Active biopolymer networks generate scale-free but euclidean clusters

We report analytical and numerical modelling of active elastic networks, motivated by experiments on crosslinked actin networks contracted by myosin motors. Within a broad range of parameters, the motor-driven collapse of active elastic networks leads to a critical state. We show that this state is qualitatively different from that of the random percolation model. Intriguingly, it possesses both euclidean and scale-free structure with Fisher exponent smaller than $2$. Remarkably, an indistinguishable Fisher exponent and the same euclidean structure is obtained at the critical point of the random percolation model after absorbing all enclaves into their surrounding clusters. We propose that in the experiment the enclaves are absorbed due to steric interactions of network elements. We model the network collapse, taking into account the steric interactions. The model shows how the system robustly drives itself towards the critical point of the random percolation model with absorbed enclaves, in agreement with the experiment.Comment: 6 pages, 7 figure

Actively stressed marginal networks

We study the effects of motor-generated stresses in disordered three dimensional fiber networks using a combination of a mean-field, effective medium theory, scaling analysis and a computational model. We find that motor activity controls the elasticity in an anomalous fashion close to the point of marginal stability by coupling to critical network fluctuations. We also show that motor stresses can stabilize initially floppy networks, extending the range of critical behavior to a broad regime of network connectivities below the marginal point. Away from this regime, or at high stress, motors give rise to a linear increase in stiffness with stress. Finally, we demonstrate that our results are captured by a simple, constitutive scaling relation highlighting the important role of non-affine strain fluctuations as a susceptibility to motor stress.Comment: 8 pages, 4 figure

Critical behaviour in the nonlinear elastic response of hydrogels

In this paper we study the elastic response of synthetic hydrogels to an applied shear stress. The hydrogels studied here have previously been shown to mimic the behaviour of biopolymer networks when they are sufficiently far above the gel point. We show that near the gel point they exhibit an elastic response that is consistent with the predicted critical behaviour of networks near or below the isostatic point of marginal stability. This point separates rigid and floppy states, distinguished by the presence or absence of finite linear elastic moduli. Recent theoretical work has also focused on the response of such networks to finite or large deformations, both near and below the isostatic point. Despite this interest, experimental evidence for the existence of criticality in such networks has been lacking. Using computer simulations, we identify critical signatures in the mechanical response of sub-isostatic networks as a function of applied shear stress. We also present experimental evidence consistent with these predictions. Furthermore, our results show the existence of two distinct critical regimes, one of which arises from the nonlinear stretch response of semi-flexible polymers.

Small scale unit for groundwater treatment

Small scale unit for groundwater treatmen

The mechanical response of semiflexible networks to localized perturbations

Previous research on semiflexible polymers including cytoskeletal networks in cells has suggested the existence of distinct regimes of elastic response, in which the strain field is either uniform (affine) or non-uniform (non-affine) under external stress. Associated with these regimes, it has been further suggested that a new fundamental length scale emerges, which characterizes the scale for the crossover from non-affine to affine deformations. Here, we extend these studies by probing the response to localized forces and force dipoles. We show that the previously identified nonaffinity length [D.A. Head et al. PRE 68, 061907 (2003).] controls the mesoscopic response to point forces and the crossover to continuum elastic behavior at large distances.Comment: 16 pages, 18 figures; substantial changes to text and figures to clarify the crossover to continuum elasticity and the role of finite-size effect

Barrier and internal wave contributions to the quantum probability density and flux in light heavy-ion elastic scattering

We investigate the properties of the optical model wave function for light heavy-ion systems where absorption is incomplete, such as $\alpha + ^{40}$Ca and $\alpha + ^{16}$O around 30 MeV incident energy. Strong focusing effects are predicted to occur well inside the nucleus, where the probability density can reach values much higher than that of the incident wave. This focusing is shown to be correlated with the presence at back angles of a strong enhancement in the elastic cross section, the so-called ALAS (anomalous large angle scattering) phenomenon; this is substantiated by calculations of the quantum probability flux and of classical trajectories. To clarify this mechanism, we decompose the scattering wave function and the associated probability flux into their barrier and internal wave contributions within a fully quantal calculation. Finally, a calculation of the divergence of the quantum flux shows that when absorption is incomplete, the focal region gives a sizeable contribution to nonelastic processes.Comment: 16 pages, 15 figures. RevTeX file. To appear in Phys. Rev. C. The figures are only available via anonynous FTP on ftp://umhsp02.umh.ac.be/pub/ftp_pnt/figscat

Development and Validation of a Quantitative Measure of Intensive Parenting Attitudes

Intensive mothering (IM) attitudes have been considered the dominant discourse of motherhood, but have only been assessed qualitatively The goal of this study was to develop a quantitative scale to assess these ideologies, their construct validity, and their relationship to relevant constructs (i.e., work status and division of household labor). An on-line questionnaire was given to 595 mothers asking 56 questions assessing different aspects of IM attitudes as well as several validation measures. An Exploratory Factor Analysis on 315 randomly selected mothers yielded a 5 factor solution. A Confirmatory Factor Analysis on the remaining 280 mothers demonstrated good fit. The five factors expressed the ideas that 1) women are inherently better at parenting than men (Essentialism), 2) parenting should be fulfilling (Fulfillment), 3) children should be cognitively stimulated by parents (Stimulation), 4) mothering is difficult (Challenging), and 5) parents should prioritize the needs of the child (Child-Centered). Scales had adequate reliability and construct validity compared to the Parental Investment in Child scale, the Parenting Sense of Competence Scale, and Beliefs about Maternal Employment. The Essentialism, Fulfillment, and Challenging scales were positively related to having more responsibility for childcare and household chores. Stay-at-home mothers had higher scores on Essentialism and lower scores on Stimulation than both part-time and full-time working mothers supporting the notion that both working and non-working mothers have intensive parenting ideologies that are manifested in different ways

In-work poverty: a systematic review

Government strategy is committed to halving child poverty in the United Kingdom (UK) by 2010 and eradicating it by 2020. A range of policy initiatives have made encouraging progress, with 600,000 children lifted out of poverty in the last ten years. Yet, the total number of poor children living in working households has stayed the same at 1.4 million (nearly half of all poor children). If the ambitious targets are to be met, it will be important that the next phase of the strategy for tackling child poverty prioritises these families. The systematic review detailed in this report was commissioned by the Department for Work and Pensions (DWP). Systematic reviewing is a specialist technique which employs standardised and explicit methods to minimise the risk of drawing the wrong or misleading conclusions from a body of evidence. Explicit reporting of how the review was conducted allows others to assess the validity of its findings