528 research outputs found
Rayleigh-Taylor instability of crystallization waves at the superfluid-solid 4He interface
At the superfluid-solid 4He interface there exist crystallization waves
having much in common with gravitational-capillary waves at the interface
between two normal fluids. The Rayleigh-Taylor instability is an instability of
the interface which can be realized when the lighter fluid is propelling the
heavier one. We investigate here the analogues of the Rayleigh-Taylor
instability for the superfluid-solid 4He interface. In the case of a uniformly
accelerated interface the instability occurs only for a growing solid phase
when the magnitude of the acceleration exceeds some critical value independent
of the surface stiffness. For the Richtmyer-Meshkov limiting case of an
impulsively accelerated interface, the onset of instability does not depend on
the sign of the interface acceleration. In both cases the effect of
crystallization wave damping is to reduce the perturbation growth-rate of the
Taylor unstable interface.Comment: 8 pages, 2 figures, RevTe
Spontaneous orbiting of two spheres levitated in a vibrated liquid
In the absence of gravity, particles can form a suspension in a liquid irrespective of the difference in density between the solid and the liquid. If such a suspension is subjected to vibration there is relative motion between the particles and the fluid which can lead to self-organization and pattern formation. Here we describe experiments carried out to investigate the behavior of two identical spheres suspended magnetically in a fluid, mimicking weightless conditions. Under vibration the spheres mutually attract and, for sufficiently large vibration amplitudes, the spheres are observed to spontaneously orbit each other. The collapse of the experimental data onto a single curve indicates that the instability occurs at a critical value of the streaming Reynolds number. Simulations repro- duce the observed behaviour qualitatively and quantitatively, and are used to identify the features of the flow that are responsible for this instability
Self-contained encrypted image folding
The recently introduced approach for Encrypted Image Folding is generalized to make it self-contained. The goal is achieved by enlarging the folded image so as to embed all the necessary information for the image recovery. The need for extra size is somewhat compensated by considering a transformation with higher folding capacity. Numerical examples show that the size of the resulting cipher image may be significantly smaller than the plain text one. The implementation of the approach is further extended to deal also with color images
Surface Region of Superfluid Helium as an Inhomogeneous Bose-Condensed Gas
We present arguments that the low density surface region of self-bounded
superfluid He systems is an inhomogeneous dilute Bose gas, with almost all
of the atoms occupying the same single-particle state at . Numerical
evidence for this complete Bose-Einstein condensation was first given by the
many-body variational calculations of He droplets by Lewart, Pandharipande
and Pieper in 1988. We show that the low density surface region can be treated
rigorously using a generalized Gross-Pitaevskii equation for the Bose order
parameter.Comment: 4 pages, 1 Postscript figur
Extracellular vesicles derived from bone marrow mesenchymal stem cells enhance myelin maintenance after cortical injury in aged rhesus monkeys
Cortical injury, such as stroke, causes neurotoxic cascades that lead to rapid death and/or damage to neurons and glia. Axonal and myelin damage in particular, are critical factors that lead to neuronal dysfunction and impair recovery of function after injury. These factors can be exacerbated in the aged brain where white matter damage is prevalent. Therapies that can ameliorate myelin damage and promote repair by targeting oligodendroglia, the cells that produce and maintain myelin, may facilitate recovery after injury, especially in the aged brain where these processes are already compromised. We previously reported that a novel therapeutic, Mesenchymal Stem Cell derived extracellular vesicles (MSC-EVs), administered intravenously at both 24 h and 14 days after cortical injury, reduced microgliosis (Go et al. 2019), reduced neuronal pathology (Medalla et al. 2020), and improved motor recovery (Moore et al. 2019) in aged female rhesus monkeys. Here, we evaluated the effect of MSC-EV treatment on changes in oligodendrocyte maturation and associated myelin markers in the sublesional white matter using immunohistochemistry, confocal microscopy, stereology, qRT-PCR, and ELISA. Compared to vehicle control monkeys, EV-treated monkeys showed a reduction in the density of damaged oligodendrocytes. Further, EV-treatment was associated with enhanced myelin maintenance, evidenced by upregulation of myelin-related genes and increases in actively myelinating oligodendrocytes in sublesional white matter. These changes in myelination correlate with the rate of motor recovery, suggesting that improved myelin maintenance facilitates this recovery. Overall, our results suggest that EVs act on oligodendrocytes to support myelination and improves functional recovery after injury in the aged brain. SIGNIFICANCE: We previously reported that EVs facilitate recovery of function after cortical injury in the aged monkey brain, while also reducing neuronal pathology (Medalla et al. 2020) and microgliosis (Go et al. 2019). However, the effect of injury and EVs on oligodendrocytes and myelination has not been characterized in the primate brain (Dewar et al. 1999; Sozmen et al. 2012; Zhang et al. 2013). In the present study, we assessed changes in myelination after cortical injury in aged monkeys. Our results show, for the first time, that MSC-EVs support recovery of function after cortical injury by enhancing myelin maintenance in the aged primate brain
XY models with disorder and symmetry-breaking fields in two dimensions
The combined effect of disorder and symmetry-breaking fields on the
two-dimensional XY model is examined. The study includes disorder in the
interaction among spins in the form of random phase shifts as well as disorder
in the local orientation of the field. The phase diagrams are determined and
the properties of the various phases and phase transitions are calculated. We
use a renormalization group approach in the Coulomb gas representation of the
model. Our results differ from those obtained for special cases in previous
works. In particular, we find a changed topology of the phase diagram that is
composed of phases with long-range order, quasi-long-range order, and
short-range order. The discrepancies can be ascribed to a breakdown of the
fugacity expansion in the Coulomb gas representation.
Implications for physical systems such as planar Josephson junctions and the
faceting of crystal surfaces are discussed.Comment: 17 pages Latex with 5 eps figures, change: acknowledgment extende
Hidden spin-current conservation in 2d Fermi liquids
We report the existence of regimes of the two dimensional Fermi liquid that
show unusual conservation of the spin current and may be tuned by varying some
parameter like the density of fermions. We show that for reasonable models of
the effective interaction the spin current may be conserved in general in 2d,
not only for a particular regime. Low temperature spin waves propagate
distinctively in these regimes and entirely new ``spin-acoustic'' modes are
predicted for scattering-dominated temperature ranges. These new
high-temperature propagating spin waves provide a clear signature for the
experimental search of such regimes.Comment: 4 pages, no figures, revised version, accepted for pub. in the PR
Models for Access to Maternal Smoking cessation Support (MAMSS): a study protocol of a quasi-experiment to increase the engagement of pregnant women who smoke in NHS Stop Smoking Services
Background: Maternal smoking is a key cause of poor outcomes for mothers, babies and children and Wales has higher rates of smoking in pregnancy than any other UK country. Despite various improvements within the NHS Stop Smoking Service to strengthen the intervention for pregnant women, referrals and successful quit attempts for this group have continued to remain extremely low. A key element of UK national guidance for smoking cessation during pregnancy is to provide a flexible and tailored service to help increase levels of engagement. This study aims to test the effectiveness of three different models of service delivery to address the gap in the evidence base about how to deliver a flexible, tailored smoking cessation service to pregnant women. Methods: This study will adopt a quasi-experimental design over a 12 month period. The setting is four of Wales’ seven Health Boards using an integrated approach between maternity services, local public health teams and the NHS Stop Smoking Service. Core recommendations from UK public health guidance are being implemented across intervention and usual care sites. Stop smoking support for pregnant women in intervention sites is being delivered more flexibly than in usual care sites. Both qualitative and quantitative approaches will be adopted to capture important contextual information and consider multiple perspectives. A health economic analysis will be undertaken using a cost-consequences analysis approach. The primary outcome measure is engagement with stop smoking services (defined as having at least one face-to-face therapeutic contact with a clinician). Discussion: Supporting pregnant women to stop smoking is a challenging area of public health. The proposed study will address several areas where there are key evidence gaps relating to smoking cessation interventions for pregnant women. Specifically, how best to encourage pregnant women to attend a specialist stop smoking support service, how to deliver the service and who should provide it
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