BRIDGMAN SOLIDIFICATION RESULTS OF EXPERIMENTS PERFORMED ON A 3D Sn/Bi ALLOYGROWN UNDER MICROGRAVITY

G. Boutet
CEA-GRENOBLE, DTA/CEREM/DEM/SPCM/LSP
17 rue des martyrs, 38054 Grenoble, FRANCE

Solidification of 3D samples on earth is never diffusion driven because the liquid zone is always submitted to natural convection fluid flows. The only way to avoid such flows is to perform microgravity experiments. Thanks to a collaboration between the National American Space Agency (NASA), the Centre National d'Etude Spatial (CNES) and the Commissariat à l'Energie Atomique (CEA), we had the opportunity to perform such experiments aboard the space shuttle Columbia in 1992 and 1996, during the USMP1 and USMP3 flights. The device used, known as MEPHISTO, has the originality of associating a classical Bridgman process with a Seebeck diagnostic, providing a thermoelectrical emf proportional to the interface temperature of the alloy during solidification. This access to the interface temperature has the particularity of being in real time as well as non perturbative, opening the door to study topics such as morphological stability of a solid/liquid front or solute transients dynamics.
Concerning morphological stability, we were able to measure with a precision of the order of 5% the critical velocity of a Sn/Bi alloy at two different compositions (0.58 and 1.60 at% bi). The comparison to theoretical predictions like those from the linear stability analysis of Mullins and Sekerka showed a systematic overstability of the interface but still, the critical velocities stay compatible thanks to large uncertainties concerning the physical properties of the alloy. In particular, the diffusion coefficient precision should be improved to draw any definitive conclusion.
The knowledge of the interface concentration at each time provides interesting data for the study of rehomogeneisation. We observed the influence of the chemical recoil after a steady state solidification by slowing rehomogeneisation down. Moreover residual convection seems to interfere all the more that pulling velocity is small. We performed a numerical approach of this solutal rehomogeneisation problem. Thanks to the symmetry of the samples as well as microgravity conditions, the problem can be numerically treated as 1 D. We used a front tracking method inspired by the works of Meyer to solve the Stefan problem.

Key-words : thermoelectricity, Bridgman solidification, microgravity, morphological stability, transients.


STUDY OF ADSORPTION DYNAMICS IN MICROGRAVITY: THE FAST EXPERIMENT ONBOARD STS-95

Passerone¹, L. Liggieri ¹, M. Ferrari¹, F. Ravera¹, R. Miller² , G. Loglio^3
1 Istituto di Chimica Fisica Applicata dei Materiali - CNR , Genoa, Italy
² Max Plank Institut für Kolloid und Grenzflächenforschung, Berlin, Germany
³ Dipartimento di Chimica Organica-Università di Firenze, Florence, Italy

The Facility for Adsorption and Surface Tension studies (FAST) has flown in October 1998 on-board the Shuttle Discovery STS-95, in the SPACEHAB module.
During the mission, three kinds of experiments have been performed, dealing with the adsorption dynamics of soluble surfactants at liquid-liquid and liquid-gas interfaces (evaluation of adsorption kinetics, dynamic surface tension, interfacial dilational modulus, surface viscosity etc.) under microgravity conditions.
Diffusion plays a fundamental role in the dynamic aspects of adsorption. Therefore, due to the large attenuation of convective fluxes and to the possibility to obtain spherical interfaces, microgravity provides the ideal conditions to better define the kinetics exchange between the bulk and the interface and to explore the behaviour of the system under higher frequency interface area excitations.
The FAST facility has been proposed by ICFAM to ESA in the framework of the Columbus Precursor Flights and developed by Officine Galileo (Italy).
The core of the facility are two Capillary Pressure tensiometers which allow the masurement of dynamic surface tension od spherical interfaces subjected to changes of the surface area.
In this contribution, the basic working principle of the Capillary Pressure Tensiometer, which is the core of the facility, and the main lines of the three selected experiments for this first flight are given together with a first evaluation of the experimental results obtained.


EXPERIMENTAL CONSIDERATIONS ON THE OSCILLATING BUBBLE TECHNIQUE

T. D. Karapantsios^* and A. I. Balouktsis
Dept. of Mechanical Engineering, Technological Educational Institution of Serres
End of Magnesia’s str., GR-62 100 Serres, Greece
^*also affiliated to the School of Mechanical & Industrial Engineering,
University of Thessaly, Pedion Areos, GR-38 334 Volos, Greece

Mathematical models and mechanistic arguments indicate that the dilatational rheological properties of fluid-fluid interfaces can have a dominant influence on the behavior of many industrial processes. In an attempt to measure these properties, the oscillating bubble technique was developed in which a gas bubble is formed on the tip of a capillary tube submerged in an aqueous substrate. The bubble size is then oscillated repeatedly while the internal bubble pressure is measured by a pressure transducer to determine the excess force required to expand and compress the interface. The present study examines the effectiveness of this technique for measuring dynamic interfacial properties as regards specific experimental difficulties.
Gravity poses a plausible restriction due to bubble deformation from its spherical shape and the natural convection noise interference. By using very small bubbles the on-earth accuracy of the technique is increased and hydrostatic head effects are diminished. Furthermore, most mathematical models are based on the assumption that the translation of the bubble center during the oscillation is negligible although it is questionable from an experimental viewpoint and such evidence is provided herein. Problems encountered with a moving contact line of the bubble at the tip of the capillary are also discussed in connection to bubble stability. Spurious pressure signals due to incomplete surfactant spreading over the bubble surface are discussed. Meticulous observations verified by numerical calculations show that for an apparatus using a gas chamber to impart volumetric oscillations to the bubble, the size of the bubble changes from cycle to cycle to an extent dependent on the value of the surface properties.
As an extension of the technique’s ability to measure really slight pressure changes an experimental configuration may incorporate an automatic compensation of surface tension effect by means of a simulated pressure signal on the reference side of the (differential) pressure transducer.


LOW RESIDUAL ACCELERATION LEVELS TO CARRY-OUT A GRAVITATIONAL EXPERIMENT ON DROP TOWER BREMEN

H. Dittus, St. Lochmann, C. Mehls
Centre of Applied Space Technology and Microgravity, University Bremen, Am Fallturm, D- 28359 Bremen, Germany

Precise gravitational experiments (e.g. to prove the Equivalence Principle, to measure the Gravitational Constant, or to test predictions from General Relativity) require a very low residual acceleration (microgravity) environment. The disturbing acceleration in the frequency range below 0.01 Hz is 10^-10 g and less, depending on the special experiment. To carry out experiments on spacecrafts, highly precise attitude control systems (drag-free systems) are necessary to provide gravitational influences of the surrounding spacecraft on the experiment. Gravitational experiments on drop towers require additional provision to avoid any disturbance influence.
On Drop Tower Bremen we attained a residual acceleration level of only 10^-8 g in the frequency range between 1 and 50 Hz by means of a specially constructed free flying platform inside the drop capsule. This enable us to carry out an experiment to prove the Weak Principle of Equivalence to a theoretical limit of about 10^-13 by measuring the relative movement of the trajectories of two cylindrical test bodies of different composition. SQUID-based sensing is used to detect differential movements in the 10^-14 m range. Therefore, the experimental set-up can also be treated as a differential accelerometer to measure accelerations down to 10^-13 m/s².
We will describe the experimental set-up report on recent experimental results. We also will discuss the perspectives for future gravitational experiments in space.


REVIEW OF LIGHT SCATTERING EXPERIMENTS IN MICROGRAVITY

J.C. Worms
European Space Science Committee do ENSPS
Parc d'Innovation, Boulevard Sébastien Brandt
F87400 ILLKIRCH, France
wormsjc@ensps.u-strasbg.fr

Light scattering by solar system dust particles is a major tool to gain access to some of the physical properties of these particles. These encompass interplanetary dust, planetary regoliths, cometary dust, etc. Measurement of the degree of polarization of the scattered light provides insight on the particle size, size distribution, and on their porosity/roughness. Most of the theoretical basis to relate these parameters to the observed scattered radiance is unfortunately lacking because of the irregular nature of the scattering grains: Mie models, for instance, only deal with spherical or spheroidal grains. Modelling studies making use of fractal aggregates and DDA-related techniques are promising but are still far from being able to compute representative scattering functions for large, irregular, aggregates. Laboratory measurements are thus essential to refine these studies. Various ground-based techniques are being used to this end (optical, electromagnetic or mechanical levitation, microwave-analog experiments, etc) but cannot be used to investigate "free" (three-dimensional) clouds of separated particles.
To address this issue, it was proposed to use microgravity to simulate the conditions prevailing for orbiting dust in the solar system. The PROGRA2 (PRopriétés Optiques des GRains Astronomiques et Atmospheriques) experiment was accepted and financed by CNES and built at Service d' Aéronomie in Verrières and LPCE in Orleans. Selected regularly on several parabolic flight campaigns on board the "Zero G" Caravelle, NASA KC-135 and Airbus A300 aircrafts, PROGRA2 has participated in 8 campaigns since 1994 with a yield of 24 polarimetric phase curves (degree of linear polarization versus phase angle) for basaltic glass, Boron and Silicium carbide, glass spheres, silicate samples, and industrial ashes and soot particles. The instrument is fully operational and can be configured to fly several times a year on short notice.
A second experiment was proposed for flight on a sounding rocket to test the evolution of the degree of polarisation with the state of aggregation of spherical glass particles: large aggregates will scatter light differently than spherical monomers. This German-French experiment, called CODAG-SRE, is financed by CNES, DLR and ESA, and will fly on the MASER-8 sounding rocket in April 1999. Apart from its light scattering aspect, this experiment will also provide insight into the aggregation processes believed to take place in the early solar system nebula.
The presentation will discuss the scientific objectives of light scattering experiments undertaken in microgravity, the results obtained so far, and future prospects.


TEMPERATURE AND WETTING BEHAVIOR OF A GAS-LIQUID SYSTEM HEATED UNDER MICROGRAVI'IY

D. Beysens* Y. Garrabos^†, R. Wunenburger^†, C. Chabot^†, J.-P. Delville^‡, V. Nikolayev^§ and J. Hegseth^§
* Départment de Recherche Fondamentale sur la Matière Condensèe, Commissariat à l’Energie Atomique, CEA/Grenoble 17, Avenue des Martyrs, F 38054 Grenobie Cedex 9, France
^†Institut de Chimie de Ia Matière Condensèe de Bordeaux, CNRS, Université de Bordeaux 1, Avenue du Dr. Schweitzer, F-33608 Pessac Cedex, France
^‡Centre de Physlque MolécuIalre, Optique et Hertzienne, CNRS UnIversité de Bordeaux 1, Cours de la Liberalion, F-33608 Talence Cedex, France
^§ University of New OrIeans, Department of Physlcs, New Orleans, Louisiana 70148, USA

Because the liquid-gas~solid interface is of great interest for many practical and fundamental reasons, much is known about its behaviour at thermal equi1ibrium. Examples where this interface is important range from capillary force to surface wetting. Less well known is the behaviour of this interface out of thermal equilibrium. Here the non equi1brium conditions were achieved by ramping the temperature from diphasic state of pure fluid SF6. These experiments were performed on the MIR space station in the ALICE II instrument using high precision temperature and density control to put a SF₆ fluid sample near its critical point. Near critica1 conditions and a microgravity environment were used to scale the diverging behaviour as the systems temperature T tends to the critical temperature T₀ and to suppress buoyancy driven flows and gravitational constraints on the liquid-gas interface during the « boiling» of the gas bubble inside the liquid.
We report local temperature measurements which show large transient temperature inhomogeneities between both phases during continuous heating or temperature steps. Moreover we found that as T is increased to T₀ the apparent contact angle is very large (up to 110°), and the gas appears 10 "wet" the solid surface. This behaviour is quite different from the well known situation of perfect wetting by the liquid phase (zero contact angle) near the critical point . We then consider the possible causes of this surprising behaviour: heat and mass transfer by the «piston» effect (adiabatic heating), vapor recoil force due to evaporation, surface tension gradients.