Phase separation in hydrogen-helium mixtures at Mbar pressures

The properties of hydrogen-helium mixtures at Mbar pressures and intermediate temperatures (4000 to 10000 K) are calculated with first-principles molecular dynamics simulations. We determine the equation of state as a function of density, temperature, and composition and, using thermodynamic integration, we estimate the Gibbs free energy of mixing, thereby determining the temperature, at a given pressure, when helium becomes insoluble in dense metallic hydrogen. These results are directly relevant to models of the interior structure and evolution of Jovian planets. We find that the temperatures for the demixing of helium and hydrogen are sufficiently high to cross the planetary adiabat of Saturn at pressures around 5 Mbar; helium is partially miscible throughout a significant portion of the interior of Saturn, and to a lesser extent in Jupiter.Comment: 6 pages, 7 figures. Published in "Proceedings of the National Academy of Sciences USA

Exercise Beliefs and Behaviours of Individuals with Joint Hypermobility Syndrome/ Ehlers Danlos Syndrome-Hypermobility Type

This is an Accepted Manuscript of an article published by Taylor & Francis Group in Disability & Rehabilitation on 10 November 2017, available online at: https://doi.org/10.1080/09638288.2017.1398278. © 2017 Informa UK Limited, trading as Taylor & Francis GroupPurpose: To explore exercise beliefs and behaviours of individuals with Joint Hypermobility syndrome/Ehlers–Danlos syndrome – hypermobility type and to explore patient experiences of physiotherapy.Methods: A cross sectional questionnaire survey design was used to collect quantitative and qualitative data from adult members of the Hypermobility Syndromes Association and Ehlers–Danlos Syndrome Support UK. Descriptive and inferential statistics were used to analyse the data. Qualitative data was analysed thematically.Results: 946 questionnaires were returned and analysed. Participants who received exercise advice from a physiotherapist were 1.75 more likely to report high volumes of weekly exercise (odds ratio [OR] = 1.75, 95% confidence interval [CI] = 1.30–2.36, p < 0.001) than those with no advice. Participants who believed that exercise is important for long-term management were 2.76 times more likely to report a high volume of weekly exercise compared to the participants who did not hold this belief (OR = 2.76, 95% CI = 1.38–5.50, p = 0.004). Three themes emerged regarding experience of physiotherapy; physiotherapist as a partner, communication – knowledge, experience and safety.Conclusion: Pain, fatigue and fear are common barriers to exercise. Advice from a physiotherapist and beliefs about the benefits of exercise influenced the reported exercise behaviours of individuals with Ehlers–Danlos syndrome – hypermobility type in this survey.Peer reviewe

Modeling of Metal(100) Homepitaxial Film Growth at Very Low Temperatures

We model the growth of Ag films deposited on Ag(100) below 140K. Our recent Variable-Temperature Scanning Tunneling Microscopy (VTSTM) studies reveal “smooth growth” from 120-140K, consistent with earlier diffraction studies. However, we also find rougher growth for lower temperatures. This unexpected behavior is modeled by describing the deposition dynamics using a “restricted downward funneling” model, wherein deposited atoms get caught on the sides of steep nanoprotrusions (which are prevalent below 120K), rather than always funneling down to lower four-fold hollow adsorption sites. At OK, where no thermal diffusion processes are operative, this leads to the formation of overhangs and internal defects (or voids). Above 40K, low barrier interlayer diffusion processes become operative, producing the observed smooth growth by 120K. We also discuss how the apparent film morphology mapped out by the STM tip “smears” features of the actual film morphology (which are small at low temperature), and also can lead to underestimation of the roughness

Morphology of multilayer Ag/Ag(100) films versus deposition temperature: STM analysis and atomistic lattice-gas modeling

Scanning tunneling microscopy is used to analyze the nanoscale morphology of 25 ML films of Ag deposited on Ag(100) at temperatures (T) between 55 and 300 K. A transition from self-affine growth to “mound formation” occurs as T increases above about 140 K. The roughness decreases with increasing T up until 140 K in the self-affine growth regime, and then increases until about 210 K before decreasing again in the mounding regime. We analyze mounding behavior via a lattice-gas model incorporating: downward funneling of depositing atoms from step edges to lower fourfold hollow adsorption sites; terrace diffusion of adatoms with a barrier of 0.40 eV leading to irreversible island formation in each layer; efficient transport of adatoms along island edges to kink sites; and downward thermal transport of adatoms inhibited by a step-edge barrier of 0.06–0.07 eV along close-packed step edges (but with no barrier along kinked or open steps). This model reasonably recovers the T-dependence of not just the roughness, but also of the mound slopes and lateral dimensions above 190 K. To accurately describe lateral dimensions, an appropriate treatment of the intralayer merging of growing islands is shown to be critical. To describe behavior below 190 K, one must account for inhibited rounding of kinks by adatoms at island edges, as this controls island shapes, and thus the extent of open steps and of easy downward transport. Elsewhere, we describe the low-T regime of self-affine growth (with no terrace diffusion) accounting for a breakdown of the simple downward funneling picture

A critique of general allometry-inspired models for estimating forest carbon density from airborne LiDAR.

There is currently much interest in developing general approaches for mapping forest aboveground carbon density using structural information contained in airborne LiDAR data. The most widely utilized model in tropical forests assumes that aboveground carbon density is a compound power function of top of canopy height (a metric easily derived from LiDAR), basal area and wood density. Here we derive the model in terms of the geometry of individual tree crowns within forest stands, showing how scaling exponents in the aboveground carbon density model arise from the height-diameter (H-D) and projected crown area-diameter (C-D) allometries of individual trees. We show that a power function relationship emerges when the C-D scaling exponent is close to 2, or when tree diameters follow a Weibull distribution (or other specific distributions) and are invariant across the landscape. In addition, basal area must be closely correlated with canopy height for the approach to work. The efficacy of the model was explored for a managed uneven-aged temperate forest in Ontario, Canada within which stands dominated by sugar maple (Acer saccharum Marsh.) and mixed stands were identified. A much poorer goodness-of-fit was obtained than previously reported for tropical forests (R2 = 0.29 vs. about 0.83). Explanations for the poor predictive power on the model include: (1) basal area was only weakly correlated with top canopy height; (2) tree size distributions varied considerably across the landscape; (3) the allometry exponents are affected by variation in species composition arising from timber management and soil conditions; and (4) the C-D allometric power function was far from 2 (1.28). We conclude that landscape heterogeneity in forest structure and tree allometry reduces the accuracy of general power-function models for predicting aboveground carbon density in managed forests. More studies in different forest types are needed to understand the situations in which power functions of LiDAR height are appropriate for modelling forest carbon stocks

Development and ordering of mounds during metal(100) homoepitaxy

Scanning-tunneling microscopy studies combined with atomistic modeling for Ag/Ag(100) homoepitaxy reveal complex growth behavior at 300 K: initial smooth growth up to ∼25 ML, where three-dimensional (3D) mounds develop from 2D islands; then an extended regime of mound steepening for ∼1000 ML producing unexpected rough growth; and finally an asymptotic regime with cooperative mound ordering and coalescence dynamics quite distinct from that in systems with up-down symmetry. The steepening regime is compressed upon lowering temperature, so while initial growth is rougher, asymptotic growth is actually smoother

Social physique anxiety and physical activity in early adolescent girls : the influence of maturation and physical activity motives

This study considered the influence of maturation on social physique anxiety (SPA), the relationship between SPA and current and future physical activity (PA) levels and the influence of motives for physical activity on this relationship in early adolescent girls (n=162; mean age=11.80±0.33 years). Participants completed the Pubertal Development Scale, the modified Social Physique Anxiety Scale and the Motives for Physical Activity Scale at baseline and the Physical Activity Questionnaire for Older Children at baseline and 6 months later. The girls became less active across the 6 months and girls in the early stages of maturation had significantly lower SPA than the girls in the middle and late stages of maturation. SPA was not related to current or future physical activity in the sample as a whole. Cluster analysis identified four groups with different motive profiles and the High Appearance and Fitness group demonstrated a moderate negative relationship between SPA and PA at phase 1, whereas the other groups did not. These findings indicate that SPA may increase with maturation and the relationship between SPA and PA is dependent on reasons for being active. For girls who are motivated to be active primarily by body-related reasons SPA is likely to lead to lower levels of PA