Differential-activity driven instabilities in biphasic active matter

Active stresses can cause instabilities in contractile gels and living tissues. Here we describe a generic hydrodynamic theory that treats these systems as a mixture of two phases of varying activity and different mechanical properties. We find that differential activity between the phases provides a mechanism causing a demixing instability. We follow the nonlinear evolution of the instability and characterize a phase diagram of the resulting patterns. Our study complements other instability mechanisms in mixtures such as differential growth, shape, motion or adhesion

Thermal Bremsstrahlung Radiation in a Two-Temperature Plasma

In the normal one-temperature plasma the motion of ions is usually neglected when calculating the Bremsstrahlung radiation of the plasma. Here we calculate the Bremsstrahlung radiation of a two-temperature plasma by taking into account of the motion of ions. Our results show that the total radiation power is always lower if the motion of ions is considered. We also apply the two-temperature Bremsstrahlung radiation mechanism for an analytical Advection-Dominated Accretion Flow (ADAF) model; we find the two-temperature correction to the total Bremsstrahlung radiation for ADAF is negligible.Comment: 5 pages, 4 figures, accepted for publication in CHJAA. Some discussions and references adde

Active elastohydrodynamics of vesicles in narrow, blind constrictions

Fluid-resistance limited transport of vesicles through narrow constrictions is a recurring theme in many biological and engineering applications. Inspired by the motor-driven movement of soft membrane-bound vesicles into closed neuronal dendritic spines, here we study this problem using a combination of passive three-dimensional simulations and a simplified semi-analytical theory for active transport of vesicles that are forced through such constrictions by molecular motors. We show that the motion of these objects is characterized by two dimensionless quantities related to the geometry and the strength of forcing relative to the vesicle elasticity. We use numerical simulations to characterize the transit time for a vesicle forced by fluid pressure through a constriction in a channel, and find that relative to an open channel, transport into a blind end leads to the formation of an effective lubrication layer that strongly impedes motion. When the fluid pressure forcing is complemented by forces due to molecular motors that are responsible for vesicle trafficking into dendritic spines, we find that the competition between motor forcing and fluid drag results in multistable dynamics reminiscent of the real system. Our study highlights the role of non-local hydrodynamic effects in determining the kinetics of vesicular transport in constricted geometries

The Rotation of M Dwarfs Observed by the Apache Point Galactic Evolution Experiment

We present the results of a spectroscopic analysis of rotational velocities in 714 M dwarf stars observed by the SDSS III Apache Point Galactic Evolution Experiment (APOGEE) survey. We use a template fitting technique to estimate $v\sin{i}$ while simultaneously estimating $\log{g}$, $[\text{M}/\text{H}]$, and $T_{\text{eff}}$. We conservatively estimate that our detection limit is 8 km s$^{-1}$. We compare our results to M dwarf rotation studies in the literature based on both spectroscopic and photometric measurements. Like other authors, we find an increase in the fraction of rapid rotators with decreasing stellar temperature, exemplified by a sharp increase in rotation near the M$4$ transition to fully convective stellar interiors, which is consistent with the hypothesis that fully convective stars are unable to shed angular momentum as efficiently as those with radiative cores. We compare a sample of targets observed both by APOGEE and the MEarth transiting planet survey and find no cases were the measured $v\sin{i}$ and rotation period are physically inconsistent, requiring $\sin{i}>1$. We compare our spectroscopic results to the fraction of rotators inferred from photometric surveys and find that while the results are broadly consistent, the photometric surveys exhibit a smaller fraction of rotators beyond the M$4$ transition by a factor of $\sim 2$. We discuss possible reasons for this discrepancy. Given our detection limit, our results are consistent with a bi-modal distribution in rotation that is seen in photometric surveys.Comment: 31 pages, 11 figures, 4 tables. Accepted for publication by A

Electronic Structure of Sr_2FeMoO_6

We have analysed the unusual electronic structure of Sr_2FeMoO_6 combining ab-initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intraatomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength (U_{eff}) at the Mo site due to these renormalised interaction strengths.Comment: To appear in Phys. Rev. Let