Motion of an Adhesive Gel in a Swelling Gradient: a Mechanism for Cell Locomotion

Motivated by the motion of nematode sperm cells, we present a model for the motion of an adhesive gel on a solid substrate. The gel polymerizes at the leading edge and depolymerizes at the rear. The motion results from a competition between a self-generated swelling gradient and the adhesion on the substrate. The resulting stress provokes the rupture of the adhesion points and allows for the motion. The model predicts an unusual force-velocity relation which depends in significant ways on the point of application of the force.Comment: 4 pages, 1 figur

Casimir stresses in active nematic films

We calculate the Casimir stresses in a thin layer of active fluid with nematic order. By using a stochastic hydrodynamic approach for an active fluid layer of finite thickness $L$, we generalize the Casimir stress for nematic liquid crystals in thermal equilibrium to active systems. We show that the active Casimir stress differs significantly from its equilibrium counterpart. For contractile activity, the active Casimir stress, although attractive like its equilibrium counterpart, diverges logarithmically as $L$ approaches a threshold of the spontaneous flow instability from below. In contrast, for small extensile activity, it is repulsive, has no divergence at any $L$ and has a scaling with $L$ different from its equilibrium counterpart

The actin cortex as an active wetting layer

Using active gel theory we study theoretically the properties of the cortical actin layer of animal cells. The cortical layer is described as a non-equilibrium wetting film on the cell membrane. The actin density is approximately constant in the layer and jumps to zero at its edge. The layer thickness is determined by the ratio of the polymerization velocity and the depolymerization rate of actin.Comment: submitted to Eur Phys Jour

"Trade and the structure of cities"

Our purpose is to investigate how the interplay between trade, commuting and communication costs shapes the economy at both the interregional and intra-urban levels. Specifically, we study how economic integration affects the internal structure of cities and show how decentralizing the production and consumption of goods in secondary employment centers allows firms located in a large city to maintain their predominance. Several new results in both economic geography and urban economics are established, which all agree with empirical evidence.

Maximal fluctuations of confined actomyosin gels: dynamics of the cell nucleus

We investigate the effect of stress fluctuations on the stochastic dynamics of an inclusion embedded in a viscous gel. We show that, in non-equilibrium systems, stress fluctuations give rise to an effective attraction towards the boundaries of the confining domain, which is reminiscent of an active Casimir effect. We apply this generic result to the dynamics of deformations of the cell nucleus and we demonstrate the appearance of a fluctuation maximum at a critical level of activity, in agreement with recent experiments [E. Makhija, D. S. Jokhun, and G. V. Shivashankar, Proc. Natl. Acad. Sci. U.S.A. 113, E32 (2016)].Comment: 12 pages, 5 figure

Development, A question of Opportunity. A critique of the 2006 World Development Report, Equity and Development

The World Bank’s World Development Report 2006 addresses Equity and Development. It defines equity as respect for equal opportunities combined with the avoidance of absolute deprivation. Even though justice theories have long been interested in equity (given that equality of opportunity is one of the recognised values of Western society), it has hitherto remained a marginal issue in development economics. Our critique presents a detailed analysis of this report in the light of recent economic studies on this subject and endeavours to place it in the context of the evolution of World Bank thinking and policies. The first part illustrates the wealth of this concept, with its downside being that it is hard to accurately define. The second part demonstrates the gap between the prospects opened up by the enlargement of the development goals beyond poverty reduction and the report’s policy recommendations, which are generally an extension of the World Bank’s traditional analyses. The future of the equity concept for development policy-making could be closely dependent on the development community’s ability to take on board both all its complexity and wealth.

Soft inclusion in a confined fluctuating active gel

We study stochastic dynamics of a point and extended inclusion within a one dimensional confined active viscoelastic gel. We show that the dynamics of a point inclusion can be described by a Langevin equation with a confining potential and multiplicative noise. Using a systematic adiabatic elimination over the fast variables, we arrive at an overdamped equation with a proper definition of the multiplicative noise. To highlight various features and to appeal to different biological contexts, we treat the inclusion in turn as a rigid extended element, an elastic element and a viscoelastic (Kelvin-Voigt) element. The dynamics for the shape and position of the extended inclusion can be described by coupled Langevin equations. Deriving exact expressions for the corresponding steady state probability distributions, we find that the active noise induces an attraction to the edges of the confining domain. In the presence of a competing centering force, we find that the shape of the probability distribution exhibits a sharp transition upon varying the amplitude of the active noise. Our results could help understanding the positioning and deformability of biological inclusions, eg. organelles in cells, or nucleus and cells within tissues.Comment: 16 pages, 9 figure

Occupational and internal labour markets in Britain and France.

Proposes a topology to capture a broad range of the country variation in such areas as recruitment and training practices, job demarcation, employee mobility and collective bargaining. Basis of the proposed topology; Implications of the empirical data on the labor markets in Great Britain and France for policy-makers.