D. Villers

Coupled buoyancy and Marangoni convection in acetone : experiments and comparison with numerical simulations

This paper presents a study of the convection in acetone due jointly to the thermocapillary (Marangoni) and thermogravitational effects. The liquid (acetone) is submitted to a horizontal temperature difference. Experiments and numerical simulations both show the existence of three different states : monocellular steady states, multicellular steady states and spatio-temporal structures. The results are discussed and compared with the linear stability analysis of Smith & Davis (1983).

Influence of interfacial tension gradients on thermal convection in two superposed immiscible liquid layers

This paper considers mainly the theoretical aspect of thermal convection in two superposed immiscible (or partially miscible) fluid layers in a rectangular cavity with differentially heated end walls. We develop a model valid for layers whose lateral extent is much larger than depth, deriving the horizontal velocity profile in each layer as a function of the various parameters such as expansion coefficients, viscosities, depth of each layer, and the interfacial tension gradient along the interface. Results are discussed, taking into account the results of experiments that we presented recently [1].

Optimization of an Hough transform algorithm for the search of a center

We present improvements of an adaptative Hough transform algorithm applied to the search of a common center of circular or partially circular components present in an image. The efficiency has been considerably optimized by a continuous update of a list of voting points, in conjunction with the evolution of the accumulator size and position. The method was implemented as a plugin for the scientific open source image processing package ImageJ. Although initially designed for X-ray diffraction analysis, numerous other applications are quoted in different other scientific field, in image measurement techniques, industrial vision, and biometry, i.e. for iris localization.

Thermal convection in superposed immiscible liquid layers

This paper deals with thermal convection in two superposed immiscible fluid layers in a rectangular cavity with differentially heated end walls. We have measured the horizontal velocity profiles in each layer as a function of elevation and we show the dominant role of interfacial tension effects. Experiments were conducted using Laser Doppler Anemometry; the system studied was water-heptanol.

Nucleation-controlled growth and normal growth: a unified view

Abstract From a unified model we propose that, upon slight modification, the system of equations proposed by Seto and Frank to describe nucleation-controlled growth of 2D crystals may also model the normal growth of 2D crystal with rough surfaces. Furthermore, we will show that the solution of these equations is equivalent to that of a Klein-Gordon equation, and we give exact analytical and numerical solutions of that equation. The profiles obtained reproduce well the observed shape of polyethylene crystals with curved facets.

Synthesis and thermal characterization of crystallizable poly(caprolactone)/poly(hexamethylene terephthalate) block copolymer

Abstract An alternating multiblock copolymer of poly(e-caprolactone) (PCL) and poly(hexamethylene terephthalate) (PHT) with a 48/52 PCL/PHT composition weight ratio has been synthesized. Its structural and morphological parameters were measured using 1H NMR, density measurements, differential scanning calorimetry and its crystallization and melting were studied by real time simultaneous WAXS and SAXS using synchrotron radiation. These results are compared with those for the pure PCL and PHT used as building blocks. In the crystallization conditions used here, the copolymer crystallizes as alternate lamellae of PHT and PCL. The crystallinity of the PCL blocks is much lower than that of pure PCL because they crystallize under stress in the semi-crystalline PHT network.

Crystals with curved edges: a unified model that mediates between the theories of nucleation-controlled and rough surface growth

Abstract In order to explain the curved outline of some polymer crystals, we start from a slightly modified Franks system of equations describing nucleation-controlled crystal growth. But we give an alternative and completely new interpretation of these equations with a view to applying them to the description of normal growth of crystals with rough surfaces. We show first that Mansfields approximate solution of such equations is not exact, does not satisfy absorbing boundary conditions and does not depend on the rate of motion of these boundaries. Because that solution describes the facets of a crystal in terms of arcs of the same ellipse, it cannot be used to describe the curvature of 200 faces of those crystals of large elongation ratio which clearly are not parts of a single ellipse. The most puzzling feature of Mansfields proposal is, however, that pronounced curvature is obtained solely if the mean distance between steps is assumed to be of the same order of magnitude as the width of a single stem. In such an eventuality, the use of the concept of nucleation-controlled growth becomes meaningless. We propose, therefore, a completely new interpretation of Franks equations, which allows us to use these equations to describe nucleation-controlled growth. Then these equations appear as the mathematical formulation of a model that mediates between the approach of Seto and Frank, and that of Gilmer and Sadler. Detailed analysis of the experimental data, mostly those of Labaig and Bassett, shows that the outline of crystals with curved habit is neither elliptic nor made of elliptical arcs. We provide new and exact solutions of our generalized equations with generalized boundary conditions. These solutions account well for the experimental data. In some cases they may be approximated by Mansfields solution (despite the fact that the change of curvature along the facets differs from that actually observed) or by the solution found independently by Toda and us (33th IUPAC Polymer Symposium at Montreal), despite the fact that it is unusual that near the tips of the crystals the outline of a crystal is well approximated by straight segments. The most important claim is, however, that our model appears as a bridge between the model of nucleation-controlled growth and that of normal growth of rough surfaces.

How geometry controls the tearing of adhesive thin films on curved surfaces.

Flaps can be detached from a thin film glued on a solid substrate by tearing and peeling. For flat substrates, it has been shown that these flaps spontaneously narrow and collapse in pointy triangular shapes. Here we show that various shapes, triangular, elliptic, acuminate, or spatulate, can be observed for the tears by adjusting the curvature of the substrate. From combined experiments and theoretical models, we show that the flap morphology is governed by simple geometric rules.

Use of the Hough transform to determine the center of digitized X-ray diffraction patterns

Abstract The use of area detector of X-ray diffraction yields digitized information. The position of the incident beam on such recorded images is needed with a high precision for a careful determination of the Bragg spacings. An application of the Hough transform is presented to obtain the coordinates of the center of the X-ray pattern with a good accuracy and without any important intervention of the operator.

New dynamical effects in spherulitic growth

Abstract Two unexpected phenomena have been observed during the study of the growth of α and β spherulites of a poly(ethylene oxide)-resorcinol complex: an overgrowth rate (denoted as G i ) of the fastest, namely the α-form, along the interface of the β-form, and a transformation of the β-form into the α-form, which propagates as a dynamic front at a constant rate G t . The evidence for these two phenomena is demonstrated, and possible explanations are discussed. Measurements of the two rates, G i and G t , are also reported.