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Biology provides the physicist with a stunning variety of patterns to explore, and several fundamental ideas in the physics of pattern formation, such as Turing instabilities and the clock-and-wave-front mechanism, are rooted in studies of biological systems. It remains unclear, however, whether these classic concepts can explain the emergence of patterns in most biological systems, or whether new and different mechanisms remain to be discovered.

Here, the authors, at the University of Michigan, Ann Arbor, examined a spatially discrete, three variable reaction-diffusion model inspired by the interactions that create a periodic pattern of gene expression in the Drosophila eye imaginal disc. This model is capable of creating a regular pattern behind a moving front, as observed in eye discs, through a novel "switch and template" mechanism. In order to better understand this mechanism, the authors performed a detailed study of the model's behaviour in one dimension, using a combination of analytic methods and numerical searches of parameter space. Using this approach, the authors find that patterns are created robustly, provided that there is an appropriate separation of time scales and that self-activation is sufficiently strong. Moreover, the paper presents explicit expressions in this limit for the front speed and the pattern wavelength. Moving fronts in pattern-forming systems near an initial linear instability generically select a unique pattern, but the P&L model operates in a strongly non-linear regime where the final pattern depends on the initial conditions as well as on parameter values. This study highlights the important role that cellularisation and cell-autonomous feedback can play in biological pattern formation.

Miniaturized non-thermal jet plasmas belong to the class of microplasmas characterized by dimensions below 1 mm. They represent an emerging technique for local surface treatment at atmospheric pressure such as surface modification and the deposition of thin functional coatings (for corrosion protection or gas diffusion barrier). The deposition of films with controlled quality requires the knowledge of the basic mechanisms, both plasma kinetics and flow dynamics that sustain and stabilize the gas discharge. This calls for a measure of the plasma parameters, validated by a suitable model. The plasma source studied here is a capacitive coupled capillary jet (27.12 MHz) operating in a distinctive regime where self-organized discharge patterns develop (Fig.: time averaged top view). This discharge regime along with the plasma source geometry puts the device in a prime position for a coating process at atmospheric pressure. The paper describes a first step towards a thorough plasma physical description of the discharge dynamics and the energy transport by determining the electron concentration in the active discharge zone. Two independent approaches were used, spectroscopic measurements of the broadening of Balmer Hβ and Hγ lines and a time-dependent, 2D fluid model of a single discharge filament. Electron concentrations between 2.2 and 3.3×10¹⁴cm^-3 have been obtained after separating the relevant spectral line broadening effects. The fluid model has confirmed these results. The relatively high electron concentration in the active jet zone can be explained by the contraction of the discharge into single filaments. The self-organization makes these filaments deterministic but not stochastic. Their steady behaviour furthermore supports the establishment of a larger concentration of excited atoms, especially metastable atoms, leading to enhanced ionization.