Parabolic focal conics and polygonal textures in lipid liquid crystals

Polygonal defect arrays in lipid liquid crystals prepared from dipalmitoyl-, dimyristoyl-, and dilaurylphosphatidylcholine with various concentrations of water have been observed and characterized using optical microscopy. These arrays appear similar to the polygonal arrays previously observed in thermotropic smectic and cholesteric liquid crystals. The parabolic focal conic model proposed by Rosenblatt, Pindak, Clark and Meyer for the polygonal arrays in thermotropic smectic liquid crystals also describes the observed properties in the smectic A phase of lipid liquid crystals. The polygonal arrays become progressively more birefringent as the strains inducing them are increased. The parabolic focal conic model does not completely describe the details of the more birefringent polygonal arrays.Les phases liquide-cristallines lyotropes de dipalmitoyl-, dimyristoyl- et dilaurylphosphatidylcholine préparées pour différentes concentrations en eau présentent des réseaux de défauts polygonaux que l'on étudie au microscope optique. Les réseaux sont semblables à ceux observés précédemment dans le cas de phases thermotropes smectiques et cholestériques. Le modèle de domaine focal parabolique proposé par Rosenblatt, Pindak, Clark et Meyer pour les réseaux des smectiques thermotropes permet aussi de rendre compte des propriétés observées dans ces phases lamellaires lyotropes. La biréfringence des réseaux polygonaux s'accroit progressivement avec les contraintes qui induisent les défauts. Le modèle de domaine focal parabolique ne décrit pas complètement les détails des réseaux polygonaux les plus biréfringents

Surface layering of liquids: The role of surface tension

Recent measurements show that the free surfaces of liquid metals and alloys are always layered, regardless of composition and surface tension; a result supported by three decades of simulations and theory. Recent theoretical work claims, however, that at low enough temperatures the free surfaces of all liquids should become layered, unless preempted by bulk freezing. Using x-ray reflectivity and diffuse scattering measurements we show that there is no observable surface-induced layering in water at T=298 K, thus highlighting a fundamental difference between dielectric and metallic liquids. The implications of this result for the question in the title are discussed.Comment: 5 pages, 4 figures, to appear in Phys. Rev. B. 69 (2004

Microscopic View on Short-Range Wetting at the Free Surface of the Binary Metallic Liquid Gallium-Bismuth: An X-ray Reflectivity and Square Gradient Theory Study

We present an x-ray reflectivity study of wetting at the free surface of the binary liquid metal gallium-bismuth (Ga-Bi) in the region where the bulk phase separates into Bi-rich and Ga-rich liquid phases. The measurements reveal the evolution of the microscopic structure of wetting films of the Bi-rich, low-surface-tension phase along different paths in the bulk phase diagram. A balance between the surface potential preferring the Bi-rich phase and the gravitational potential which favors the Ga-rich phase at the surface pins the interface of the two demixed liquid metallic phases close to the free surface. This enables us to resolve it on an Angstrom level and to apply a mean-field, square gradient model extended by thermally activated capillary waves as dominant thermal fluctuations. The sole free parameter of the gradient model, i.e. the so-called influence parameter, $\kappa$, is determined from our measurements. Relying on a calculation of the liquid/liquid interfacial tension that makes it possible to distinguish between intrinsic and capillary wave contributions to the interfacial structure we estimate that fluctuations affect the observed short-range, complete wetting phenomena only marginally. A critical wetting transition that should be sensitive to thermal fluctuations seems to be absent in this binary metallic alloy.Comment: RevTex4, twocolumn, 15 pages, 10 figure

Observation of the Smectic C -- Smectic I Critical Point

We report the first observation of the smectic C--smectic I (C--I) critical point by Xray diffraction studies on a binary system. This is in confirmity with the theoretical idea of Nelson and Halperin that coupling to the molecular tilt should induce hexatic order even in the C phase and as such both C and I (a tilted hexatic phase) should have the same symmetry. The results provide evidence in support of the recent theory of Defontaines and Prost proposing a new universality class for critical points in layered systems.Comment: 9 pages Latex and 5 postscript figures available from [email protected] on request, Phys.Rev.Lett. (in press

X-ray scattering: Liquid metal/vapor interfaces

We will review the principal x-ray scattering measurements that have been carried out on the free surface of liquid metals over the past two decades. For metals such as K, Ga, In Sn, Bi etc the surface induces well-defined layering with atomic spacing ‘d’ that penetrates into the bulk a distance of the order of the bulk liquid correlation length. As a consequence the angular dependence of the surface structure factor observed by x-ray reflectivity displays a broad peak at wavevector transfer ∼ 2π/d with a half width that is comparable to the width of the bulk liquid structure factor. Quantitative measurement of this surface structure factor requires correction for a singular Debye-Waller like effect arising from thermally excited capillary waves. For liquid metal alloys the layering is accompanied by chemical segregation (i.e. Gibbs absorption) that can be characterized from the energy dependence of the reflectivity. Particularly interesting are the temperature dependence and elasticity of the two-dimensional surface frozen phases that form on the surface of the Au82Si18 liquid eutectic. Surface freezing, although not observed near the eutectic points of alloys such as Au-Ge, Pd-Ge and Pd-Si, has been observed at the free surface of the glass forming alloy Au49Ag5.5Pd2.3Cu26.9Si16.3

Landau theory of the reentrant nematic-smectic A phase transition

The reentrant, nematic to smectic A phase transition is shown to follow from the Landau theory if one assumes the existence of an optimum density for smectic ordering. The shape of the coexistence line in the P-T plane is fit exactly by this theory. The effects of concentration on reentrant behaviour are also explained.Nous montrons que le diagramme de phase rentrant de la transition nématique-smectique A, découle naturellement de la théorie de Landau si l'on admet qu'il existe une densité optimale pour l'obtention de l'ordre smectique. La forme de la ligne de coexistence dans le plan P-T, est correctement prédite par la théorie. Nous décrivons aussi le comportement rentrant des mélanges