Layering, freezing and re-entrant melting of hard spheres in soft confinement

Confinement can have a dramatic effect on the behavior of all sorts of particulate systems and it therefore is an important phenomenon in many different areas of physics and technology. Here, we investigate the role played by the softness of the confining potential. Using grand canonical Monte Carlo simulations, we determine the phase diagram of three-dimensional hard spheres that in one dimension are constrained to a plane by a harmonic potential. The phase behavior depends strongly on the density and on the stiffness of the harmonic confinement. Whilst we find the familiar sequence of confined hexagonal and square-symmetric packings, we do not observe any of the usual intervening ordered phases. Instead, the system phase separates under strong confinement, or forms a layered re-entrant liquid phase under weaker confinement. It is plausible that this behavior is due to the larger positional freedom in a soft confining potential and to the contribution that the confinement energy makes to the total free energy. The fact that specific structures can be induced or suppressed by simply changing the confinement conditions (e.g. in a dielectrophoretic trap) is important for applications that involve self-assembled structures of colloidal particles.Comment: 5 pages, 5 figure

An inverse optimization strategy to determine single crystal mechanical behavior from polycrystal tests: Application to AZ31 Mg alloy

An inverse optimization strategy was developed to determine the single crystal properties from experimental results of the mechanical behavior of polycrystals. The polycrystal behavior was obtained by means of the finite element simulation of a representative volume element of the microstructure in which the dominant slip and twinning systems were included in the constitutive equation of each grain. The inverse problem was solved by means of the Levenberg-Marquardt method, which provided an excellent fit to the experimental results. The iterative optimization process followed a hierarchical scheme in which simple representative volume elements were initially used, followed by more realistic ones to reach the final optimum solution, leading to important reductions in computer time. The new strategy was applied to identify the initial and saturation critical resolved shear stresses and the hardening modulus of the active slip systems and extension twinning in a textured AZ31 Mg alloy. The results were in general agreement with the data in the literature but also showed some differences. They were partially explained because of the higher accuracy of the new optimization strategy but it was also shown that the number of independent experimental stress-strain curves used as input is critical to reach an accurate solution to the inverse optimization problem. It was concluded that at least three independent stress-strain curves are necessary to determine the single crystal behavior from polycrystal tests in the case of highly textured Mg alloys

Microsaccade generation requires a foveal anchor

Visual scene characteristics have the ability to affect various aspects of saccade and microsaccade dynamics. For example, blank visual scenes are known to elicit diminished saccade and microsaccade production, compared to natural scenes. Similarly, microsaccades are less frequent in the dark. Yet, the extent to which foveal and peripheral visual information contribute to microsaccade production remains unclear: because microsaccade are directed to covert attention locations as per the superior colliculus activation map, it follows that peripheral stimulation could suffice to produce regular microsaccade dynamics, even without foveal stimulation being present. Here we compared the characteristics of microsaccades generated in the presence or absence of foveal and/or peripheral visual stimulation, while human subjects conducted four types of oculomotor tasks (fixation, free-viewing, guided-viewing and fixation during passive viewing). Foveal information was either available, or made unavailable by the presentation of both solid and blurred scotomas. We found foveal stimulation to be critical for microsaccade production, and peripheral stimulation, by itself, to be insufficient to yield microsaccades. Our results indicate that a foveal visual anchor is necessary for microsaccade generation.  

Vitamin D deficiency among children and adolescents living in sunny South Texas

Background: Exposure to sunlight is essential to produce Vitamin D (ViD). Recent studies suggest obesity is associated with low ViD concentration. Living in South Texas with 220 sunny days a year should be enough to maintain adequate ViD levels. We aimed to analyze ViD levels and obesity in children and adolescents. Methods: We included 1239 pediatric (1.5 to 18.8 years old) participants (primary care clinic from Laredo) with registered CDC percentiles of BMI (pBMI) and serum concentrations of ViD (Atellica™). Data are described as median (p25, p75), Loess correlation between pBMI and ViD, ANCOVA to adjust by age, sex, and pBMI. We used the program Stata v16.1. The size of effects is expressed as Cohen-d and eta squared (eta2). Results: The median age was 12.5 (9.5, 15.1) years, pBMI was 94 (80, 98), 49% females (n=611). The pBMI showed small differences by sex (M 82.1±24 vs M 84.5±23, Cohen-d 0.14, p,0.001). The Loess showed an inverse relationship between pBMI with a rapid drop of ViD from p90. The ANCOVA coefficients were negative for sex (b=- 0.32 for females p=0.007, eta2=0.03) and pBMI (b=-0.001, p=0.025, eta2=0.15) on ViD concentration. Conclusion: We conclude obesity and female are related to low concentration VitD in sunny Laredo. Perhaps participants with more pBMI have less outdoor physical activity and increased sequester of ViD from adipose tissue. Future research should analyze the effect of these findings on adulthood morbidity

Serendipitous discovery of RR Lyrae stars in the Leo V ultra-faint galaxy

During the analysis of RR Lyrae stars discovered in the High cadence Transient Survey (HiTS) taken with the Dark Energy Camera at the 4-m telescope at Cerro Tololo Inter-American Observatory, we found a group of three very distant, fundamental mode pulsator RR Lyrae (type ab). The location of these stars agrees with them belonging to the Leo V ultra-faint satellite galaxy, for which no variable stars have been reported to date. The heliocentric distance derived for Leo V based on these stars is 173 +/- 5 kpc. The pulsational properties (amplitudes and periods) of these stars locate them within the locus of the Oosterhoff II group, similar to most other ultra-faint galaxies with known RR Lyrae stars. This serendipitous discovery shows that distant RR Lyrae stars may be used to search for unknown faint stellar systems in the outskirts of the Milky Way.Comment: Accepted in ApJ Letter

Effect of smoking on body weight: longitudinal analysis of the SUN cohort

Our aim was to investigate prospectively the association between two major cardiovascular risk factors: smoking and weight gain. METHODS: We prospectively evaluated 7565 individuals taking part in a dynamic cohort study over a median follow-up period of 50 months. Self-reported weight and physical activity levels had been validated previously. The adjusted mean difference in weight gain relative to never-smokers (the reference group) was estimated for different levels of tobacco exposure. RESULTS: After adjusting for age, baseline body mass index, sedentary lifestyle, changes in physical activity level, total energy intake, fiber intake, food consumption between meals, and sugary soft drink, fast food and alcohol consumption, it was found that individuals who stopped smoking during follow-up had a greater relative weight gain: men 1.63 kg (95% confidence interval [CI], 1.07-2.19 kg), and women 1.51 kg (95% CI, 1.11-1.91 kg). In addition, active smokers had a greater weight gain than never-smokers: men 0.49 kg (95% CI, 0.11-0.87 kg), and women 0.36 kg (95% CI, 0.07-0.65 kg). CONCLUSIONS: Individuals who stopped smoking during follow-up and active smokers both experienced significantly greater weight gains than never-smokers. This association between cardiovascular risk factors should be taken into account when developing prevention programs

Entropy based parametrization of soils: Models and Tools

Particle-size distribution (PSD) is a fundamental soil physical property. The PSD is commonly reported in terms of the mass percentages of sand, silt and clay present