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A diffusion-reaction model for the growth of bacterial colonies is presented. The often observed cooperative behavior developed by bacteria which increases their motility in adverse growth conditions is here introduced as a nonlinear diffusion term. The presence of this mechanism depends on a response which can present hysteresis. By changing only the concentrations of agar and initial nutrient, numerical integration of the proposed model reproduces the different patterns shown by Bacillus subtilis OG-01.

Despite the important role of electromechanical alternans in cardiac arrhythmogenesis, its molecular origin is not well understood. The appearance of calcium alternans has often been associated to fluctuations in the sarcoplasmic reticulum (SR) Ca loading. However, cytosolic calcium alternans observed without concurrent oscillations in the SR Ca content suggests an alternative mechanism related to a dysfunction in the dynamics of the ryanodine receptor (RyR2). We have investigated the effect of SR release refractoriness in the appearance of alternans, using a mathematical model of a single human atrial cell, based on the model by Nygren et al. (30), where we modified the dynamics of the RyR2 and of SR Ca release. The genesis of calcium alternans was studied stimulating the cell for different periods and values of the RyR2 recovery time from inactivation. At fast rates cytosolic calcium alternans were obtained without concurrent SR Ca content fluctuations. A transition from regular response to alternans was also observed, changing the recovery time from inactivation of the RyR2. This transition was found to be hysteretic, so for a given set of parameters different responses were observed. We then studied the relevance of RyR2 refractoriness for the generation of alternans, reproducing the same protocols as in recent experiments. In particular, restitution of Ca release during alternans was studied with a S1S2 protocol, obtaining a different response if the S2 stimulation was given after a long or a short release. We show that the experimental results can be explained by RyR2 refractoriness, arising from a slow RyR2 recovery from inactivation, stressing the role of the RyR2 in the genesis of alternans.

A general asymptotic analysis of the Gunn effect in n-type GaAs under general boundary conditions for metal-semiconductor contacts is presented. Depending on the parameter values in the boundary condition of the injecting contact, different types of waves mediate the Gunn effect. The periodic current oscillation typical of the Gunn effect may be caused by moving charge-monopole accumulation or depletion layers, or by low-or high-field charge-dipole solitary waves. A new instability caused by multiple shedding of ͑low-field͒ dipole waves is found. In all cases the shape of the current oscillation is described in detail: we show the direct relationship between its major features ͑maxima, minima, plateaus, etc.͒ and several critical currents ͑which depend on the values of the contact parameters͒. Our results open the possibility of measuring contact parameters from the analysis of the shape of the current oscillation. ͓S1063-651X͑97͒04708-9͔ PACS number͑s͒: 05.45.ϩb, 72.20.Ht, 85.30.Fg

BackgroundRapid pacing rates induce alternations in the cytosolic calcium concentration caused by fluctuations in calcium released from the sarcoplasmic reticulum (SR). However, the relationship between calcium alternans and refractoriness of the SR calcium release channel (RyR2) remains elusive.Methodology/Principal FindingsTo investigate how ryanodine receptor (RyR2) refractoriness modulates calcium handling on a beat-to-beat basis using a numerical rabbit cardiomyocyte model. We used a mathematical rabbit cardiomyocyte model to study the beat-to-beat calcium response as a function of RyR2 activation and inactivation. Bi-dimensional maps were constructed depicting the beat-to-beat response. When alternans was observed, a novel numerical clamping protocol was used to determine whether alternans was caused by oscillations in SR calcium loading or by RyR2 refractoriness. Using this protocol, we identified regions of RyR2 gating parameters where SR calcium loading or RyR2 refractoriness underlie the induction of calcium alternans, and we found that at the onset of alternans both mechanisms contribute. At low inactivation rates of the RyR2, calcium alternans was caused by alternation in SR calcium loading, while at low activation rates it was caused by alternation in the level of available RyR2s.Conclusions/SignificanceWe have mapped cardiomyocyte beat-to-beat responses as a function of RyR2 activation and inactivation, identifying domains where SR calcium load or RyR2 refractoriness underlie the induction of calcium alternans. A corollary of this work is that RyR2 refractoriness due to slow recovery from inactivation can be the cause of calcium alternans even when alternation in SR calcium load is present.

This work evaluates, for the first time, the possibility of producing multifunctional alkali-activated composites combining ultra-low density, low thermal conductivity, high acoustic absorption, and good moisture buffering capacity. The composites were prepared using cork as a lightweight aggregate. This novel material might promote energy savings and tackle the CO 2 emissions of the building sector, while simultaneously improve the comfort for inhabitants (e.g. humidity levels regulation and sound pollution reduction). The composites apparent density (as low as 168 kg/m 3 ) and thermal conductivity (as low as 68 mW/m K) are amongst the lowest ever reported for alkali-activated materials (AAM) composites and foams, while their sound absorption ability is comparable to the best performing AAM foams reported to date, but in addition these eco-friendly composites also show good ability to passively adjust the humidity levels inside buildings. The multifunctional properties shown by the cork -AAM composites set them apart from other conventional building materials and might contribute to the global sustainability of the construction sector.

Green noise barriers have become an alternative means of reducing urban traffic noise. In this paper, the acoustic performance of a modular greenery noise barrier is evaluated. In situ measurements of noise reflection were performed using an experimental prototype to estimate the sound absorption coefficients. These coefficients were found to have values of approximately 0.7, higher than those previously found in laboratory measurements for a similar system with a lower vegetation density. The obtained values were input into software for predicting environmental noise to analyse the expected performance of such barriers, particularly in the case of a pair of parallel barriers. A comparison with the results for reflective barriers indicates a significant improvement in sound attenuation of up to 4 dBA. The values are similar and even superior to results reported by other authors regarding the effectiveness of absorptive treatments applied to parallel barriers, and furthermore, the proposed barriers offer an aesthetic element for environmental integration.

The Gunn effect consists of time-periodic oscillations of the current flowing through an external purely resistive circuit mediated by solitary wave dynamics of the electric field on an attached appropriate semiconductor. By means of an asymptotic analysis, it is argued that Gunn-like behavior occurs in specific classes of model equations. As an illustration, an example related to the constrained Cahn-Allen equation is analyzed. ͓S1063-651X͑97͒15508-8͔PACS number͑s͒: 03.40. Kf, 05.60.ϩw, 07.50.Ek In semiconductors where the local current density as a function of the local electric field is N shaped, the Gunn effect is a ubiquitous phenomenon ͓1-5͔. The Gunn effect ͓6͔ consists of time-periodic oscillations of the electric current flowing through an external purely resistive circuit attached to a semiconductor sample subject to dc voltage bias. The current oscillations correspond to the generation, onedimensional motion, and annihilation of solitary waves of the electric field inside the semiconductor. Besides this, the onset of the Gunn effect can be quite interesting, as the current may display intermittency accompanied by spatiotemporal structures of the electric field inside the semiconductor ͓7͔. Recently the onset of the Gunn instability was analyzed by singular perturbation methods which provide the governing amplitude equation for long semiconductors ͓8͔. Gunn-like phenomena may also explain the experimentally observed self-sustained oscillations of the current in doped weakly coupled superlattices ͓9͔ whose dominant transport mechanism is resonant tunneling between adjacent quantum wells ͓10͔. In these cases, the oscillations are due to recycling of electric-field wave fronts ͑charge monopoles͒ instead of solitary waves ͓10͔. The difference in the type of waves may be tracked to the boundary condition at the injecting contact ͓11,12͔. Gunn-like phenomena have also been numerically observed in a driven diffusive lattice-gas model of hopping conductivity ͓13͔.A natural question that comes to mind in relation with these phenomena concerns their universality: Given that the Gunn instability appears in widely different semiconductor systems and models, what are the features a given model has to have in order to present the Gunn instability? Notice that the Gunn effect is in principle a nonequilibrium phenomenon which may happen far from any bifurcation points. Thus the question of its universality may not be related to linearization about fixed points of a renormalization transformation. Nevertheless an asymptotic analysis allows us to understand deeply the Gunn effect and to try to give a precise meaning to the notion of universality far from equilibrium. This paper tries to give an answer to the universality question, and it also puts the Gunn instability into perspective by comparing it to phenomena occurring in other pattern forming systems ͓14͔.From the study of the Gunn instability in semiconductor models, we can extract the following common features that seem to be necessary for its occurrence.͑1͒ T...