The study of physical phenomena in microgravity for scientific or technological purposes has become an aspect of spatial research that is conducted at a national level in France by a well-structured scientific community. This particular aspect of spatial research first emerged at the same time as the first manned space flights.

In recent times such studies have successfully used the microgravity environment to obtain high quality scientific results, some of which have been recognized by research assessment bodies. In addition to the Edmond Brun prize (2003) and a bronze medal from CNRS (2002), two "grands prix" from the French Academy of Science were awarded in this field in 1996 and 2000.

Implementation difficulties encountered when the International Space Station is at full thrust led the scientific community to use sounding rockets and parabolic flights with equal success. Finally, given that all microgravity research in the fluid physics field is placed upstream from very important spatial technology issues, its scientific scheduling takes account of an increase in research themes whose reach extends to issues concerning transport technologies in low orbit and, with regards to planetary exploration, automatic or manned orbit.

Microgravity is the dynamic environment: in relation to a linear coordinate linked to their centre of gravity, material objects move in a field of gravity uniquely affected by their initial speed. The inertial force constantly compensates the weight of a material object with the centre of gravity position and the total mass of the system. On Earth, microgravity can be reproduced by releasing objects in "freefall" from towers or in redeveloped disused mine shafts, or even by flying them in aircraft in parabolic flight.

Microgravity can therefore influence fluid phases where it deletes hydrostatic pressure, sedimentation and convection. It can be used to study the purely diffusive transport conditions of physical phenomena, which, on the ground, are hidden, deformed or even rendered unfit for use by the presence of perturbing movements in the fluid phases. Microgravity applications therefore provide significant means used to obtain knowledge that would be impossible to acquire on the ground, both with regards to scientific research and information that contributes to improving industrial processes. Microgravity is also an essential characteristic of the spatial environment to which launch vehicles and satellites must adapt. It is here that the second aspect emerges as a crucial technological requirement that is closely linked to research into material sciences.

Solidification and crystal growth

The ability to solidify samples in levitation with no contact with the crucible makes it possible to demonstrate in greater detail the ephemeral metastable phases in refractory materials.

Removing convection also makes it possible to measure thermophysical coefficients in purely diffusive conditions, particularly diffusion coefficients in liquid metals – the precise characteristics of which are very much in demand with regards to digital simulation in industrial processes.

Microgravity is used to study the transition between columnar growth (of dendritic origin) and equiaxed growth (germs introduced in the original ingot cross over in all directions), without grain convection or sedimentation.

Microgravity is also used to study, in purely diffusive conditions, structures that form at the interface between a liquid and a solid. With regards to transparent alloy models, preparatory studies conducted on the ground for experiments that will be carried out in the French DECLIC facility (Dispositif pour l'Etude de la Croissance et des Liquides Critiques: Facility for the Study of the Growth and Fluids near Critical Point) have made it possible to correctly identify perturbations caused by convection which, using the temperature and concentration gradients that it produces along the interface, leads to structural transitions that are difficult to characterize and observe from the ground.

Top: oblique view of eutectic solidification interfaces in a CBr4-C2C16 sample, 300 µm thick. The liquid on top and the pull rate is 0.25 µm.s-1.
Bottom: filtered and resized image (the bar represents µm).

Combustion

In the field of combustion, microgravity presents many advantages since it is possible to levitate small drops of fuel, then apply the spherical symmetry from theoretical studies to them and check basic theories. It is thus possible to measure the regression rate of the drops’ diameter according to time parameters, one of the key parameters of combustion models. Furthermore, on the ground, when the surface tension becomes very weak, for example as critical conditions are approached, it is no longer possible to position the small drops on supports. Microgravity is therefore the only means of conducting combustion experiments on drops at high pressure. Several experiments of this kind have been conducted during parabolic aircraft flights or in freefall towers. All this research has very practical benefits since they provide information required to understand the ignition and appearance of instability in rocket engines.

Another benefit of microgravity for combustion is the absence of convective movements and therefore of draught phenomena. It therefore becomes possible blow on a surface in combustion at a far lower speed than would be required on the ground by draught (convection) and study the basic physics. Determining extinction and inflammability conditions can also contribute to fire safety onboard orbital vehicles.

Fluid interfaces and foams

In contrast to solidification or crystalline growth, which involve studying the interface between a solid and a liquid, fluid interfaces are interfaces between two fluids, regardless of whether they are expanded interfaces or those that are closed in on themselves, as in the case of foams.

In this field, microgravity is used to observe the evolutions of fluid phases and surface tension forces when affected by a single diffusive transport. Foam with a high volumetric liquid fraction, known as wet foams, has a higher service life in microgravity because gravity drainage on the interfaces no longer exists and only the capillary disproportion and flows are still present. For example, microgravity makes it possible to observe the formation of plate edges as heavy foam is drained from the interfaces during the resource phase (1.8 g) on the A300 Zero G aircraft in parabolic flight. These flights were also used to develop the device for the controlled production of foam in microgravity.

Levitated objects and diphasic flow

Levitated objects are objects that, in the absence of sedimentation, remain in suspension in the continuous phase (still or moving) in which they are contained (emulsions for example). They can also be granular matter, particles in a flow or diphastic flows in a tube. With regards to diphasic flows, the absence of shifting speed between the gas and the liquid means that almost all turbulence caused by the air bubbles disappears, whilst the drag and lift forces become very weak, which in turn simplifies the interaction mechanisms between the air bubbles and the continuous phase (in contrast to on the ground where gravity and turbulence act together).

In the field of granular matter, results from experiments conducted in parabolic flights have made it possible to obtain the equation of state for granular gases and to demonstrate the emergence of compact clusters in a sufficiently dense gas of vibrated grains whose subjacent mechanisms could play a fundamental part in the formation of the planetary rings.

One of the more surprising effects of the anelasticity
of the collisions is shown in figures a and b.
The gas of homogeneous particles (a) can become unstable;
particles form a cluster that rebound on the walls like a solid body.

Critical fluids

On approaching the critical point, fluid density becomes like that of a liquid, whist keeping the transport properties of gas, which obtains specific properties for supercritical fluids with regards to selective solvation, solubility and reactivity phenomena.

The area surrounding the critical point of pure fluids is host to significant anomalies with regards to thermodynamic parameters (hyper compressibility) and transport properties. On the ground, these anomalies cause high levels of sedimentation and highly intense convective movements that make any observation attempts difficult. Eliminating these anomalies in microgravity makes it possible to conduct observations in exceptional conditions. New mechanisms, like the piston effect (thermo-acoustic transfer of heat, quicker than other known transfer types) become dominant. Their signature is found in situations that would have remained unexplained or paradoxical if it were not for their discovery in microgravity.

Critical point phenomena find applications in the energy sector and in research into higher thermal efficiency. The space industry faces challenges with regards to both proximity to critical point (cryogenic engines) and microgravity.

The DECLIC facility, for critical fluids, in integration.
© EADS-Sodern/C. Sabater

Microgravity applications

Special emphasis in microgravity research programmes is now placed on the management of fluids in space: heat control in spacecraft and high-efficiency heat exchanges, orbital propulsion, storage of fluids in space for increasingly long periods of time. Material sciences for space can also be material sciences for space.

With regards to mass and heat transfers with low or zero gravity, the most important aspect are the phase transition phenomena at the liquid/gas/solid interfaces, where the solid can be flat or divided and the liquid cryogenic or supercritical. These challenges are related to the following issues: boil-off crisis, capillary phenomena in the presence of phase and flow transitions, system miniaturisation (diphasic microfluidics, phase transition), cryogenic cooling using pulsed tubes and the effects of compressibility (thermo-acoustic coupling, adiabatic heat transfers using piston effects) in hyper compressible supercritical fluids.

With regards to flow mechanics, in the launch vehicle sector, important aspects are large movements of interfaces in cryogenic fluids (hydrogen, oxygen) or noble gases (helium, xenon, argon), triggered by instabilities that could be Rayleigh-Taylor types for example (geyser phenomenon) or convection phenomena in hyper compressible mediums and drops in thermoconvective pressure.

Finally, with regards to basic physicochemical data, it should be noted that there is often a severe lack of knowledge for those of fluids used in space and there is a real need to improve knowledge in this area. The same can be said for state equations, particularly for hydrogen and oxygen.