Supported bilayers: combined specular and diffuse x-ray scattering

A new method is proposed for the analysis of specular and off-specular reflectivity from supported lipid bilayers. Both thermal fluctuations and the "static" roughness induced by the substrate are carefully taken into account. Examples from supported bilayers and more complex systems comprising a bilayer adsorbed or grafted on the substrate and another "floating" bilayer are given. The combined analysis of specular and off-specular reflectivity allows the precise determination of the structure of adsorbed and floating bilayers, their tension, bending rigidity and interaction potentials. We show that this new method gives a unique opportunity to investigate phenomena like protusion modes of adsorbed bilayers and opens the way to the investigation of more complex systems including different kinds of lipids, cholesterol or peptides

Easy orientation of diblock copolymers on self-assembled monolayers using UV irradiation

A simple method based on UV/ozone treatment is proposed to control the surface energy of dense grafted silane layers for orientating block copolymer mesophases. Our method allows one to tune the surface energy down to a fraction of a mN/m. We show that related to the surface, perpendicular orientation of a lamellar phase of a PS-PMMA diblock copolymer (neutral surface) is obtained for a critical surface energy of 23.9-25.7 mN/m. Perpendicular cylinders are obtained for 24.6 mN/m and parallel cylinders for 26.8 mN/m.Comment: 3 figures, 1 tabl

Aging mechanism in tunable Pickering emulsion

We study the stability of a model Pickering emulsion system. A special counter-flow microfluidics set-up was used to prepare monodisperse Pickering emulsions, with oil droplets in water. The wettability of the monodisperse silica nanoparticles (NPs) could be tuned by surface grafting and the surface coverage of the droplets was controlled using the microfluidics setup. A surface coverage as low as 23$\%$ is enough to stabilize the emulsions and we evidence a new regime of Pickering emulsion stability where the surface coverage of emulsion droplets of constant size increases in time, in coexistence with a large amount of dispersed phase. Our results demonstrate that the previously observed limited coalescence regime where surface coverage tends to control the average size of the final droplets must be put in a broader perspective

Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases

The linear compressibility of two-dimensional fatty acid mesophases has determined by grazing incidence x-ray diffraction. Surface pressure vs molecular area isotherms were reconstructed from these measurements, and the linear compressibility (relative distortion along a given direction for isotropic applied stress) was determined both in the sample plane and in a plane normal to the aliphatic chain director (transverse plane). The linear compressibilities range over two orders of magnitude from 0.1 to 10 m/N and are distributed depending on their magnitude in 4 different sets which we are able to associate with different molecular mechanisms. The largest compressibilities (10m/N) are observed in the tilted phases. They are apparently independent of the chain length and could be related to the reorganization of the headgroup hydrogen-bounded network, whose role should be revalued. Intermediate compressibilities are observed in phases with quasi long-range order (directions normal to the molecular tilt in L_2 or L_2' phases, S phase), and could be related to the ordering of these phases. The lowest compressibilities are observed in the solid untilted CS phase and for 1 direction of the S and L_2'' phases. They are similar to the compressibility of crystalline polymers and correspond to the interactions between methyl groups in the crystal. Finally, negative compressibilities are observed in the transverse plane for L_2' and L_2'' phases and can be traced to subtle reorganizations upon untilting.Comment: 24 pages, 17 figure

Controlling interactions in supported bilayers from weak electrostatic repulsion to high osmotic pressure

Understanding interactions between membranes requires measurements on well-controlled systems close to natural conditions, in which fluctuations play an important role. We have determined, by grazing incidence X-ray scattering, the interaction potential between two lipid bilayers, one adsorbed on a solid surface and the other floating close by. We find that interactions in this highly hydrated model system are two orders of magnitude softer than in previously reported work on multilayer stacks. This is attributed to the weak electrostatic repulsion due to the small fraction of ionized lipids in supported bilayers with a lower number of defects. Our data are consistent with the Poisson-Boltzmann theory, in the regime where repulsion is dominated by the entropy of counter ions. We also have unique access to very weak entropic repulsion potentials, which allowed us to discriminate between the various models proposed in the literature. We further demonstrate that the interaction potential between supported bilayers can be tuned at will by applying osmotic pressure, providing a way to manipulate these model membranes, thus considerably enlarging the range of biological or physical problems that can be addressed.Comment: 14 pages, 8 figure

L’alliance dravidienne au singulier

Alors que dans le système de parenté dravidien la dimension diachronique est constitutive de l’alliance, quelles alliances reproduit-on dans une société indifférenciée en l’absence de groupes intermariés ? Telle est la question de départ que suscite le cas des Chimane amazoniens, dont le système de parenté, cognatique et dravidien, présente l’apparent paradoxe d’une formule globale implicite sans que l’on puisse distinguer chez eux de relations d’alliance collectives susceptibles d’être reproduites. Or, s’ils n’ont pas des alliances, c’est en fait qu’ils n’en ont qu’une, présente sous la forme d’un continuum – et donc d’un type différent. Sa formulation conduit à proposer, dans l’ensemble des systèmes dravidiens, un nouveau pôle indifférencié défini en contrepoint d’un pôle unilinéaire, la constante résidant dans l’homologie qui apparaît entre les caractéristiques de l’alliance et celles de son support.Dravidian Marriage Alliances in the Singular. – The diachronic dimension is a constitutive aspect of affinity in Dravidian kinship systems. However, in the case of cognatic societies lacking intermarrying groups, exactly which matrimonial alliances are being reproduced? This is the question raised by the case of the Chimane of Amazonia whose cognatic, Dravidian kinship system presents the apparent paradox of having an implicit global alliance structure in the absence of clear-cut, reproducible, collective relations of affinity. The author suggests that if the Chimane do not have such marriage alliances, it is because they have a single alliance, which takes the form of a continuum. In accounting for this novel alliance type, the author argues for the existence, within the set of Dravidian systems, of a new, undifferentiated pole, defined in contrast to a unilineal pole, each case being specified by a homologous relationship between the characteristics of alliance and those of its descent-related grounds

Ils sont comme nous, mais... Relations de parenté et de genre entre Chimane et " gens de dedans "

RÉSUMÉIls sont comme nous. mais... Relations de parenté et de genre entre Chimane et « gens de dedans »Parmi les êtres surnaturels qui peuplent l'univers des Chimane d'Amazonie bolivienne, les « gens de dedans » occupent une place essentielle. Leur connaissance est assurée par les chamanes qui trouvent chez eux des auxiliaires et des épouses dont ils ont des enfants. Ces esprits cumulent le statut de « parents » et d'« autres gens », deux catégories habituellement exclusives avec lesquelles les relations se jouent sur deux plans différents : celui de la parenté et celui des relations interethniques, lequel s'articule à l'ordre des sexes. Pour être pleinement saisies, les relations avec les « gens de dedans » doivent donc être envisagées sur ces deux plans, qui s'avèrent cependant congruents et concourent à mettre en place un tableau singulièrement équilibré.Mots clés : Daillant. chamanisme. parenté, alliances, relations hommes-femmes. Amazonie. BolivieABSTRACTThey Are Just Like Us. But... Kinship and Gender Relations between Chimane and " Inside People "Amongst the supematural beings that inhabit the world of the Chimane in the Bolivian Amazonia. the so-called " inside people " occupy a central place. Links with them are established through the shamans who seek and find among them spirit helpers as well as spouses with whom they have children. These spirits are endowed with the dual status of " relatives " and " other people ". two categories that are normally mutually exclusive and with which relations occur on two différent planes : that of kinship and that of inter-ethnic relations, itself geared to the pattern ordering the sexes. To be fully understood. relations with " inside people " must therefore be considered on both these planes which reveal themselves to be congruent. their conjunction forming a remarkably balanced picture.Key words : Daillant. shamanism. kinship. alliances, men-women relations. Amazonia. Bolivi

Potential of lichens for monitoring iodine-129 and chlorine-36

Chlorine-36 (half life 3.01 × 105 year), a beta emitter, is produced naturally but its presence has been enhanced by atmospheric weapons testing and other nuclear activities. Iodine-129 has a half life of 1.57 × 107 years and is also produced by nuclea

Effect of an electric field on a floating lipid bilayer: a neutron reflectivity study

We present here a neutron reflectivity study of the influence of an alternative electric field on a supported phospholipid double bilayer. We report for the first time a reproducible increase of the fluctuation amplitude leading to the complete unbinding of the floating bilayer. Results are in good agreement with a semi-quantitative interpretation in terms of negative electrostatic surface tension.Comment: 12 pages, 7 figures, 1 table accepted for publication in European Physical Journal E Replaced with with correct bibliograph