J. Als-Nielsen

Bio: J. Als-Nielsen is an academic researcher. The author has contributed to research in topics: Liquid crystal & Phase transition. The author has an hindex of 2, co-authored 2 publications receiving 35 citations.

Antiferroelectric surface layers in a liquid crystal as observed by synchrotron X-ray scattering

Abstract: The X-ray reflectivity form the surface of a liquid crystal with terminally polar (cyano substituted) molecules has been studied using a high-resolution triple-axis X-ray spectrometer in combination with a synchrotron source. It is demonstrated that at the surface of the smectic A1 phase a few antiferroelectric double layers develop that can be distinguished from the bulk single layer structure. A model is developed that separates the electron density in a contribution from the molecular form factor, and from the structure factor of the mono- and the bilayers, respectively. With only a few adjustable parameters it accounts for the rather complex observed reflection curve. It shows that (i) the first molecular layer has tails up rather than heads up, (ii) the smectic order parameter of the first mono- and bilayer is saturated, (iii) the antiferroelectric bilayering does decay rather abruptly and not exponentially Etude RX de la surface d'un cristal liquide pres de la transition nematique-smectique A1. Developpement en surface de quelques doubles couches antiferroelectriques qui peuvent etre distinguees de la structure en simples couches du materiau massif

Surface effects and anchoring in liquid crystals

Abstract: As their name indicates, liquid crystals simultaneously exhibit some characteristics common to both ordinary isotropic liquids and solid crystals. This ambivalence is also found in the effects of surfaces on these systems which lead to a great diversity of phenomena. These phenomena are reviewed focusing on nematic liquid crystals which have the simplest structure among the many existing types and which have been the most extensively studied. Three main kinds of effects can be distinguished. The first concerns the perturbation of the liquid crystalline structure close to the surface. Beyond this transition region, the bulk liquid crystalline structure is recovered with an orientation which is fixed by the surface: this phenomenon of orientation of liquid crystals by surfaces is the so-called anchoring. Finally, close to bulk phase transitions, critical adsorption or wetting can occur at surfaces as is also seen in isotropic systems.

Structure and fluctuations of smectic membranes

Abstract: Smectic membranes are perfect model systems for studying low-dimensional phase transitions and the associated fluctuations. During the last two decades we have seen important progress in the understanding of the structure and fluctuation behavior of these systems, driven by both new experimental techniques and theoretical developments. Phase transitions are reviewed involving liquid, hexatic, and crystalline layers, which provide several types of model system for low-dimensional melting. The authors discuss the influence of the surfaces on the physical properties of the membranes as well as the crossover from threeto two-dimensional behavior. The layer-displacement fluctuations in smectic membranes have been investigated by specular and diffuse x-ray reflectivity. Theoretical and experimental aspects of the displacement-displacement correlation function are discussed. Of special interest is the quenching or enhancement of fluctuations at surfaces, which is directly related to the phenomenon of surface ordering. The authors consider the conditions under which fluctuations are conformal throughout a membrane, and then the dynamic aspects of the layer-displacement correlation function, which include the effects of finite size, surface tension, and viscous dissipation. This leads in smectic membranes to a discrete spectrum of elastic and viscous relaxation modes, which have been studied experimentally with coherent x rays at third-generation synchrotron sources. The fluctuating character of crystalline-B membranes is also considered. Finally, the article looks briefly at thinning transitions, smectic membranes of chiral molecules, smectic films on substrates, and applications to biologically relevant systems. Open questions and future trends in the field are discussed.

Structure of symmetric polyolefin block copolymer thin films

Abstract: The microstructure of thin films of nearly symmetric poly(ethylene–propylene)–poly(ethylethylene) (PEP‐PEE) diblock copolymers (f=0.55, where f is the volume fraction of PEP) was characterized by neutron reflectometry (NR). A symmetric film structure in which the PEE block segregates preferentially to both interfaces is observed above and below the bulk order–disorder transition (ODT). Measurements at room temperature for several chain lengths, N, provide a real‐space picture of the change in interdomain interfacial profiles associated with the crossover between the strong and weak segregation limits. The polymer/air and substrate/polymer interfaces are observed to induce an ordered microstructure even when the center of the film is disordered. The characteristic dimension of this near surface microstructure is larger than the corresponding bulk value for values of χN lying between those of the bulk Gaussian‐to‐stretched‐coil and order–disorder transitions, where χ is the segment–segment interaction param...

Synchrotron x-ray studies of liquid-vapor interfaces

Abstract: The variation of density across the liquid-vapor interface from essentially zero density far out in the vapor phase to a homogeneous density deep in the liquid phase can be determined by X-ray reflectivity measurements1)at grazing angles θ well beyond the critical angle for total reflection, θC. We shall first derive the relation between the density variation, ρ(z), and the reflectivity R(θ). Next we consider as an experimental example the roughness caused by thermal excitations of capillary waves on simple liquids like water and carbon-tetra-chloride2). Then we consider the smectic layering of liquid crystal molecules of the free surface, first in a system with a spontaneous first order phase transition from the isotropic phase to a smectic A phase3), and afterwards in a system with a second order phase transition from the nematic phase to the smectic A phase4–6).