Daniel Beysens

Magnetic Gravity Compensation

Magnetic gravity compensation in fluids is increasingly popular as a means to achieve low-gravity for physical and life sciences studies. We explain the basics of the magnetic gravity compensation and analyze its advantages and drawbacks. The main drawback is the spatial heterogeneity of the residual gravity field. We discuss its causes. Some new results concerning the heterogeneity estimation and measurement are presented. A review of the existing experimental installations and works involving the magnetic gravity compensation is given for both physical and life sciences.

Study of fluid behaviour under gravity compensated by a magnetic field

Fluids, and especially cryogenic fluids like hydrogen and oxygen, are widely used in space technology for propulsion and cooling. The knowledge of fluid behaviour during the acceleration variation and under reduced gravity is necessary for an efficient management of fluids in space. Such a management also rises fundamental questions about thermo-hydrodynamics and phase change once buoyancy forces are cancelled. For security reasons, it is nearly impossible to use the classical microgravity means to experiment with such cryofluids. However, it is possible to counterbalance gravity by using the paramagnetic (O2) or diamagnetic (H2) properties of fluids. By applying a magnetic field gradient on these materials, a volume force is created that is able to impose to the fluid a varying effective gravity, including microgravity. We have set up a magnetic levitation facility for H2 in which numerous experiments have been performed. A new facility for O2 is under construction. It will enable fast change in the effective gravity by quenching down the magnetic field. The facilities and some particularly representative experimental results are presented.

Bubble spreading during the boiling crisis: modelling and experimenting in microgravity

Boiling is a very efficient way to transfer heat from a heater to the liquid carrier. We discuss the boiling crisis, a transition between two regimes of boiling: nucleate and film boiling. The boiling crisis results in a sharp decrease in the heat transfer rate, which can cause a major accident in industrial heat exchangers. In this communication, we present a physical model of the boiling crisis based on the vapor recoil effect. Under the action of the vapor recoil the gas bubbles begin to spread over the heater thus forming a germ for the vapor film. The vapor recoil force not only causes its spreading, it also creates a strong adhesion to the heater that prevents the bubble departure, thus favoring the further spreading. Near the liquid-gas critical point, the bubble growth is very slow and allows the kinetics of the bubble spreading to be observed. Since the surface tension is very small in this regime, only microgravity conditions can preserve a convex bubble shape. In the experiments both in the Mir space station and in the magnetic levitation facility, we directly observed an increase of the apparent contact angle and spreading of the dry spot under the bubble. Numerical simulations of the thermally controlled bubble growth show this vapor recoil effect too thus confirming our model of the boiling crisis.

Gas wets a solid wall in orbit

When coexisting gas and liquid phases of a pure fluid are heated through their critical point, large-scale density fluctuations make the fluid extremely compressible and expandable and slow the diffusive transport. These properties lead to perfect wetting by the liquid phase (zero contact angle) near the critical temperature Tc. However, when the systems temperature T is increased to Tc, so that it is slightly out of equilibrium, the apparent contact angle is very large (up to 110°), and the gas appears to “wet” the solid surface. These experiments were performed and repeated on several missions on the Mir space station using the Alice-II instrument, to suppress buoyancy-driven flows and gravitational constraints on the liquid–gas interface. These unexpected results are robust, i.e., they are observed under either continuous heating (ramping) or stepping by positive temperature quenches, for various morphologies of the gas bubble and in different fluids (SF6 and CO2). Possible causes of this phenomenon include both a surface-tension gradient, due to a temperature gradient along the interface, and the vapor recoil force, due to evaporation. It appears that the vapor recoil force has a more dominant divergence and explains qualitatively the large apparent contact angle far below Tc.

Vapour spreading and the boiling crisis

The boiling crisis (BC) is well known in the world of heat and mass transfer. It is a transition from nucleate boiling (i.e. boiling in its usual sense) to film boiling, where the heater is covered by a continuous vapour film. The BC is observed when the heat flux from the heater exceeds a critical value. Heat exchange then falls down and endangers the exchanger whose temperature rises abruptly. The physical mechanism of the BC is still under debate. We propose the recoil force (the thrust of vapour production) at the solid–liquid–vapour contact line to lie at the origin of the BC. At large heat flux, the recoil force tends to spread the vapour bubble that otherwise would not wet the solid. We give both analytical and numerical analysis in support of this idea. We also report experiments under microgravity conditions performed with near-critical fluids (SF6 and CO2). The absence of gravity effects and the vicinity of the critical point where the liquid–vapour surface tension vanishes emphasize the influence of the recoil force: during heating, the vapour drop is indeed seen to spread.

Phase separation in critical and off-critical fluids: specific experimental behaviours at criticality

The phase separation in fluids close to a critical point can be observed in the form of either an interconnected pattern (critical case) or a disconnected pattern (off-critical case). These two regimes have been investigated in different ways. First, a sharp change in pattern is shown to occur very close to the critical point when the composition is varied. No crossover has been observed between the t1 behaviour (interconnected) and a t1/3 behaviour (disconnected), where t is time. This latter growth law, which occurs in the case of compact droplets, will be discussed. Second, it has been observed that a growing interconnected pattern leaves a signature in the form of small droplets. The origin of such a distribution will be discussed in terms of coalescence of domains. No distribution of this kind is observed in the off-critical case.

Direct observation of critical fluctuations

By performing careful observations very near the critical point of binary fluids or microemulsions using optical microscopy it is possible to obtain a resolution of the order of the correlation length and observe fluctuations in the order parameter (concentration). The origin of these fluctuations is discussed by comparing the picture element to a spin block variable within 3D Ising model. It follows that the free energy of the configuration can be obtained from the histogram of the fluctuation amplitudes. Black and white domains can be defined by clipping these fluctuations relative to a mean value. Domains are seen to be self-similar in shape, with a fractal dimension of 2.8. The origin of this self-similarity is discussed and a possible relation with percolation model is envisaged.

Growth of a Droplet Pattern (Breath Figures) on a Surface

The nucleation and growth of “Breath Figures”, the patterns formed when vapor condenses onto a cold surface, are investigated for water. They have been studied by simultaneous microscopic and light-scattering observations. The parameters of the growth: contact angle θ of the drops with the surface, incident gas flow, degree of supersaturation, have been varied. For θ = 90., the growth of the pattern (after an initial period during which the surface coverage and the droplet polydispersity reach a constant value), is self-similar in time. The radius of an average droplet grows with a power law with exponent np ≃ 0.75, whereas the growth of a single droplet between two coalescences obeys a power law with a lower exponent ns ≃ 0.23. Comparisons are made between the experiments, theory s and numerical simulations.