Mean-field study of itinerant ferromagnetism in trapped ultracold Fermi gases: Beyond the local density approximation

We theoretically investigate the itinerant ferromagnetic transition of a spherically trapped ultracold Fermi gas with spin imbalance under strongly repulsive interatomic interactions. Our study is based on a self-consistent solution of the Hartree-Fock mean-field equations beyond the widely used local density approximation. We demonstrate that, while the local density approximation holds in the paramagnetic phase, after the ferromagnetic transition it leads to a quantitative discrepancy in various thermodynamic quantities even with large atom numbers. We determine the position of the phase transition by monitoring the shape change of the free energy curve with increasing the polarization at various interaction strengths.Comment: 7 pages, 5 figures; published version in Phys. Rev.

Hydrodynamic and entropic effects on colloidal diffusion in corrugated channels

In the absence of advection, confined diffusion characterizes transport in many natural and artificial devices, such as ionic channels, zeolites, and nanopores. While extensive theoretical and numerical studies on this subject have produced many important predictions, experimental verifications of the predictions are rare. Here, we experimentally measure colloidal diffusion times in microchannels with periodically varying width and contrast results with predictions from the Fick-Jacobs theory and Brownian dynamics simulation. While the theory and simulation correctly predict the entropic effect of the varying channel width, they fail to account for hydrodynamic effects, which include both an overall decrease and a spatial variation of diffusivity in channels. Neglecting such hydrodynamic effects, the theory and simulation underestimate the mean and standard deviation of first passage times by 40\% in channels with a neck width twice the particle diameter. We further show that the validity of the Fick-Jakobs theory can be restored by reformulating it in terms of the experimentally measured diffusivity. Our work thus demonstrates that hydrodynamic effects play a key role in diffusive transport through narrow channels and should be included in theoretical and numerical models.Comment: 7 pages, 4 figure

Viscous vortex flows

Several computational studies are currently being pursued that focus on various aspects of representing the entire lifetime of the viscous trailing vortex wakes generated by an aircraft. The formulation and subsequent near-wing development of the leading-edge vortices formed by a delta wing are being calculated at modest Reynolds numbers using a three-dimensional, time-dependent Navier-Stokes code. Another computational code was developed to focus on the roll-up, trajectory, and mutual interaction of trailing vortices further downstream from the wing using a two-dimensional, time-dependent, Navier-Stokes algorithm. To investigate the effect of a cross-wind ground shear flow on the drift and decay of the far-field trailing vortices, a code was developed that employs Euler equations along with matched asymptotic solutions for the decaying vortex filaments. And finally, to simulate the conditions far down stream after the onset of the Crow instability in the vortex wake, a full three-dimensional, time-dependent Navier-Stokes code was developed to study the behavior of interacting vortex rings

Stripe formation in bacterial systems with density-suppressed motility

Engineered bacteria in which motility is reduced by local cell density generate periodic stripes of high and low density when spotted on agar plates. We study theoretically the origin and mechanism of this process in a kinetic model that includes growth and density-suppressed motility of the cells. The spreading of a region of immotile cells into an initially cell-free region is analyzed. From the calculated front profile we provide an analytic ansatz to determine the phase boundary between the stripe and the no-stripe phases. The influence of various parameters on the phase boundary is discussed.Comment: 5 pages, 3 figures. Phys. Rev. Lett. in press (2012

Amino Acids Were Not All Created Equal

The contention that amino acids were not all created equal is based on the simple premise that intestinal uptakes of non-bound (synthetic, crystalline, feed-grade) amino acids are more rapid than their protein-bound counterparts. The post-enteral ramifications of this difference in bioequivalence are amplified in broiler chickens given their express growth rates and just one complication is the post-prandial oxidation of amino acids. The lack of bioequivalence between non-bound and protein-bound amino acids is a real obstacle to the development and adoption of reduced-crude protein diets that have the potential to promote sustainable chicken-meat production Thus, the purpose of this paper is to examine our contention that amino acids were no longer created equal with the introduction of non-bound amino acids

Amino Acids Were Not All Created Equal

The contention that amino acids were not all created equal is based on the simple premise that intestinal uptakes of non-bound (synthetic, crystalline, feed-grade) amino acids are more rapid than their protein-bound counterparts. The post-enteral ramifications of this difference in bioequivalence are amplified in broiler chickens given their express growth rates and just one complication is the post-prandial oxidation of amino acids. The lack of bioequivalence between non-bound and protein-bound amino acids is a real obstacle to the development and adoption of reduced-crude protein diets that have the potential to promote sustainable chicken-meat production Thus, the purpose of this paper is to examine our contention that amino acids were no longer created equal with the introduction of non-bound amino acids