. We observe an increase in device efficiency with three different material systems over a number of devices. Using both electrical characterization and numerical modeling we show that the increase in efficiency is caused by optical absorption in the pure polymer layer and hence efficient charge separation at the polymer bulkheterojunction interface.

While the existence of polar ordered states in active systems is well established, the dynamics of the self-assembly processes are still elusive. We study a lattice gas model of self-propelled elongated particles interacting through excluded volume and alignment interactions, which shows a phase transition from an isotropic to a polar ordered state. By analyzing the ordering process we find that the transition is driven by the formation of a critical nucleation cluster and a subsequent coarsening process. Moreover, the time to establish a polar ordered state shows a power-law divergence.

Length regulation of microtubules (MTs) is essential for many cellular processes. Molecular motors like kinesin-8, which move along MTs and also act as depolymerases, are known as key players in MT dynamics. However, the regulatory mechanisms of length control remain elusive. Here, we investigate a stochastic model accounting for the interplay between polymerization kinetics and motor-induced depolymerization. We determine the dependence of MT length and variance on rate constants and motor concentration. Moreover, our analyses reveal how collective phenomena lead to a well-defined MT length.

Populations of competing biological species exhibit a fascinating interplay between the nonlinear dynamics of evolutionary selection forces and random fluctuations arising from the stochastic nature of the interactions. The processes leading to extinction of species, whose understanding is a key component in the study of evolution and biodiversity, are influenced by both of these factors. Here, we investigate a class of stochastic population dynamics models based on generalized Lotka-Volterra systems. In the case of neutral stability of the underlying deterministic model, the impact of intrinsic noise on the survival of species is dramatic: It destroys coexistence of interacting species on a time scale proportional to the population size. We introduce a new method based on stochastic averaging which allows one to understand this extinction process quantitatively by reduction to a lower-dimensional effective dynamics. This is performed analytically for two highly symmetrical models and can be generalized numerically to more complex situations. The extinction probability distributions and other quantities of interest we obtain show excellent agreement with simulations.