Photoelastic force measurements in granular materials

Photoelastic techniques are used to make both qualitative and quantitative measurements of the forces within idealized granular materials. The method is based on placing a birefringent granular material between a pair of polarizing filters, so that each region of the material rotates the polarization of light according to the amount of local of stress. In this review paper, we summarize past work using the technique, describe the optics underlying the technique, and illustrate how it can be used to quantitatively determine the vector contact forces between particles in a 2D granular system. We provide a description of software resources available to perform this task, as well as key techniques and resources for building an experimental apparatus

Evolution of displacements and strains in sheared amorphous solids

The local deformation of two-dimensional Lennard-Jones glasses under imposed shear strain is studied via computer simulations. Both the mean squared displacement and mean squared strain rise linearly with the length of the strain interval $\Delta \gamma$ over which they are measured. However, the increase in displacement does not represent single-particle diffusion. There are long-range spatial correlations in displacement associated with slip lines with an amplitude of order the particle size. Strong dependence on system size is also observed. The probability distributions of displacement and strain are very different. For small $\Delta \gamma$ the distribution of displacement has a plateau followed by an exponential tail. The distribution becomes Gaussian as $\Delta \gamma$ increases to about .03. The strain distributions consist of sharp central peaks associated with elastic regions, and long exponential tails associated with plastic regions. The latter persist to the largest $\Delta \gamma$ studied.Comment: Submitted to J. Phys. Cond. Mat. special volume for PITP Conference on Mechanical Behavior of Glassy Materials. 16 Pages, 8 figure

Spectral catalogue of the intermediate ionization states of iron in the extreme ultraviolet

Using precisely controlled laboratory conditions we have begun to establish a spectral catalogue of the intermediate ionization states of iron, Fe IX - Fe XXIV, in the extreme ultraviolet. The measurements are being performed in support of the development of reliable modeling codes for the analysis of data from the Extreme Ultraviolet Explorer and future space astrophysics missions sensitive to extreme ultraviolet radiation. They aim to resolve the controversies surrounding the short-wavelength spectra of stellar coronae. Preliminary measurements showing the wealth of iron lines in the 50-120 {Angstrom} region are presented

Force distributions in a triangular lattice of rigid bars

We study the uniformly weighted ensemble of force balanced configurations on a triangular network of nontensile contact forces. For periodic boundary conditions corresponding to isotropic compressive stress, we find that the probability distribution for single-contact forces decays faster than exponentially. This super-exponential decay persists in lattices diluted to the rigidity percolation threshold. On the other hand, for anisotropic imposed stresses, a broader tail emerges in the force distribution, becoming a pure exponential in the limit of infinite lattice size and infinitely strong anisotropy.Comment: 11 pages, 17 figures Minor text revisions; added references and acknowledgmen

Memory of the Unjamming Transition during Cyclic Tiltings of a Granular Pile

Discrete numerical simulations are performed to study the evolution of the micro-structure and the response of a granular packing during successive loading-unloading cycles, consisting of quasi-static rotations in the gravity field between opposite inclination angles. We show that internal variables, e.g., stress and fabric of the pile, exhibit hysteresis during these cycles due to the exploration of different metastable configurations. Interestingly, the hysteretic behaviour of the pile strongly depends on the maximal inclination of the cycles, giving evidence of the irreversible modifications of the pile state occurring close to the unjamming transition. More specifically, we show that for cycles with maximal inclination larger than the repose angle, the weak contact network carries the memory of the unjamming transition. These results demonstrate the relevance of a two-phases description -strong and weak contact networks- for a granular system, as soon as it has approached the unjamming transition.Comment: 13 pages, 15 figures, soumis \`{a} Phys. Rev.

Improved electron-beam ion-trap lifetime measurement of the Ne8+ 1s2s3S1 level

An earlier electron-beam ion-trap (EBIT) lifetime measurement of the Ne8+ 1s2s3S1 level has been improved upon, reducing the uncertainties to less than the scatter in the existing theoretical calculations. The new result, 91.7±0.4 μs, agrees with the previous value, but is more precise by a factor of 4. The new value distinguishes among theoretical values, as agreement is obtained only with those calculations that employ "exact" nonrelativistic or relativistic wave functions. Routes to measurements with even higher accuracy are discussed

Quasiperiodic Tip Splitting in Directional Solidification

We report experimental results on the tip splitting dynamics of seaweed growth in directional solidification of succinonitrile alloys with poly(ethylene oxide) or acetone as solutes. The seaweed or dense branching morphology was selected by solidifying grains which are oriented close to the {111} plane. Despite the random appearance of the growth, a quasiperiodic tip splitting morphology was observed in which the tip alternately splits to the left and to the right. The tip splitting frequency f was found to be related to the growth velocity V as a power law f V^{1.5}. This finding is consistent with the predictions of a tip splitting model that is also presented. Small anisotropies are shown to lead to different kinds of seaweed morphologies.Comment: 4 pages, 7 figures, submitted to Physical Review Letter

Effect of low dose, short-term creatine supplementation on muscle power output in elite youth soccer players

Background: To determine the effects of a low dose, short-term Creatine monohydrate (Cr) supplementation (0.03 g.kg.d-1 during 14 d) on muscle power output in elite youth soccer players. Methods: Using a two-group matched, double blind, placebo-controlled design, nineteen male soccer players (mean age = 17.0 ± 0.5 years) were randomly assigned to either Cr (N = 9) or placebo (N = 10) group. Before and after supplementation, participants performed a 30s Wingate Anaerobic Test (WAnT) to assess peak power output (PPO), mean power output (MPO), fatigue index (FI), and total work. Results: There were significant increases in both PPO and MPO after the Cr supplementation period (P ≤ 0.05) but not the placebo period. There were also significant increases in total work, but not FI, after the Cr supplementation and placebo periods (P ≤ 0.05). Notably, there were differences in total work between the Cr and placebo groups after (P ≤ 0.05) but not before the 14 d supplementation period. Conclusion: There is substantial evidence to indicate that a low-dose, short-term oral Cr supplementation beneficially affected muscle power output in elite youth soccer players

Experimental M1 Transition Rates of Coronal Lines from Ar X, Ar XIV, and Ar XV

Transition probabilities of three magnetic dipole (M1) transitions in multiply charged ions of Ar have been measured using the Livermore electron-beam ion trap. Two of the transitions are in the ground configurations of Ar XIV (B-like) and Ar IX (F-like), and are associated with the coronal lines at 4412.4 and 5533.4 ÅŽ , respectively. The third is in the excited 2s2p configuration of Be-like Ar XV and produces the coronal line at 5943.73 Å. Our results for the three atomic level lifetimes are 9.32^0.12 ms for the Ar X 2s22p5 2P1/2 level, 9.70^0.15 ms for the Ar XIV 2s22p level, and 15.0^0.8 ms for the Ar XVo 2P3/2o 2s2p level. These results diff†er significantly from earlier measurements and are the most accurate ones to date

Youth Gambling: The health and wellbeing of New Zealand secondary school students in 2012

Gambling has become a widely available activity in today’s society (Hardoon & Derevensky, 2002; Turchi & Derevensky, 2006). In fact, many researchers “have noted that an entire generation has now grown up in an era when lottery and casino gambling is widely available and heavily advertised” (Volberg, Gupta, Griffiths, Ólason, & Delfabbro, 2010, p. 3). Evidence suggests that it has become a popular past-time not only for adults, but also for children and young people (Derevensky & Gupta, 2000; Gupta & Derevensky, 1998a; Hardoon & Derevensky, 2002; Jacobs, 2000; Splevins, Mireskandari, Clayton, & Blaszczynski, 2010; Turchi & Derevensky, 2006). Moreover, research indicates that gambling is one of the first risky activities that adolescents become involved with (i.e. they begin gambling prior to experimentation with alcohol, drugs, sexual behaviour) (Volberg, et al., 2010). Whilst for many youth involvement in gambling does not result in problematic behaviour, others go on to experience serious problems (Dickson, Derevensky, & Gupta, 2003). A vast range of adolescent gambling prevalence studies that have been undertaken over the past 25 years, across different countries, and incorporating general populations as well as youth specifically. Rates of youth problem gambling have often been found to be higher than the rates identified for adults (Huang & Boyer, 2007; Shaffer & Hall, 1996; Welte, Barnes, Tidwell, & Hoffman, 2008; Williams, Page, Parke, & Rigbye, 2008), with some estimating them to be more than double those of adults (Gupta & Derevensky, 1998a; Jackson, Dowling, Thomas, Bond, & Patton, 2008; Lesieur, et al., 1991), or up to three times as high (Rigbye, 2010). However, it has also been recognised that there is far less research in this field compared to that which has explored other youth risk behaviours such as substance use (Blinn-Pike, Worthy, & Jonkman, 2010). The gap in New Zealand-based information regarding prevalence of youth gambling has been identified previously (Bellringer, et al., 2003; Rossen, Tse, & Vaidya, 2009) and in 2003 it was recommended that research be undertaken to measure the involvement of New Zealand youth in gambling as well as associated factors and gambling-related problems (Bellringer, et al., 2003). A limited body of research has since employed various sources of data to consider youth gambling in New Zealand (e.g. Gray, 2010; Ministry of Health, 2008, 2009; Rossen, 2008; Rossen, Butler, & Denny, 2011). An extremely valuable source of information on New Zealand youth is the University of Auckland’s (UoA) National youth health and wellbeing surveys. To date, the UoA’s Adolescent Health Research Group (AHRG) has completed three National youth health and wellbeing surveys. The Youth2000 Survey Series aim to provide nationally representative information on the health and wellbeing of young people attending New Zealand secondary schools. The Survey Series includes a wide range of questions about issues that contribute to the health and wellbeing of young people (such as substance use, injuries and violence, home and family) and allow researchers to take an ecological approach to identifying overall risk and protective factors in young people’s lives. Youth’12, a survey of 8,500 secondary school students throughout New Zealand, is the most recent survey to be undertaken by the AHRG. The inclusion of gambling items in the Youth’12 survey provides a unique opportunity to examine the impacts of gambling and problem gambling on secondary school students throughout New Zealand within an ecological framework. This report was commissioned by the Ministry of Health and begins with a comprehensive review of the local and international youth gambling literature, followed by an overview of the Youth2000 Survey Series and methodology for Youth’12. A thorough analysis of Youth’12 gambling items was undertaken with results being reported under the following eight categories (detailed results for each set of analyses are also provided in the appendices): - Students and their own gambling (Section Five); - Unhealthy gambling amongst students (Section Six); - Attitudes and motivating factors towards gambling (Section Seven); - The impacts of others’ gambling on students (Section Eight); and, - Risk and protective factors for student gambling (Section Nine); - Gambling and Māori taitamariki in Aotearoa (Section 10); - Gambling and Pacific young people in New Zealand (Section 11); and, - Gambling and Asian young people in New Zealand (Section 12). Finally, a discussion chapter provides an overview of the findings and implications