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

Culture, Policies and Labor Market Outcomes

We study whether cultural attitudes towards gender, the young, and leisure are significant determinants of the evolution over time of the employment rates of women and of the young, and of hours worked in OECD countries. Beyond controlling for a larger menu of policies, institutions and structural characteristics of the economy than has been done so far, our analysis improves upon existing studies of the role of "culture" for labor market outcomes by dealing explicitly with the endogeneity of attitudes, policies and institutions, and by allowing for the persistent nature of labor market outcomes. When we do all this we find that culture still matters for women employment rates and for hours worked. However, policies and other institutional or structural characteristics are also important. Attitudes towards youth independence, however, do not appear to be important in explaining the employment rate of the young. In the case of women employment rates, the policy variable that is significant along with attitudes, is the OECD index of employment protection legislation. For hours worked the policy variables that play a role, along with attitudes, are the tax wedge and unemployment benefits. The quantitative impact of these policy variables is such that changes in policies have at least the potential to undo the effect of variations in cultural traits on labor market outcomes.culture, policies, institutions, employment, hours

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

Dark Field Differential Dynamic Microscopy enables the accurate characterization of the roto-translational dynamics of bacteria and colloidal clusters

Micro- and nanoscale objects with anisotropic shape are key components of a variety of biological systems and inert complex materials, and represent fundamental building blocks of novel self-assembly strategies. The time scale of their thermal motion is set by their translational and rotational diffusion coefficients, whose measurement may become difficult for relatively large particles with small optical contrast. Here we show that Dark Field Differential Dynamic Microscopy is the ideal tool for probing the roto-translational Brownian motion of shape anisotropic particles. We demonstrate our approach by successful application to aqueous dispersions of non-motile bacteria and of colloidal aggregates of spherical particles

Correcting artifacts from finite image size in Differential Dynamic Microscopy

Differential Dynamic Microscopy (DDM) analyzes traditional real-space microscope images to extract information on sample dynamics in a way akin to light scattering, by decomposing each image in a sequence into Fourier modes, and evaluating their time correlation properties. DDM has been applied in a number of soft-matter and colloidal systems. However, objects observed to move out of the microscope's captured field of view, intersecting the edges of the acquired images, can introduce spurious but significant errors in the subsequent analysis. Here we show that application of a spatial windowing filter to images in a sequence before they enter the standard DDM analysis can reduce these artifacts substantially. Moreover, windowing can increase significantly the accessible range of wave vectors probed by DDM, and may further yield unexpected information, such as the size polydispersity of a colloidal suspension

Culture, Policies and Labor Market Outcomes

We study whether cultural attitudes towards gender, the young, and leisure are significant determinants of the evolution over time of the employment rates of women and of the young, and of hours worked in OECD countries. Beyond controlling for a larger menu of policies, institutions and structural characteristics of the economy than has been done so far, our analysis improves upon existing studies of the role of "culture" for labor market outcomes by dealing explicitly with the endogeneity of attitudes, policies and institutions, and by allowing for the persistent nature of labor market outcomes. When we do all this we find that culture still matters for women employment rates and for hours worked. However, policies and other institutional or structural characteristics are also important. Attitudes towards youth independence, however, do not appear to be important in explaining the employment rate of the young. In the case of women employment rates, the policy variable that is significant along with attitudes, is the OECD index of employment protection legislation. For hours worked the policy variables that play a role, along with attitudes, are the tax wedge and unemployment benefits. The quantitative impact of these policy variables is such that changes in policies have at least the potential to undo the effect of variations in cultural traits on labor market outcomes.

Differential dynamic microscopy microrheology of soft materials: A tracking-free determination of the frequency-dependent loss and storage moduli

Particle-tracking microrheology (PT-μr) exploits the thermal motion of embedded particles to probe the local mechanical properties of soft materials. Despite its appealing conceptual simplicity, PT-μr requires calibration procedures and operating assumptions that constitute a practical barrier to its wider application. Here we demonstrate differential dynamic microscopy microrheology (DDM-μr), a tracking-free approach based on the multiscale, temporal correlation study of the image intensity fluctuations that are observed in microscopy experiments as a consequence of the translational and rotational motion of the tracers. We show that the mechanical moduli of an arbitrary sample are determined correctly over a wide frequency range provided that the standard DDM analysis is reinforced with an iterative, self-consistent procedure that fully exploits the multiscale information made available by DDM. Our approach to DDM-μr does not require any prior calibration, is in agreement with both traditional rheology and diffusing wave spectroscopy microrheology, and works in conditions where PT-μr fails, providing thus an operationally simple, calibration-free probe of soft materials

Vascular Disrupting Activity of Tubulin-Binding 1,5-Diaryl-1H-imidazoles

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