Kinetic Heterogeneities at Dynamical Crossovers

We perform molecular dynamics simulations of a model glass-forming liquid to measure the size of kinetic heterogeneities, using a dynamic susceptibility $\chi_{\rm ss}(a, t)$ that quantifies the number of particles whose dynamics are correlated on the length scale $a$ and time scale $t$. By measuring $\chi_{\rm ss}(a, t)$ as a function of both $a$ and $t$, we locate local maxima $\chi^\star$ at distances $a^\star$ and times $t^\star$. Near the dynamical glass transition, we find two types of maxima, both correlated with crossovers in the dynamical behavior: a smaller maximum corresponding to the crossover from ballistic to sub-diffusive motion, and a larger maximum corresponding to the crossover from sub-diffusive to diffusive motion. Our results indicate that kinetic heterogeneities are not necessarily signatures of an impending glass or jamming transition.Comment: 6 pages, 4 figure

On a result of R. R. Hall

AbstractAn upper bound for the mean value of a non-negative submultiplicative function by R. R. Hall [3] is sharpened and generalised. Hall's inequality implies a certain rather accurate upper sieve estimate, and this aspect of Hall's result is exploited in deriving good lower bounds for π(x) via the sieve

About the determination of critical exponents related to possible phase transitions in nuclear fragmentation

We introduce a method based on the finite size scaling assumption which allows to determine numerically the critical point and critical exponents related to observables in an infinite system starting from the knowledge of the observables in finite systems. We apply the method to bond percolation in 2 dimensions and compare the results obtained when the bond probability p or the fragment multiplicity m are chosen as the relevant parameter.Comment: 12 pages, TeX, 4 figure

Dynamic facilitation explains democratic particle motion of metabasin transitions

Transitions between metabasins in supercooled liquids seem to occur through rapid "democratic" collective particle rearrangements. Here we show that this apparent homogeneous particle motion is a direct consequence of dynamic facilitation. We do so by studying metabasin transitions in facilitated spin models and constrained lattice gases. We find that metabasin transitions occur through a sequence of locally facilitated events taking place over a relatively short time frame. When observed on small enough spatial windows these events appear sudden and homogeneous. Our results indicate that metabasin transitions are essentially "non-democratic" in origin and yet another manifestation of dynamical heterogeneity in glass formers.Comment: 6 pages, 6 figure

Snowier Winters Extend Autumn Availability of High-quality Forage for Caribou in Arctic Alaska

Caribou (Rangifer tarandus) rely on the short Arctic growing season to restore body condition, support the demands of lactation, and prepare for the long arctic winter, making them susceptible to even small changes in forage availability or quality. Body condition in the summer and autumn is linked to winter survival rates and fecundity in cows, critical factors in the productivity of caribou populations. Climate change predictions of warmer and wetter northern winters suggest increased snowfall over Alaska’s North Slope, which has recently been verified between 1995 and 2017. However, a comprehensive analysis of how deeper snow will affect caribou forage quality is absent across Alaska. In this study, we quantify how snow depth alters the quality and seasonality of caribou forage using a long-term (24 yr) International Tundra Experiment snow depth manipulation to evaluate how winter climate change scenarios may affect tussock tundra systems in northern Alaska. Deeper snow in prior winters leads to increases in growing season leaf N and digestible protein (DP) in deciduous shrubs (and Betula spp.) and graminoids (Carex spp. and Eriophorum spp.), but not evergreen dwarf shrubs (Rhododendron spp. and Vaccinium spp.). Dry matter digestibility varied among species with small differences (\u3c5%) associated with snow depth. Most striking was the discovery that deeper snow in the prior winter increased the duration of DP levels above the minimum threshold for protein gain in caribou by as much as 25 d in Salix pulchra and 6–9 d in Betula nana and Carex bigelowii in late summer and early autumn. Consequently, deeper winter snow may provide an extended window of opportunity for foraging and the accumulation of lean body mass and fat reserves which promote winter survival and successful calving the following spring and potentially improve the productivity of caribou in northern Alaska

Surface probe measurements of the elasticity of sectioned tissue, thin gels and polyelectrolyte multilayer films : correlations between substrate stiffness and cell adhesion

Surface probe measurements of the elasticity of thin-film matrices as well as biological samples prove generally important to understanding cell attachment across such systems. To illustrate this, sectioned arteries were probed by Atomic Force Microscopy (AFM) within the smooth muscle cell (SMC)-rich medial layer, yielding an apparent Young’s modulus Emedia ~ 5-8 kPa. Polyacrylamide gels with Egel spanning several-fold above and below this range were then cast 5-70 μm thick and coated with collagen: SMC spreading shows a hyperbolic dependence in projected cell area versus Egel. The modulus that gives half-max spreading is E1/2-spread ~ 8-10 kPa, proving remarkably close to Emedia. More complex, layer-by-layer micro-films of poly(L-lysine)/hyaluronic acid were also tested and show equivalent trends of increased SMC spreading with increased stiffness. Adhesive spreading of cells thus seems to correlate broadly with the effective stiffness of synthetic materials and tissues

Experimental search for dynamic heterogeneities in molecular glass formers

We have measured the linear dielectric susceptibility of two molecular glass formers close to Tg in order to estimate the size of the dynamically correlated clusters of molecules which are expected to govern the physics of glass formation. This size has been shown to be related to the dynamic dielectric susceptibility dEps(w)/dT (Eps : dielectric susceptibility, T : temperature, w&#61472;: frequency). To allow for an accurate determination of the T derivative, we scanned the interval 192 < T < 232 K every 1 K for glycerol and 159 < T < 179 K every 0.5 K for propylene carbonate. The resolution on T variations was about 1 mK. The result for glycerol is that the number of correlated molecules increases by a factor 3 when T goes from 226 to 195 K. It has been shown that the non-linear susceptibility provides a direct measurement of dynamic correlations. To measure it, we used a standard Lockin technique yielding the third harmonic of the current flowing out of a capacitor. We obtained only an upper limit on the ratio of the third to the first harmonic, due to the non-linear response of standard electronics.Comment: 7 page

Finite size effects and the order of a phase transition in fragmenting nuclear systems

We discuss the implications of finite size effects on the determination of the order of a phase transition which may occur in infinite systems. We introduce a specific model to which we apply different tests. They are aimed to characterise the smoothed transition observed in a finite system. We show that the microcanonical ensemble may be a useful framework for the determination of the nature of such transitions.Comment: LateX, 5 pages, 5 figures; Fig. 1 change

Combinatorial sieves of dimension exceeding one

AbstractA general sieve for each dimension κ > 1 is given which improves the sieve estimates of Ankeny and Onishi. The work depends on a combinatorial identity which is invariant under Buchstab iteration and on the solution of a pair of differential-difference equations with side conditions