Pearling instability of nanoscale fluid flow confined to a chemical channel

We investigate the flow of a nano-scale incompressible ridge of low-volatility liquid along a "chemical channel": a long, straight, and completely wetting stripe embedded in a planar substrate, and sandwiched between two extended less wetting solid regions. Molecular dynamics simulations, a simple long-wavelength approximation, and a full stability analysis based on the Stokes equations are used, and give qualitatively consistent results. While thin liquid ridges are stable both statically and during flow, a (linear) pearling instability develops if the thickness of the ridge exceeds half of the width of the channel. In the flowing case periodic bulges propagate along the channel and subsequently merge due to nonlinear effects. However, the ridge does not break up even when the flow is unstable, and the qualitative behavior is unchanged even when the fluid can spill over onto a partially wetting exterior solid region.Comment: 17 pages, 12 figures, submitted to Physics of Fluids, fixed equation numbering after Eq. (17

Quantum interface unbinding transitions

We consider interfacial phenomena accompanying bulk quantum phase transitions in presence of surface fields. On general grounds we argue that the surface contribution to the system free energy involves a line of singularities characteristic of an interfacial phase transition, occurring below the bulk transition temperature T_c down to T=0. This implies the occurrence of an interfacial quantum critical regime extending into finite temperatures and located within the portion of the phase diagram where the bulk is ordered. Even in situations, where the bulk order sets in discontinuously at T=0, the system's behavior at the boundary may be controlled by a divergent length scale if the tricritical temperature is sufficiently low. Relying on an effective interfacial model we compute the surface phase diagram in bulk spatial dimensionality $d\geq 2$ and extract the values of the exponents describing the interfacial singularities in $d\geq 3$

Out-of-equilibrium critical dynamics at surfaces: Cluster dissolution and non-algebraic correlations

We study nonequilibrium dynamical properties at a free surface after the system is quenched from the high-temperature phase into the critical point. We show that if the spatial surface correlations decay sufficiently rapidly the surface magnetization and/or the surface manifold autocorrelations has a qualitatively different universal short time behavior than the same quantities in the bulk. At a free surface cluster dissolution may take place instead of domain growth yielding stationary dynamical correlations that decay in a stretched exponential form. This phenomenon takes place in the three-dimensional Ising model and should be observable in real ferromagnets.Comment: 4 pages, 4 figure

Automatic, fast and robust characterization of noise distributions for diffusion MRI

Knowledge of the noise distribution in magnitude diffusion MRI images is the centerpiece to quantify uncertainties arising from the acquisition process. The use of parallel imaging methods, the number of receiver coils and imaging filters applied by the scanner, amongst other factors, dictate the resulting signal distribution. Accurate estimation beyond textbook Rician or noncentral chi distributions often requires information about the acquisition process (e.g. coils sensitivity maps or reconstruction coefficients), which is not usually available. We introduce a new method where a change of variable naturally gives rise to a particular form of the gamma distribution for background signals. The first moments and maximum likelihood estimators of this gamma distribution explicitly depend on the number of coils, making it possible to estimate all unknown parameters using only the magnitude data. A rejection step is used to make the method automatic and robust to artifacts. Experiments on synthetic datasets show that the proposed method can reliably estimate both the degrees of freedom and the standard deviation. The worst case errors range from below 2% (spatially uniform noise) to approximately 10% (spatially variable noise). Repeated acquisitions of in vivo datasets show that the estimated parameters are stable and have lower variances than compared methods.Comment: v2: added publisher DOI statement, fixed text typo in appendix A

Health-seeking behaviours by gender among adolescents in Soweto, South Africa

KIMBACKGROUND: Adolescents are an important age-group for preventing disease and supporting health yet little is known about their health-seeking behaviours. OBJECTIVE: We describe socio-demographic characteristics and health-seeking behaviours of adolescents in Soweto, South Africa, in order to broaden our understanding of their health needs. DESIGN: The Botsha Bophelo Adolescent Health Study was an interviewer-administered cross-sectional survey of 830 adolescents (14-19 years) conducted in Soweto from 2010 to 2012. Health-seeking behaviours were defined as accessing medical services and/or being hospitalised in the 6 months prior to the survey. Chi-square analysis tested for associations between gender, other socio-demographic and behavioural characteristics, and health-seeking behaviours. RESULTS: Of 830 adolescents, 57% were female, 50% were aged 17-19 years, 85% were enrolled in school, and 78% reported experiencing medium or high food insecurity. Males were more likely than females to report sexual debut (64% vs. 49%; p<0.0001) and illicit drug use (11% vs. 3%; p<0.0001). Approximately 27% (n=224) and 8% (n=65) reported seeking healthcare or being hospitalised respectively in the previous 6 months, with no significant differences by gender. Services were most commonly sought at medical clinics (75%), predominantly because of flu-like symptoms (32%), followed by concerns about HIV (10%). Compared to females, males were more likely to seek healthcare for condom breakage (8% vs. 2%; p=0.02). Relative to males, a significantly higher proportion of females desired general healthcare services (85% vs. 78%; p=0.0091), counselling (82% vs. 70%; p<0.0001), and reproductive health services (64% vs. 56%; p=0.02). CONCLUSIONS: A quarter of male and female adolescents accessed health services in the 6 months prior to the interview. Adolescents reported a gap between the availability and the need for general, reproductive, and counselling services. Integrated adolescent-friendly, school-based health services are recommended to bridge this gap

On the Solution of the Number-Projected Hartree-Fock-Bogoliubov Equations

The numerical solution of the recently formulated number-projected Hartree-Fock-Bogoliubov equations is studied in an exactly soluble cranked-deformed shell model Hamiltonian. It is found that the solution of these number-projected equations involve similar numerical effort as that of bare HFB. We consider that this is a significant progress in the mean-field studies of the quantum many-body systems. The results of the projected calculations are shown to be in almost complete agreement with the exact solutions of the model Hamiltonian. The phase transition obtained in the HFB theory as a function of the rotational frequency is shown to be smeared out with the projection.Comment: RevTeX, 11 pages, 3 figures. To be published in a special edition of Physics of Atomic Nuclei (former Sov. J. Nucl. Phys.) dedicated to the 90th birthday of A.B. Migda