Twisting and buckling: a new undulation mechanism for artificial swimmers

We present an artificial swimmer consisting in a long cylinder of ferrogel which is polarized transversely and in opposite directions at each extremity. When it is placed on a water film and submitted to a transverse oscillating magnetic field, this artificial worm undulates and swims. Whereas symmetry breaking is due to the field gradient, the undulations of the worm result from a torsional buckling instability as the polarized ends tend to align with the applied magnetic field. The critical magnetic field above which buckling and subsequent swimming is observed may be predicted using elasticity equations including the effect of a magnetic torque. As the length of the worm is varied, several undulation modes are observed which are in good agreement with the bending modes of an elastic rod with free ends

Capillary origami: spontaneous wrapping of a droplet with an elastic sheet

The interaction between elasticity and capillarity is used to produce three dimensional structures, through the wrapping of a liquid droplet by a planar sheet. The final encapsulated 3D shape is controlled by tayloring the initial geometry of the flat membrane. A 2D model shows the evolution of open sheets to closed structures and predicts a critical length scale below which encapsulation cannot occur, which is verified experimentally. This {\it elastocapillary length} is found to depend on the thickness as $h^{3/2}$, a scaling favorable to miniaturization which suggests a new way of mass production of 3D micro- or nano-scale objects.Comment: 5 pages, 5 figure

On the spectral problem of N=4 SYM with orthogonal or symplectic gauge group

We study the spectral problem of N=4 SYM with gauge group SO(N) and Sp(N). At the planar level, the difference to the case of gauge group SU(N) is only due to certain states being projected out, however at the non-planar level novel effects appear: While 1/N-corrections in the SU(N) case are always associated with splitting and joining of spin chains, this is not so for SO(N) and Sp(N). Here the leading 1/N-corrections, which are due to non-orientable Feynman diagrams in the field theory, originate from a term in the dilatation operator which acts inside a single spin chain. This makes it possible to test for integrability of the leading 1/N-corrections by standard (Bethe ansatz) means and we carry out various such tests. For orthogonal and symplectic gauge group the dual string theory lives on the orientifold AdS5xRP5. We discuss various issues related to semi-classical strings on this background.Comment: 25 pages, 3 figures. v2: Minor clarifications, section 5 expande

Évaluation interdisciplinaire des impacts du CICE en matière d’emplois et de salaires:Rapport du Laboratoire Interdisciplinaire d’Évaluation des Politiques Publiques (LIEPP) de Sciences Po en réponse à l’appel à évaluation de France Stratégie

Le crédit d’impôt pour la compétitivité et l’emploi (CICE) a été institué avec l’objectif d’améliorer la compétitivité des entreprises. Pour étudier ses différents effets potentiels sur l’emploi et les salaires, l’évaluation présentée ici s’appuie d’une part sur une analyse économique, et d’autre part sur une étude sociologique, dont les résultats qualitatifs avaient été détaillés lors du rapport remis le 29 septembre 2016 par le LIEPP de Sciences Po à France Stratégie. L’analyse micro-économique basée sur les données fiscales et sociales des entreprises fait tout d’abord apparaître que, au niveau de l’emploi, les décisions des nouvelles embauches des entreprises n’ont pas été affectées par la présence de la nette discontinuité du CICE. De plus, comparativement aux entreprises moins intensément ciblées par la mesure, les entreprises les plus intensément ciblées par le CICE n’ont pas connu de hausse de l’emploi entre 2013 et 2015, et cela quelle que soit la catégorie socio-professionnelle. Concernant l’effet sur les salaires, notre analyse montre que la mesure n’a pas eu d’impact détectable sur la distribution des hausses de salaires mais il apparaît toutefois qu’au niveau de l’entreprise, les sommes allouées dans le cadre du CICE ont été en partie reversées aux salariés, sous forme de hausses de salaires, en particulier aux cadres, professions intellectuelles supérieures et professions intermédiaires. Il faut garder à l’esprit que les conclusions de cette évaluation ne portent que sur les trois premières années de mise en place du CICE, les données de 2016 et 2017 n’étant pas encore disponibles au moment de l’évaluation. Il convient enfin de préciser la difficulté de toute étude portant sur le CICE : celui-ci n’a pas été conçu de manière à être évalué par un dispositif expérimental

Instability of the origami of a ferrofluid drop in a magnetic field

Capillary origami is the wrapping of an usual fluid drop by a planar elastic membrane due to the interplay between capillary and elastic forces. Here, we use a drop of magnetic fluid whose shape is known to strongly depend on an applied magnetic field. We study the quasi-static and dynamical behaviors of such a magnetic capillary origami. We report the observation of an overturning instability that the origami undergoes at a critical magnetic field. This instability is triggered by an interplay between magnetic and gravitational energies in agreement with the theory presented here. Additional effects of elasticity and capillarity on this instability are also discussed.Comment: in press in PRL (2011

Origami Capillaire

Le pelage d'un chien qui sort de l'eau s'agrège en touffes : ceci est un exemple commun de l'effet des forces capillaires sur des structures élastiques. D'un point de vue pratique, la déformation de structures flexibles par les forces de tension de surface peut conduire à de graves dommages sur des microsystèmes mécaniques (à petite échelle, les forces capillaires deviennent prépondérantes). Cependant ce collage permet également l'auto-association de microstructures selon des motifs bien définis. Au delà de la flexion de tiges, que se passe-t-il si on pose une goutte d'eau sur une feuille très flexible ? La feuille peut-elle enrober spontanément la goutte ? Nous déterminerons quel est le critère d'enrobage et décrirons les differentes formes obtenues. En particulier, nous montrerons comment un tel mécanisme d'origami capillaire peut s'avérer pertinent pour l'élaboration de micro-structures tridimensionnelles à partir de patrons bidimensionnels

Capillary origami

International audienceThe hairs of a wet dog rushing out from a pond assemble into bundles; this is a common example of the effect of capillary forces on flexible structures. From a practical point of the deformation and adhesion of compliant structures induced by interfacial forces may lead to disastrous effects in mechanical microsystems

Nager grâce à un mécanisme de torsion/flexion

Nous présentons un nageur artificiel qui progresse grâce à un mécanisme original. Il consiste en un cylindre de ferrogel polarisé transversalement et en sens opposé à ses extrémités. Placé à la surface d'un film d'eau et soumis à un champ magnétique oscillant, le ver ondule et nage. Alors que la brisure de symétrie résulte de l'inhomogénéité du champ magnétique, les ondulations du ver sont dues à une instabilité de torsion/flexion liée à l'alignement des moments magnétiques des extrémités avec le champ appliqué. Les ondes de flexion ainsi excitées régissent alors les modes de nage du ver

Outcome and quality of life after surgically treated ankle fractures in patients 65 years or older

<p>Abstract</p> <p>Background</p> <p>Despite high incidence of ankle fractures in the elderly, studies evaluating outcome and impact of quality of life in this age group specifically are sparse. The aim of this study was to evaluate outcome and quality of life 6 and 12 months after injury in patients 65 years or older who had been operated on due to an ankle fracture.</p> <p>Methods</p> <p>Sixty patients 65 years or older were invited to participate in the study. 6 and 12 months after the injury a questionnaire including inquiry to participate, the Olerud-Molander Ankle Score (OMAS), Short-Form 36 (SF-36), Linear Analogue Scale (LAS), Self-rated Ankle Function and some supplementary questions was sent home to the patients. The supplementary questions concerned subjective experience of ankle instability, sporting and physical activity level before injury and recaptured activity level at follow-ups, need of walking aid before injury, state of living before injury and at follow-ups and co-morbidities. After the 12-month follow-up the patients were also called for a radiological examination.</p> <p>Results</p> <p>Fifty patients (83%) answered the questionnaire at 6-month and 46 (77%) at the 12-month follow-up. Although, 45 (90%) fractures were low-energy trauma 44 (88%) were bi- or trimalleolar and post-operative reduction results were complete in 23 (46%) ankles. The median OMAS improved from 60 (Interquartile range (IQR) 36) at 6-month to 70 (IQR 35) at 12-month (p = 0.002), but at 12-month still sixty percent or more of the patients reported pain, swelling, problems when stair-climbing and reduced activities of daily life. Twenty (40%) rated their ankle function as 'good' or 'very good' at 6-month and 30 (60%) at 12-month. Forty-one (82%) were physically active before injury but still one year after only 18/41 had returned to their pre-injury physical activity level. According to SF-36 four dimensions differed from the age- and gender matched normative data of the Swedish population, 'physical function', 'role physical' and 'role emotional' were below norms at 6-month for women (p = 0.010, p = 0.024 and 0.031) and 'general health' was above norms at 12-month for men (p = 0.044).</p> <p>Conclusion</p> <p>One year after surgically treated ankle fractures a majority of patients continue to have symptoms and reported functional limitations. However, SF-36 scores indicate that only females had functional status below the age- and gender matched normative data of the Swedish population.</p

Manipulation of Cell:Cell Contacts and Mesoderm Suppressing Activity Direct Lineage Choice from Pluripotent Primitive Ectoderm-Like Cells in Culture

In the mammal, the pluripotent cells of embryo differentiate and commit to either the mesoderm/endoderm lineages or the ectoderm lineage during gastrulation. In culture, the ability to direct lineage choice from pluripotent cells into the mesoderm/endoderm or ectoderm lineages will enable the development of technologies for the formation of highly enriched or homogenous populations of cells. Here we show that manipulation of cell:cell contact and a mesoderm suppressing activity in culture affects the outcome of pluripotent cell differentiation and when both variables are manipulated appropriately they can direct differentiation to either the mesoderm or ectoderm lineage. The disruption of cell:cell contacts and removal of a mesoderm suppressor activity results in the differentiation of pluripotent, primitive ectoderm-like cells to the mesoderm lineage, while maintenance of cell:cell contacts and inclusion, within the culture medium, of a mesoderm suppressing activity results in the formation of near homogenous populations of ectoderm. Understanding the contribution of these variables in lineage choice provides a framework for the development of directed differentiation protocols that result in the formation of specific cell populations from pluripotent cells in culture