Hyperscroll dynamics: Vortices in four-dimensional networks.

We investigate a network of excitable nodes diffusively coupled to their neighbors along four orthogonal directions. This regular network effectively forms a four-dimensional reaction-diffusion system and has rotating wave solutions. We analyze some of the general features of these hyperscroll waves, which rotate around surfaces such as planes, spheres, or tori. The surfaces evolve according to local curvatures and a system-specific surface tension. They have associated local phases and phase gradients tend to decrease over time. We also discuss the robustness of these network states against the removal of random node connections and report an example of hyperscroll turbulence.

Asymmetric synchronization in lattices of pinned spiral waves.

Networks of coupled oscillators show a wealth of fascinating dynamics and are capable of storing and processing information. In biological and social networks, the coupling is often asymmetric. We use the chirality of rotating spiral waves to introduce this asymmetry in an excitable reaction-diffusion model. The individual vortices are pinned to unexcitable disks and arranged at a constant spacing L along straight lines or simple geometric patterns. In the case of periodic boundaries or pinning disks arranged along the edge of a closed shape, small L values lead to synchronization via repeated wave collisions. The rate of synchronization as a function of L shows a single maximum and is determined by the dispersion behavior of a continuous wave train traveling along the system boundary. For finite systems, spirals are affected by their upstream neighbor, and a single dominant spiral exists along each chain. Specific initial conditions can decouple neighboring vortices even for small L values. We also present a time-delay differential equation that reproduces the phase dynamics in periodic systems.

Flow-Pattern Characterization of Biphasic Electrochemical Cells by Magnetic Resonance Imaging under Forced Hydrodynamic Conditions.

The fluid dynamics of a liquid|liquid system inside a four-electrode electrochemical cell were studied by velocimetry magnetic resonance imaging (MRI) and flow propagator measurements. To characterize this system fully, three different cell configurations operating at two rotational frequencies were analyzed. Quantitative information about the stability of the liquid|liquid interface and the dynamics of the organic phase were determined. The NMR spectroscopy results were in agreement with the electrochemical measurements performed by using the same experimental setup.

Mechanochemical stability of sub-nm ZnO chains.

Formation of monoatomic chains by axial stretching of zinc oxide nanowires is investigated using molecular dynamics and supported by density functional calculations. Special focus is made on the mechanical properties of these structures. Using a state-of-the-art force field it was found that O2 species are commonly formed within the chain. This species drastically weakens the chain strength. Previous simulations, based on a pair potential, failed to predict O2 formation. Moreover, the superductility of zinc oxide nanowires observed in earlier studies, was found to be an artifact of the pair potential. Simulations revealed that the chain length before rupture (usually of 6 atoms) is independent of the nanowire diameter. The electronic structure and the charge distribution of the chains were also studied.