Geometry for a `penguin-albatross' rookery

We introduce a simple ecological model describing the spatial organization of two interacting populations whose individuals are indifferent to conspecifics and avoid the proximity to heterospecifics. At small population densities $\Phi$ a non-trivial structure is observed where clusters of individuals arrange into a rhomboidal bipartite network with an average degree of four. For $\Phi\rightarrow0$ the length scale, order parameter and susceptibility of the network exhibit power-law divergences compatible with hyper-scaling, suggesting the existence of a zero density - non-trivial - critical point. At larger densities a critical threshold $\Phi_{c}$ is identified above which the evolution toward a partially ordered configuration is prevented and the system becomes jammed in a fully mixed state

Heterodyne Near-Field Scattering

We describe an optical technique based on the statistical analysis of the random intensity distribution due to the interference of the near-field scattered light with the strong transmitted beam. It is shown that, from the study of the two-dimensional power spectrum of the intensity, one derives the scattered intensity as a function of the scattering wave vector. Near-field conditions are specified and discussed. The substantial advantages over traditional scattering technique are pointed out, and is indicated that the technique could be of interest for wave lengths other than visible light.Comment: 3 pages, 2 figure

Soret driven convection in a colloidal solution heated from above at very large solutal Rayleigh number

Convection in a colloidal suspension with a large negative separation ratio is studied experimentally by heating from above. Shadowgraph observation at very large solutal Rayleigh numbers are reported as a function of time. Fast relaxation oscillations are reported for the root mean square value of the shadowgraph intensity. While pure fluids exhibit a transition to turbulent convection for Rayleigh number of about 10^6, stable spoke-pattern planform with up and down columnar flows are observed up to solutal Rayleigh numbers of the order of 10^9. It is suggested that the surprising stability of the planform against turbulence is due to nonlinear focusing arising from the concentration dependence of the diffusion coefficient.Comment: 10 pages, 4 figure

South African youth in the age of freedom

The social role of youth is an issue that, in the last twenty years, has become a key point of the political agenda of many African nations. In South Africa, the consequences of segregationist politics, market economy, and migrations, have profoundly shaped the social and cultural role of youth, both in urban and rural contexts. Moreover, the end of apartheid has opened a new period of wide transformation. Based on my ethnographic research in kwaMashabane, a remote South African rural area, this paper analyses how the social role of male youth is shaped by national state policy and by local dynamics. Moreover, I will focus on the relationship between models of adulthood, and the strategies that youth adopt to cope with conflicts and continuities. This analysis will show how post-apartheid freedom and the constraints of the local social structure are negotiated, and how society is coping with the complex relationships between cultural reproduction and social change

Dynamic scaling for the growth of non-equilibrium fluctuations during thermophoretic diffusion in microgravity

Diffusion processes are widespread in biological and chemical systems, where they play a fundamental role in the exchange of substances at the cellular level and in determining the rate of chemical reactions. Recently, the classical picture that portrays diffusion as random uncorrelated motion of molecules has been revised, when it was shown that giant non-equilibrium fluctuations develop during diffusion processes. Under microgravity conditions and at steady-state, non-equilibrium fluctuations exhibit scale invariance and their size is only limited by the boundaries of the system. In this work, we investigate the onset of non-equilibrium concentration fluctuations induced by thermophoretic diffusion in microgravity, a regime not accessible to analytical calculations but of great relevance for the understanding of several natural and technological processes. A combination of state of the art simulations and experiments allows us to attain a fully quantitative description of the development of fluctuations during transient diffusion in microgravity. Both experiments and simulations show that during the onset the fluctuations exhibit scale invariance at large wave vectors. In a broader range of wave vectors simulations predict a spinodal-like growth of fluctuations, where the amplitude and length-scale of the dominant mode are determined by the thickness of the diffuse layer.Comment: To appear in Scientific Report

Equilibrium and non-equilibrium concentration fluctuations in a critical binary mixture

When a macroscopic concentration gradient is present across a binary mixture, long-ranged non-equilibrium concentration fluctuations (NCF) appear as a consequence of the coupling between the gradient and spontaneous equilibrium velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF) may be also observed when the mixture is close to a critical point. Here we study the interplay between NCF and critical ECF in a near critical mixture aniline/cyclohexane in the presence of a vertical concentration gradient. To this aim, we exploit a commercial optical microscope and a simple, custom-made, temperature-controlled cell to obtain simultaneous static and dynamic scattering information on the fluctuations. We first characterise the critical ECF at fixed temperature $T$ above the upper critical solution temperature $T_{c}$, in the wide temperature range $T-T_{c}\in[0.1,30]$ $^{o}$C. In this range, we observe the expected critical scaling behaviour for both the scattering intensity and the mass diffusion coefficient and we determine the critical exponents $\gamma$, $\nu$ and $\eta$, which are found in agreement with the 3D Ising values. We then study the system in the two-phase region ($T<T_{c}$). In particular, we characterise the interplay between ECF and NCF when the mixture, initially at a temperature $T_{i}$, is rapidly brought to a temperature $T_{f}>T_{i}$. During the transient, a vertical diffusive mass flux is present that causes the onset of NCF, whose amplitude vanishes with time, as the flux goes to zero. We also study the time dependence of the equilibrium scattering intensity $I_{eq}$, of the crossover wave-vector $q_{co}$ and of the diffusion coefficient $D$ during diffusion and find that all these quantities exhibit an exponential relaxation enslaved to the diffusive kinetics.Comment: 11 pages, 4 figure

Equilibrium and nonequilibrium fluctuations at the interface between two fluid phases

We have performed small-angle light-scattering measurements of the static structure factor of a critical binary mixture undergoing diffusive partial remixing. An uncommon scattering geometry integrates the structure factor over the sample thickness, allowing different regions of the concentration profile to be probed simultaneously. Our experiment shows the existence of interface capillary waves throughout the macroscopic evolution to an equilibrium interface, and allows to derive the time evolution of surface tension. Interfacial properties are shown to attain their equilibrium values quickly compared to the system's macroscopic equilibration time.Comment: 10 pages, 5 figures, submitted to PR

Synthesis and characterization of (cyclopentadienone)iron tricarbonyl complexes and their application to stereoselective catalytic transformations.

In recent years there has been a growing interest for developing efficient and selective homogeneous iron catalysts, thanks to the far lower cost and greater abundance of iron over the more precious metals. During my PhD thesis, in order to improve the catalytic activities or achieve novel reactivity, we decided to modify the framework of the Kn\uf6lker-type complex and to develop a small library of ten new chiral pre-catalysts, bearing a binaphthyl-derived backbone with different 3,3\u2019-substituents, for the enantioselective hydrogenation of ketones (up to 52% ee). (Eur. J. Org. Chem., 2015, 5526-5536; Eur. J. Org. Chem., 2015, 1887-1893). In addition to the ongoing development of chiral variants of the Kn\uf6lker motif, our group was also interested in opening new vistas in catalyst design by accessing new catalyst structures easily to synthesize and with different reactivity profiles. A new [bis(hexamethylene)cyclopentadienone]iron tricarbonyl complex was synthesized, which showed more stability and better reproducible results respect to the known Kn\uf6lker-Casey catalyst underlined by TON, TOF and kinetic measurements. Recently, we also developed a new generation of planar chiral (cyclopentadienone)iron tricarbonyl pre-catalyst, the results are promising but the research is still under investigation