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To understand the boiling crisis mechanism, one can take advantage of the slowing down of boiling at high pressures, in the close vicinity of the liquid-vapor critical point of the given fluid. To preserve conventional bubble geometry, such experiments need to be carried out in low gravity. We report here two kinds of saturated boiling experiments. First we discuss the spatial experiments with SF at 46 C. Next we address two ground-based experiments under magnetic gravity compensation with H at 33 K. We compare both kinds of experiments and show their complementarity. The dry spots under vapor bubbles are visualized by using transparent heaters made with metal oxide films. We evidence two regimes of the dry spots growth: the regime of circular dry spots and the regime of chain coalescence of dry spots that immediately precedes the heater dryout. A recent H experiment is shown to bridge the gap between the near-critical and low pressure boiling experiments. [ABSTRACT FROM AUTHOR]

We present an experimental setup (OLGA: Oxygen Low Gravity Apparatus) designed to compensate, partially or totally, gravity forces in a sample of O by means of magnetic forces. The compensation remains within 1 % in a 5 cm volume and within 5 % in a 40 cm volume. The experiments can be carried out at intermediate controlled gravity levels, between Earth (1g ) and zero (0g ) gravity by adjusting the magnetic field intensity. The setup can also be used to perform fast variations of gravity, with a time constant of 340ms, from -0.5g maximum (overcompensation) to 0g , or from 0g to 0.4g maximum. The sharp acceleration or deceleration of spacecrafts can thus be reproduced to investigate in detail for instance, the transient behavior of life support systems or, when properly rescaled, fuel in rocket tanks. Numerical simulations and experiments also show that high conductive materials (copper) near the cell can modify the magnetic forces during the transient and then the accelerations. In contrast, low conductive materials (titanium) have negligible effect. [ABSTRACT FROM AUTHOR]

The turbidity ( $$\tau $$ ) measurements of Güttinger and Cannell (Phys Rev A 24:3188-3201, ) in the temperature range $$28\,\text {mK}\le T-T_{c}\le 29\,\text {K}$$ along the critical isochore of homogeneous xenon are reanalyzed. The singular behaviors of the isothermal compressibility ( $$\kappa _{T}$$ ) and the correlation length ( $$\xi $$ ) predicted from the master crossover functions are introduced in the turbidity functional form derived by Puglielli and Ford (Phys Rev Lett 25:143-146, ). We show that the turbidity data are thus well represented by the Ornstein-Zernike approximant, within 1 % precision. We also introduce a new crossover master model (CMM) of the parametric equation of state for a simple fluid system with no adjustable parameter. The CMM model and the phenomenological crossover parametric model are compared with the turbidity data and the coexisting liquid-gas density difference ( $$\Delta \rho _{LV}$$ ). The excellent agreement observed for $$\tau $$ , $$\kappa _{T}$$ , $$\xi $$ , and $$\Delta \rho _{LV}$$ in a finite temperature range well beyond the Ising-like preasymptotic domain confirms that the Ising-like critical crossover behavior of xenon can be described in conformity with the universal features estimated by the renormalization-group methods. Only 4 critical coordinates of the vapor-liquid critical point are needed in the (pressure, temperature, molecular volume) phase surface of xenon. [ABSTRACT FROM AUTHOR]

Large bubbles of oxygen are magnetically levitated inside a liquid column of oxygen. Then the magnetic field is rapidly quenched resulting in the formation of a geyser. This configuration reproduces the conditions of rocket re-ignition in orbit. Two bubbles with fill factors 6 % and 15 % were used. Two-dimensional numerical simulations based on VOF-PLIC method are also carried out. Comparison of the experimental, numerical and theoretical results shows good agreement. The method can thus be used for further more focused studies with oxygen for various gravity quenches, fill ratios and pressure values. [ABSTRACT FROM AUTHOR]

Some tenebrionind beetles inhabiting the Namib desert are known for using their body to collect water droplets from wind-blown fogs. We aim to determine whether dew water collection is also possible for desert insects. For this purpose, we investigated the infra-red emissivity, and the wetting and structural properties, of the surface of the elytra of a preserved specimen of Physasterna cribripes (Tenebrionidæ) beetle, where the macro-structure appears as a series of 'bumps', with 'valleys' between them. Dew formation experiments were carried out in a condensation chamber. The surface properties (infra-red emissivity, wetting properties) were dominated by the wax at the elytra surface and, to a lower extent, its micro-structure. We performed scanning electron microscope on histological sections and determined the infra-red emissivity using a scanning pyrometer. The emissivity measured (0.95±0.07 between 8-14μm) was close to the black body value. Dew formation occurred on the insect's elytra, which can be explained by these surface properties. From the surface coverage of the condensed drops it was found that dew forms primarily in the valleys between the bumps. The difference in droplet nucleation rate between bumps and valleys can be attributed to the hexagonal microstructure on the surface of the valleys, whereas the surface of the bumps is smooth. The drops can slide when they reach a critical size, and be collected at the insect's mouth. Graphical abstract: [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The CEA Grenoble, through the ESEME/SBT team, has developed a new ground based facility providing magnetic compensation of gravity in oxygen. A 2T superconducting magnetic coil has been used to create the magnetic field. The installation is described. Well adapted to the heat and mass transfer studies, for example of the various boiling regimes, it permits to enhance the understanding of these phenomena in reduced gravity and gives a convenient way to reproduce space conditions on the ground. The first experimental results are presented. [ABSTRACT FROM AUTHOR]

The heating of coexisting gas and liquid phases of pure fluid through its critical point makes the fluid extremely compressible, expandable, slows the diffusive transport, and decreases the contact angle to zero (perfect wetting by the liquid phase). We have performed experiments on near-critical fluids in a variable volume cell in the weightlessness of an orbiting space vehicle, to suppress buoyancy-driven flows and gravitational constraints on the liquid-gas interface. The high compressibility, high thermal expansion, and low thermal diffusivity lead to a pronounced adiabatic heating called the piston effect. We have directly visualized the near-critical fluid’s boundary layer response to a volume quench when the external temperature is held constant. We have found that when the system’s temperature T is increased at a constant rate past the critical temperature T c , the interior of the fluid gains a higher temperature than the hot wall (overheating). This extends previous results in temperature quenching experiments in a similarly prepared system when the gas is clearly isolated from the wall. Large elliptical wetting film distortions are also seen during these ramps. By ray tracing through the elliptically shaped wetting film, we find very thick wetting film on the walls. This wetting film is at least one order of magnitude thicker than films that form in the Earth’s gravity. The thick wetting film isolates the gas bubble from the wall allowing gas overheating to occur due to the difference in the piston effect response between gas and liquid. Remarkably, this overheating continues and actually increases when the fluid is ramped into the single-phase supercritical phase. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The results of two series of space experiments carried out onboard the MIR in 1999–2000 are presented. The experiments were undertaken to study the effect of microgravity environment as well as specially excited vibration on heat spread from a point source inside a cell filled with SF6 near its critical point. New space experiments are proposed to continue the series interrupted by the deorbiting of the MIR. These proposals are the central point of the CRIT project planned for the realization onboard the Russian Segment of the ISS. [ABSTRACT FROM AUTHOR]

Experiments on non interacting balls in a vibrated box are reported. In a first experiment with an electromagnetic vibrator on earth or in board of Airbus A300 of CNES, the 1-ball dynamics exhibit little transverse motion and an intermittent quasi periodic motion along the direction parallel to the vibration. This behaviour proves a significant reduction of the phase space dimension of this billiard-like system from 11- d to 3- d or 1- d. It is caused by dissipation, which generates non ergodic dynamics. This experiment exemplifies the coupling between translation and rotation degrees of freedom during the collisions with the walls, due to solid friction at contacts. This eliminates ball rotation and freezes transverse velocity fluctuations. This trend is confirmed by 3d simulations with JJ Moreau discrete element code. A two-ball experiment performed under zero-g conditions in the Maxus 5 flight confirms the trend; the quasi-periodicity is found much greater, which is probably due to an improvement of experimental conditions. The two balls are not in perfect synchronisation showing the effect of small random noise; but the particles has never collided. This is then the normal dynamics of a gas of non-interacting dilute spherical grains in a vibrated container. [ABSTRACT FROM AUTHOR]

Close to the liquid gas critical point, the linear treatment of the symmetrical onecomponent θ[sup 4] model to observe the fluid-restricted universality of the subclass of pure fluids is reversed. The comparison with the fitting results obtained from the recent applications of the crossover description to CO[sub 2], CH[sub 4], C[sub 2]H[sub 4], C[sub 2]H[sub 6], R134a, SF[sub 6], and H[sub 2]O confirms that the dimensionless characteristic two scale factors involved in this description are: (a) the critical compressibility factor and (b) the slope at the critical point of the reduced potential P/T T[sub c]/P[sub c] along the critical isochore. For the two-phase domain along the critical isochore, a precise formulation for the extension range of the fluid-restricted universality is given in terms of the reduced scaling size ... of the critical density fluctuations, expressed as a function of the dilated scaling field which measures the distance to the critical point below T[sub c]. The explicit definition of the microscopic length scale a[sub c] = ... which characterizes the short-range of the microscopic interac- tion, gives a correlative estimation of the crossover domain when ξ[sup -] a[sub c]. [ABSTRACT FROM AUTHOR]

An experimental study of the thermal response to a stepwise rise of the wall temperature of two-phase near-critical SF6 in low gravity for an initial temperature ranging from 0.1 to 10.1 K from the critical temperature is described. The change in the vapor temperature with time considerably exceeds the change in the wall temperature (overheating by up to 23% of the wall temperature rise). This strong vapor overheating phenomenon results from the inhomogeneous adiabatic heating process occurring in the two-phase near-critical fluid while the vapor bubble is thermally isolated from the thermostated walls by the liquid. One-dimensional numerical simulations of heat transfer in near-critical two-phase 3He confirm this explanation. The influence of heat and mass transfer between gas and liquid occurring at short time scales on the thermal behavior is analyzed. A model for adiabatic heat transfer, which neglects phase change but accounts for the difference between the thermophysical properties of the vapor and those of the liquid, is presented. A new characteristic time scale of adiabatic heat transfer is derived, which is found to be larger than that in a one-phase liquid and vapor. [ABSTRACT FROM AUTHOR]

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 system's 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. [ABSTRACT FROM AUTHOR]

On the basis of the data obtained in experiments onboard the Mirorbital complex (OC) in March and April 1999 with the ALICE-2 instrument, the influence of vibrations on heat propagation from a point-like source in the hexafluoride sulfur (SF6) under supercritical parameters is analyzed. The appearance of two different heat propagation regimes is found: along and across the direction of vibrations. Upon the occurrence of the second-type regime, the heating region first stretches in the direction perpendicular to the direction of vibrations and then propagates at a rate typical for the undisturbed case. The temperature relaxation process is analyzed, which takes place near the heater after its turn-off. The relaxation rate grows as the intensity of vibrations increases and the initial temperature tends to the critical one. The determining dimensionless parameters of the problem are suggested. [ABSTRACT FROM AUTHOR]

More than two decades ago, in a seminal paper John Cahn proposed scaling arguments for the possibility of a wetting transition in two coexisting fluid phases near the critical point. Since then, Cahn's model has been tested in many fluid systems and further refined by including the real interactions between the fluid and the solid wall. A fascinating consequence of the existence of a wetting transition is the possibility for a transition from weak to strong adsorption in the homogeneous phase. The situation is further enriched in nonstandard geometries having special geometrical constraints. The subject of this review concerns one such situation, where charge-stabilized colloidal particles are suspended in the homogeneous region of a binary liquid mixture. In this case, the preferential adsorption of one of the liquid components on to the colloid surface completely modifies the stability of the particles leading to an aggregation process. Although the exact mechanism underlying the adsorption phenomenon is still debated, it is closely related to the wetting transition. Recent experimental developments concerning the static and dynamic aspects of this phenomenon are reviewed. In addition, the main findings of a theoretical model based on the adsorption-modified electrostatic interactions between the colloidal particles are discussed. [ABSTRACT FROM AUTHOR]