Spatially localized states in Marangoni convection in binary mixtures

Two-dimensional Marangoni convection in binary mixtures is studied in periodic domains with large spatial period in the horizontal. For negative Soret coefficients convection may set in via growing oscillations which evolve into standing waves. With increasing amplitude these waves undergo a transition to traveling waves, and then to more complex waveforms. Out of this state emerge stable stationary spatially localized structures embedded in a background of small amplitude standing waves. The relation of these states to the time-independent spatially localized states that characterize the so-called pinning region is investigated by exploring the stability properties of the latter, and the associated instabilities are studied using direct numerical simulation in time.     

Un nouveau préconditionneur pour les problèmes elliptiques à coefficients variablesA new preconditonner for general elleptic problems

In this Note we describe a preconditionner for iteratively solving the linear system arising from the discretization of a general nonseparable elliptic problem by spectral element method. This preconditionner is constructed from approximating the original problem with the closest (in some sense) separable elliptic problem. A direct method is then used to invert the preconditionner. To cite this article: M. Azaïez et al., C. R. Mecanique 331 (2003).     

Soret effect and slow mass diffusion as a catalyst for overstability in Marangoni-Bénard flows

We study the onset of time dependent Marangoni-Bénard convection in binary mixtures subject to Soret effect by numerical computation of linear instability thresholds in infinite fluid layers and two-dimensional boxes. The calculations are done for positive Marangoni numbers (Ma > 0) and negative Marangoni Soret parameters S _M = –(D _S γ _c )/(Dγ _T ) where D _S and D are the Soret and mass diffusion coefficients, respectively, and γ _T , γ _c are the first derivatives of the surface tension with respect to temperature and concentration. Our purpose is to understand why for particular choices of Prandtl and Schmidt numbers, the increase of the stabilizing solutal contribution leads to a decrease of the critical temperature difference, a phenomenon already reported by Chen & Chen [5] and Skarda et al. [12] For various choices of Prandtl and Schmidt numbers we analyze the evolution of the critical Marangoni number Ma _c , critical wavenumber k _c and angular frequency ω _c with S _M and compute the corresponding eigenvectors. We next propose a physical mechanism which explains how the stabilizing solutal contribution acts as a catalyst for overstability. Finally, we extend our results to two dimensional boxes of small aspect ratio.     

Periodic and localized states in natural doubly diffusive convection

Numerical continuation is used to follow branches of steady doubly diffusive convection in a vertical slot driven by imposed horizontal temperature and concentration gradients. No-slip boundary conditions are used on the lateral walls; periodic boundary conditions with large spatial period are used in the vertical direction. A variety of different states, both spatially periodic and spatially localized, are identified and the profusion of the resulting solution branches is linked to a phenomenon known as homoclinic snaking.     

Design and synthesis of a series of novel, cationic liposaccharide derivatives as potential penetration enhancers for oral drug delivery

Oral delivery remains a challenge for a number of drug candidates with low absorption profile (poor membrane permeability, lack of stability, solubility issues or susceptibility to enzymatic degradation) and various methodologies have been investigated to overcome it. The approach presented here consists of associating, by ion-pairing, a hydrophilic, ionizable model drug to a series of synthetic counter-ionic entities with a controlled degree of lipophilicity in order to enhance its penetration of biomembranes and offer some protection against in situ degradation, while preserving its biologically active chemical structure. We report herein the synthesis of a series of positively charged potential penetration enhancers designed from d-glucose, 2-aminododecanoic acid, and additional lipophilic amino acids, and converted afterward into quaternary ammoniums in an optimized, innovative one-step reaction. Each liposaccharide derivative synthesized was fully characterized and their ability to generate micelles in solution was assessed by isothermal titration calorimetry.     

Influence of Vertical Vibrations on the Stability of a Binary Mixture in a Horizontal Porous Layer Subjected to a Vertical Heat Flux

Macroscale three-dimensional modeling of fluid flow in a thin porous layer under unsaturated conditions is a challenging task. One major issue is that such layers do not satisfy the representative elementary volume length-scale requirement. Recently, a new approach, called reduced continua model (RCM), has been developed to describe multiphase fluid flow in a stack of thin porous layers. In that approach, flow equations are formulated in terms of thickness-averaged variables and properties. In this work, we have performed a set of experiments, where a wet \(260\hbox {-}\upmu \hbox {m}\)-thin porous layer was placed on top of a dry layer of the same material. We measured the change of average saturation with time using a single-sided low-field nuclear magnetic resonance device known as NMR-MOUSE. We have employed both RCM and the traditional Richards equation-based models to simulate our experimental results. We found that the traditional unsaturated flow model cannot simulate experimental results satisfactorily. Very close agreement was obtained by including the dynamic capillary term as postulated by Hassanizadeh and Gray in the traditional equations. The reduced continua model was found to be in good agreement with the experimental result without adding dynamic capillarity term. Moreover, the computational effort needed for RCM simulations was one order of magnitude less than that of traditional models.     

Cumulative mechanical moments and microstructure deformation induced by growth shape in columnar solidification

The dynamical interaction between columnar interface microstructure and self-stress, resulting in unforeseen mechanical deformation phenomena, is brought to light by means of in situ and real-time synchrotron x-ray topography during directional solidification of dilute aluminum alloys. Beyond long-known local mechanical stresses, global mechanical constraints are found to be active. In particular, column rotation results from deformation caused by the mechanical moments associated with the very growth shape, namely, the cumulative torque acting together with the cumulative bending moment under gravity. A basic model allowing for a qualitative explanation of the observed distinctive features of the self-stress effects on microstructure dynamics is proposed.     

Comment on the Paper: The CRONE Suspension: Management of the Dilemma Comfort-Road Holding (by Moreau et al.)

The linearisation procedure used in the modelling of the hydro-pneumatic suspension system by Moreau et al. contains an inaccuracy. Hence, the conclusion that the weight of the sprung mass is normally absent of the dynamic behaviour of the sprung mass is demonstrated to be false.In Nonlinear Dynamics 38: 461–484, 2004, Kluwer Academics Publishers.     

In situ and real-time analysis of TGZM phenomena by synchrotron X-ray radiography

The development of brilliant third-generation synchrotron X-ray sources, together with advances in X-ray optics and detectors, has provided timely efficient tools for in-depth understanding of physical phenomena in a broad spectrum of situations. Synchrotron X-ray radiography enables in situ and real-time observation of microstructure evolution, i.e. a direct access to dynamical phenomena which could not be anticipated from post-mortem analysis. Dedicated experiments are carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France) in Al-based alloys to study the dynamics of temperature gradient zone melting (TGZM) phenomenon. TGZM occurs when a liquid–solid zone is submitted to a temperature gradient and leads to the migration of liquid droplets or channels through the solid, up the temperature gradient. The thorough characterisation of both the initial solid during the thermal stabilisation phase prior to solidification (static TGZM) in Al–3.5 wt% Ni alloy and the dendritic microstructure in the later stage of solidification in Al–7.0 wt% Si alloy is performed. Based on experimental observations, quantitative data (in particular liquid-migration velocity) are measured and a very good agreement is found with theoretical analysis.