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Chiral polyalkylthiophenes are noncentrosymmetric organic materials which can be used both in second harmonic-generation devices and in polarized light emitting diodes. In this work we present the synthesis and the characterization of a polyalkylthiophene with a chiral center very close to the conjugated backbone: poly(3-[(S)-2-methylbutyl]thiophene) (PMBT). Circular dichroism (CD) measurements have been carried out to ascertain the chirality of these materials. The CD spectra show intense signals both in mixed solvents and in the solid state. The strong Cotton effect can be associated to a highly ordered aggregated phase whose nature is still under investigation. We also present the photo and electroluminescence characterization of single layer light emitting diode (LED) with the following configuration: ITO (Tin Indium Oxide)/PMBT/Al.
A finite volume computer model of the continuous casting
process for steel flat products has been developed. In this first
stage, the model concentrates on the hydrodynamic aspects
of the process and in particular the dynamic behavior of
the metal/slag interface. The model was validated against
experimental measurements obtained in a water model apparatus.
This paper presents the first experimental results on Marangoni–Bénard instability
in a symmetrical three-layer system. A pure thermocapillary phenomenon has been
observed by performing the experiment in a microgravity environment where buoyancy
forces can be neglected. This configuration enables the hydrodynamic stability
of two identical liquid–liquid interfaces subjected to a normal gradient of temperature
to be studied. The flow is driven by one interface only and obeys the criterion
based on the heat diffusivity ratio proposed by Scriven & Sternling (1959) and Smith
(1966). The measured critical temperature difference for the onset of convection is
compared to the value obtained from two-dimensional numerical simulations. The
results of the simulations are in reasonable agreement with the velocimetry and the
thermal experimental data for moderate supercriticality. Numerically and experimentally,
the convective pattern exhibits a transition between different convective regimes
for similar temperature gradients. Their common detailed features are discussed.
Thermocapillary convection in three-dimensional rectangular finite containers with rigid lateral walls is studied. The upper surface of the fluid layer is assumed to be flat and non-deformable but is submitted to a temperature-dependent surface tension. The realistic ‘no-slip’ condition at the sidewalls makes the method of separation of variables inapplicable for the linear problem. A spectral Tau method is used to determine the critical Marangoni number and the convective pattern at the threshold as functions of the aspect ratios of the container. The influence on the critical parameters of a non-vanishing gravity and a non-zero Biot number at the upper surface is also examined. The nonlinear regime for pure Marangoni convection (Ra = 0) and for Pr = 104, Bi = 0 is studied by reducing the dynamics of the system to the dynamics of the most unstable modes of convection. Owing to the presence of rigid walls, it is shown that the convective pattern above the threshold may be quite different from that predicted by the linear approach. The theoretical predictions of the present study are in very good agreement with the experiments of Koschmieder & Prahl (1990) and agree also with most of Dijkstra's (1995a, b) numerical results. Important differences with the analysis of Rosenblat, Homsy & Davis (1982b) on slippery walls containers are emphasized.
A nonlinear analysis of Bénard–Marangoni convection in a horizontal fluid layer of infinite extent is proposed. The nonlinear equations describing the fields of temperature and velocity are solved by using the Gorkov–Malkus–Veronis technique, which consists of developing the steady solution in terms of a small parameter measuring the deviation from the marginal state. This work generalizes an earlier paper by Schlüter, Lortz & Busse wherein only buoyancy-driven instabilities were handled. In the present work both buoyancy and temperature-dependent surface-tension effects are considered. The band of allowed steady states of convection near the onset of convection is determined as a function of the Marangoni number and the wavenumber. The influence of various dimensionless quantities like Rayleigh, Prandtl and Biot numbers is examined. Supercritical as well as subcritical zones of instability are displayed. It is found that hexagons are allowable flow patterns.