Pattern Formation In Time Research Articles

A Combination of Actin Treadmilling and Cross-Linking Drives Contraction of Random Actomyosin Arrays

We investigate computationally the self-organization and contraction of an initially random actomyosin ring. In the framework of a detailed physical model for a ring of cross-linked actin filaments and myosin-II clusters, we derive the force balance equations and solve them numerically. We find that to contract, actin filaments have to treadmill and to be sufficiently cross linked, and myosin has to be processive. The simulations reveal how contraction scales with mechanochemical parameters. For example, they show that the ring made of longer filaments generates greater force but contracts slower. The model predicts that the ring contracts with a constant rate proportional to the initial ring radius if either myosin is released from the ring during contraction and actin filaments shorten, or if myosin is retained in the ring, while the actin filament number decreases. We demonstrate that a balance of actin nucleation and compression-dependent disassembly can also sustain contraction. Finally, the model demonstrates that with time pattern formation takes place in the ring, worsening the contractile process. The more random the actin dynamics are, the higher the contractility will be.

Marangoni convection in binary mixtures

Marangoni instabilities in binary mixtures in the presence of the Soret effect and evaporation are different from those in pure liquids. In contrast to a large amount of experimental work on Marangoni convection in pure liquids, such experiments in binary mixtures are not available in the literature, to our knowledge. Using binary mixtures of NaCl/water in an open system, evaporation of water molecules at the liquid-vapor interface is inevitable. We have systematically investigated the pattern formation for a set of substrate temperatures and solute concentrations in an open system. The flow patterns evolve with time, driven by surface-tension fluctuations due to evaporation and the Soret effect, while the air-liquid interface does not deform. A shadow-graph method is used to follow the pattern formation in time. The patterns are mainly composed of polygons and rolls. The mean pattern size first decreases slightly, and then gradually increases during the evolution. Evaporation affects the pattern formation mainly at the early stages and the local evaporation rate tends to become spatially uniform at the film surface. The Soret effect becomes important at the later stages and affects the mixture for a large mean solute concentration where the Soret number is significantly above zero. The strength of convection increases with the initial solute concentration and the substrate temperature. Our findings differ from the theoretical predictions in which evaporation is neglected.

Understanding and Controlling Pore Etching in Semiconductors

The paper first compares pore formation in semiconductors, and then turns to some common features found in all semiconductors. In particular, the appearance of some kind of self- organization, expressed as pattern formation in time (e.g. self- induced current oscillations) or in space (e.g. pore crystal formation), will be discussed. A stochastic model of the electrode dissolution via oxidation, implemented in a Monte- Carlo algorithm, can reproduce some of these phenomena quite closely, and the paper will give some new results. For a deeper understanding of pore etching as well as for some control of etching parameters over etching time, it is necessary to have some in-situ measurements that contain data about what is going on at the pore tips deep inside the sample. It will be shown that Fast-Fourier Transform (FFT) impedance spectroscopy (IS) is principally suited to the task. The method gains even more power if photo impedance , i.e. modulation of the backside or frontside illumination and monitoring the current response, is used in conjunction with suitable theoretical models. Extending FFTIS to other semiconductor like InP or to basic processes like anodic oxidation provides new insights; this will be demonstrated for the case of anodic oxidation in a range of organic electrolytes often used for pore etching in p-type semiconductors.