Nonuniqueness in a minimal model for cell motility

Two–phase flow models have been used previously to model cell motility, however these have rapidly become very complicated, including many physical processes, and are opaque. Here we demonstrate that even the simplest one–dimensional, two–phase, poroviscous, reactive flow model displays a number of behaviours relevant to cell crawling. We present stability analyses that show that an asymmetric perturbation is required to cause a spatially uniform, stationary strip of cytoplasm to move, which is relevant to cell polarization. Our numerical simulations identify qualitatively distinct families of travelling–wave solution that co–exist at certain parameter values. Within each family, the crawling speed of the strip has a bell–shaped dependence on the adhesion strength. The model captures the experimentally observed behaviour that cells crawl quickest at intermediate adhesion strengths, when the substrate is neither too sticky nor too slippy

Thinking territory historically.

BACKGROUND: While the randomised controlled trial (RCT) is generally regarded as the design of choice for assessing the effects of health care, within the social sciences there is considerable debate about the relative suitability of RCTs and non-randomised studies (NRSs) for evaluating public policy interventions. // OBJECTIVES: To determine whether RCTs lead to the same effect size and variance as NRSs of similar policy interventions; and whether these findings can be explained by other factors associated with the interventions or their evaluation. // METHODS: Analyses of methodological studies, empirical reviews, and individual health and social services studies investigated the relationship between randomisation and effect size of policy interventions by: 1) Comparing controlled trials that are identical in all respects other than the use of randomisation by 'breaking' the randomisation in a trial to create non-randomised trials (re-sampling studies). 2) Comparing randomised and non-randomised arms of controlled trials mounted simultaneously in the field (replication studies). 3) Comparing similar controlled trials drawn from systematic reviews that include both randomised and non-randomised studies (structured narrative reviews and sensitivity analyses within meta-analyses). 4) Investigating associations between randomisation and effect size using a pool of more diverse RCTs and NRSs within broadly similar areas (meta-epidemiology). // RESULTS: Prior methodological reviews and meta-analyses of existing reviews comparing effects from RCTs and nRCTs suggested that effect sizes from RCTs and nRCTs may indeed differ in some circumstances and that these differences may well be associated with factors confounded with design. Re-sampling studies offer no evidence that the absence of randomisation directly influences the effect size of policy interventions in a systematic way. No consistent explanations were found for randomisation being associated with changes in effect sizes of policy interventions in field trials

One Hour of Chemical Demonstrations

This article describes a diverse set of chemistry demonstrations especially selected to encourage student interaction and to be easily transported. The demonstrations may be presented at a level that can be tailored to any audience– from very young children to high school students planning careers in science. An ideal environment is a small classroom with 20-30 students where everyone can take part in the discussion. Once the chemicals are prepared, the collection of demonstrations takes about ten minutes to set-up, and one hour (or less) to perform. Very little is needed at the visiting site, no more than a table and a pitcher of water. A single electrical outlet is useful, but not essential. In Table 2 th

Three-Dimensional Propagation of Magnetohydrodynamic Waves in Solar Coronal Arcades

We numerically investigate the excitation and temporal evolution of oscillations in a two-dimensional coronal arcade by including the three-dimensional propagation of perturbations. The time evolution of impulsively generated perturbations is studied by solving the linear, ideal magnetohydrodynamic (MHD) equations in the zero-beta approximation. As we neglect gas pressure the slow mode is absent and therefore only coupled MHD fast and Alfven modes remain. Two types of numerical experiments are performed. First, the resonant wave energy transfer between a fast normal mode of the system and local Alfven waves is analyzed. It is seen how, because of resonant coupling, the fast wave with global character transfers its energy to Alfvenic oscillations localized around a particular magnetic surface within the arcade, thus producing the damping of the initial fast MHD mode. Second, the time evolution of a localized impulsive excitation, trying to mimic a nearby coronal disturbance, is considered. In this case, the generated fast wavefront leaves its energy on several magnetic surfaces within the arcade. The system is therefore able to trap energy in the form of Alfvenic oscillations, even in the absence of a density enhancement such as that of a coronal loop. These local oscillations are subsequently phase-mixed to smaller spatial scales. The amount of wave energy trapped by the system via wave energy conversion strongly depends on the wavelength of perturbations in the perpendicular direction, but is almost independent from the ratio of the magnetic to density scale heights.Comment: 27 pages, 11 figure

Three-dimensional oblique water-entry problems at small\ud deadrise angles

This paper extends Wagner theory for the ideal, incompressible normal impact of rigid bodies that are nearly parallel to the surface of a liquid half-space. The impactors considered are three-dimensional and have an oblique impact velocity. A variational formulation is used to reveal the relationship between the oblique and corresponding normal impact solutions. In the case of axisymmetric impactors, several geometries are considered in which singularities develop in the boundary of the effective wetted region. We present the corresponding pressure profiles and models for the splash sheets

Reduced dimensionality in layered quantum dimer magnets: Frustration vs. inhomogeneous condensates

Motivated by recent experiments on BaCuSi2O6, we investigate magnetic excitations and quantum phase transitions of layered dimer magnets with inter-layer frustration. We consider two scenarios, (A) a lattice with one dimer per unit cell and perfect inter-layer frustration, and (B) an enlarged unit cell with inequivalent layers, with and without perfect frustration. In all situations, the critical behavior at asymptotically low temperatures is three-dimensional, but the corresponding crossover scale may be tiny. Magnetic ordering in case (B) can be discussed in terms of two condensates; remarkably, perfect frustration renders the proximity effect ineffective. Then, the ordering transition will be generically split, with clear signatures in measurable properties. Using a generalized bond-operator method, we calculate the low-temperature magnetic properties in the paramagnetic and antiferromagnetic phases. Based on the available experimental data on BaCuSi2O6, we propose that scenario (B) with inequivalent layers and imperfect frustration is realized in this material, likely with an additional modulation of the inter-layer couling along the c axis.Comment: 23 pages, 18 figs, (v2) new fig for bandwidths, (v3) triplon binding energy discussed, (v4) small changes for clarification, accepted (PRB