Finger behavior of a shear thinning fluid in a Hele-Shaw cell

We make a theoretical study of the behavior of a simple fluid displacing a shear thinning fluid confined in a Hele-Shaw cell. To study the Saffman-Taylor instability when the displaced fluid is non Newtonian we face the problem of having a field which is non laplacian. By means of an hodographic transformation we are able to solve the problem in the case of weak shear thinning while taking into account the non laplacian character of the equation. Our results predict that the finger width decreases towards zero for small values of the surface tension parameter which is inversely proportional to the finger velocity.Comment: 4 pages, 1 figure, RevTeX, to appear in Phys. Rev. Let

Time integration and steady-state continuation for 2d lubrication equations

Lubrication equations allow to describe many structurin processes of thin liquid films. We develop and apply numerical tools suitable for their analysis employing a dynamical systems approach. In particular, we present a time integration algorithm based on exponential propagation and an algorithm for steady-state continuation. In both algorithms a Cayley transform is employed to overcome numerical problems resulting from scale separation in space and time. An adaptive time-step allows to study the dynamics close to hetero- or homoclinic connections. The developed framework is employed on the one hand to analyse different phases of the dewetting of a liquid film on a horizontal homogeneous substrate. On the other hand, we consider the depinning of drops pinned by a wettability defect. Time-stepping and path-following are used in both cases to analyse steady-state solutions and their bifurcations as well as dynamic processes on short and long time-scales. Both examples are treated for two- and three-dimensional physical settings and prove that the developed algorithms are reliable and efficient for 1d and 2d lubrication equations, respectively.Comment: 33 pages, 16 figure

Evaporation of a thin film: diffusion of the vapour and Marangoni instabilities

The stability of an evaporating thin liquid film on a solid substrate is investigated within lubrication theory. The heat flux due to evaporation induces thermal gradients; the generated Marangoni stresses are accounted for. Assuming the gas phase at rest, the dynamics of the vapour reduces to diffusion. The boundary condition at the interface couples transfer from the liquid to its vapour and diffusion flux. A non-local lubrication equation is obtained; this non-local nature comes from the Laplace equation associated with quasi-static diffusion. The linear stability of a flat film is studied in this general framework. The subsequent analysis is restricted to moderately thick films for which it is shown that evaporation is diffusion limited and that the gas phase is saturated in vapour in the vicinity of the interface. The stability depends only on two control parameters, the capillary and Marangoni numbers. The Marangoni effect is destabilising whereas capillarity and evaporation are stabilising processes. The results of the linear stability analysis are compared with the experiments of Poulard et al (2003) performed in a different geometry. In order to study the resulting patterns, the amplitude equation is obtained through a systematic multiple-scale expansion. The evaporation rate is needed and is computed perturbatively by solving the Laplace problem for the diffusion of vapour. The bifurcation from the flat state is found to be a supercritical transition. Moreover, it appears that the non-local nature of the diffusion problem unusually affects the amplitude equation

Cell motility: a viscous fingering analysis of active gels

The symmetry breaking of the actin network from radial to longitudinal symmetry has been identified as the major mechanism for keratocytes (fish cells) motility on solid substrate. For strong friction coefficient, the two dimensional actin flow which includes the polymerisation at the edge and depolymerisation in the bulk can be modelled as a Darcy flow, the cell shape and dynamics being then modelled by standard complex analysis methods. We use the theory of active gels to describe the orientational order of the filaments which varies from the border to the bulk. We show analytically that the reorganisation of the cortex is enough to explain the motility of the cell and find the velocity as a function of the orientation order parameter in the bulk.Comment: 15 pages, 4 figures, accepted for publication in EPJ - Plu

Les psychotropes criminogènes

Les psychotropes altèrent le psychisme d’un individu et peuvent affecter les perceptions, l’humeur, la conscience, le comportement et diverses fonctions psychologiques et physiques. Ils se divisent en cinq catégories : les dépresseurs du système nerveux central (SNC), les stimulants du SNC, les perturbateurs du SNC, les médicaments psychothérapeutiques et les androgènes et stéroïdes anabolisants.L’abus de certains psychotropes peut conduire à la tolérance, la dépendance psychologique, la dépendance physique et la toxicomanie. Un des phénomènes liés à la toxicomanie est la criminalité. Les psychotropes criminogènes directs par excellence sont l’alcool, les amphétamines, la cocaïne et la phencyclidine (PCP). Leurs propriétés pharmacologiques et toxicologiques sont variées. La relation intoxication-criminalité est complexe et dépend de divers facteurs biopsychosociaux.Psychotropics alter the psychism of an individual and can affect perceptions, mood, thinking, behaviour and various psychological and physical functions. They are divided in five categories: central nervous system (CNS) depressants, CNS stimulants, CNS disturbers, psychotherapeutic medications and androgens and anabolic steroids.Abuse of some psychotropic substances can induce tolerance, psychological dependence, physical dependence and toxicomania. One of the phenomenons related to toxicomania is criminality. Direct criminogenic psychotropics are typically alcohol, amphetamines, cocaine and phencyclidine (PCP). Their pharmacological and toxicological properties are diverse. The intoxication-criminality relationship is complex and depends on several bio-psychosocial factors

Women's political participation in Tunisia. Arab Citizenship Review No. 14, 21 January 2016

The collapse of the authoritarian regime in Tunisia in 2011 has given women new opportunities to participate in political life and in civil society activities, standing for elections (2011 and 2014) or becoming members of political parties and associations. Nevertheless, despite these advances and the already positive point of de- parture thanks to the legacy of Bourguiba, the “liberator of the country and the women”, participation of women remains unsatisfactory. While Tunisian women have enjoyed extended individual rights, especially compared to Arab women in general, since the country became independent in 1956, their political participation has remained controlled by the state.T he challenge of increasing the political participation of women, even in a democratic phase of the country’s political life, remains.The new electoral laws from 2011 and 2014 endorse parity and women rights, now guaranteed by the Constitution. However, the implementation of these rights is still at stake

Collective chemotaxis and segregation of active bacterial colonies

International audienceStill recently, bacterial fluid suspensions have motivated a lot of works, both experimental and theoretical, with the objective to understand their collective dynamics from universal and simple rules. Since some species are active, most of these works concern the strong interactions that these bacteria exert on a forced flow leading to instabilities, chaos and turbulence. Here, we investigate the self-organization of expanding bacterial colonies under chemotaxis, proliferation and eventually active-reaction. We propose a simple model to understand and quantify the physical properties of these living organisms which either give cohesion or on the contrary dispersion to the colony. Taking into account the diffusion and capture of morphogens complicates the model since it induces a bacterial density gradient coupled to bacterial density fluctuations and dynamics. Nevertheless under some specific conditions, it is possible to investigate the pattern formation as a usual viscous fingering instability. This explains the similarity and differences of patterns according to the physical bacterial suspension properties and explain the factors which favor compactness or branching

Direct Torque Control of a Doubly Fed Induction Generator

In this paper a direct torque control system is proposed and is applied to doubly fed induction generator (DFIG) based wind power generation systems. In this control system the rotor flux and the electromagnetic torque are estimated based on the rotor voltage and currents measurements. The validity and effectiveness of this control is demonstrated by some simulation results

Some fixed point theorems and existence of weak solutions of Volterra integral equation under Henstock-Kurzweil-Pettis integrability

summary:In this paper we examine the set of weakly continuous solutions for a Volterra integral equation in Henstock-Kurzweil-Pettis integrability settings. Our result extends those obtained in several kinds of integrability settings. Besides, we prove some new fixed point theorems for function spaces relative to the weak topology which are basic in our considerations and comprise the theory of differential and integral equations in Banach spaces