We have studied homogeneous cavitation in liquid nitrogen and normal liquid helium. We monitor the fluid content in a large number of independent mesopores with an ink-bottle shape, either when the fluid in the pores is quenched to a constant pressure or submitted to a pressure decreasing at a controlled rate. For both fluids, we show that, close enough to their critical point, the cavitation pressure threshold is in good agreement with the Classical Nucleation Theory (CNT). In contrast, at lower temperatures, deviations are observed, consistent with a reduction of the surface tension for bubbles smaller than two nanometers in radius. For nitrogen, we could accurately measure the nucleation rate as a function of the liquid pressure down to the triple point, where the critical bubble radius is about one nanometer. We find that CNT still holds, provided that the curvature dependence of the surface tension is taken into account. Furthermore, we evaluate the first- and second-order corrections in curvature, which are in reasonable agreement with recent calculations for a Lennard-Jones fluid.

We report on the evaporation of hexane from porous alumina and silicon membranes. These membranes contain billions of independent nanopores tailored to an ink-bottle shape, where a cavity several tens of nanometers in diameter is separated from the bulk vapor by a constriction. For alumina membranes with narrow enough constrictions, we demonstrate that cavity evaporation proceeds by cavitation. Measurements of the pressure dependence of the cavitation rate follow the predictions of the bulk, homogeneous, classical nucleation theory, definitively establishing the relevance of homogeneous cavitation as an evaporation mechanism in mesoporous materials. Our results imply that porous alumina membranes are a promising new system to study liquids in a deeply metastable state.

We report optical measurements of the imbibition of liquid helium in a sample of silica aerogel with 90 % porosity. Both direct imaging and light scattering experiments were performed to determine the dynamics and the properties of the liquid-gas interface in both the normal and superfluid phases of liquid helium. In the normal phase, a classical Lucas Washburn behavior is observed for the rise of the imbibition front while the behavior in the superfluid phase is markedly different, as the fluid invades the sample from all sides with a constant speed. In both phases, the interface is rough, leading to light scattering. In addition, condensation ahead of the imbibition front is observed at low temperature in the superfluid phase.

We combine high-resolution isotherms measurements and light scattering technics to study over a broad temperature range collective effects during the condensation and evaporation of helium from Vycor, a prototypic disordered porous material. For evaporation, our results provide the first direct evidence for a crossover from a percolation collective mechanism at low temperature to a local cavitation mechanism at high temperature. No long-range collective effects are detected during condensation. We compare these results to recent theoretical predictions emphasizing the specific role of disorder, and discuss their relevance for determining pores sizes distributions in disordered porous materials.

We describe an experimental study of the phenomenon of spray cooling in the case of liquid helium, either normal or superfluid, and its relationship to the heat transfer between an atomized two-phase flow contained in a long pipe, and the pipe walls. This situation is discussed in the context of the cooling of the superconducting magnets of the Large Hadron Collider (LHC). Experiments were conducted in a test loop reproducing the LHC cooling system, in which the vapor velocity and temperature could be varied in a large range. Shear induced atomization results in the generation of a droplet mist which was characterized by optical means. The thickness of the thin liquid film deposited on the walls by the mist was measured using interdigitated capacitors. The cooling power of the mist was measured using thermal probes, and correlated to the local mist density. Analysis of the results shows that superfluidity has only a limited influence on both the film thickness and the mist cooling power. Using a simple model, we show that the phenomenon of spray cooling accounts for the measured non-linearity of the global heat transfer. Finally, we discuss the relevance of our results for cooling the final focus magnets in an upgraded version of the LHC. (C) 2013 Elsevier Ltd. All rights reserved.

We have studied 4 He confined in a 95 % porosity silica aerogel in the vicinity of the bulk liquid gas critical point. Both thermodynamic measurements and light scattering experiments were performed to probe the effect of a quenched disorder on the liquid gas transition, in relation with the Random Field Ising Model (RFIM). We find that the hysteresis between condensation and evaporation present at lower temperatures disappears at a temperature T ch between 25 and 30 mK below the critical point. Slow relaxations are observed for temperatures slightly below T ch , indicating that some energy barriers, but not all, can be overcome. Above T ch , no density step is observed along the (reversible) isotherms, showing that the critical behavior of the equilibrium phase transition in presence of disorder, if it exists, is shifted to smaller temperatures, where it cannot be observed due to the impossibility to reach equilibrium. Above T ch , light scattering exhibits a weak maximum close to the pressure where the isotherm slope is maximal. This behavior can be accounted for by a simple model incorporating the compression of 4 He close to the silica strands.

In an optical cryostat, the sample temperature is limited by the 300 K thermal radiation transmitted through the observation windows. As discussed in this paper, the corresponding heat load should be brought much below the μW level by using suitable cold IR filters. However, there are situations where one wants to directly check the absence of IR heating by running the same experiment with the cold observation windows either open or totally blocked. We describe a particular example which is the study of helium condensation in a silica aerogel. On this aim, we developed a simple and compact magnetic system able to move metallic shutters inside our optical cryostat. This allows us to set an upper bound of 1 μW for the effect of 300 K thermal radiation on our sample. We discuss the relevance of this result to the operation of optical dilution refrigerators.

We have investigated the formation of helium droplets in two physical situations. In the first one, droplets are atomised from superfluid or normal liquid by a fast helium vapour flow. In the second, droplets of normal liquid are formed inside porous glasses during the process of helium condensation. The context, aims, and results of these experiments are reviewed, with focus on the specificity of light scattering by helium. In particular, we discuss how, for different reasons, the closeness to unity of the index of refraction of helium allows in both cases to minimise the problem of multiple scattering and obtain results which it would not be possible to get using other fluids.

We report on thermodynamic and optical measurements of the condensation process of ^4He in three silica aerogels of different microstructures. For the two base-catalysed aerogels, the temperature dependence of the shape of adsorption isotherms and of the morphology of the condensation process show evidence of a disorder driven transition, in agreement with recent theoretical predictions. This transition is not observed for a neutral-catalysed aerogel, which we interpret as due to a larger disorder in this case.

We describe a set-up for studying adsorption of helium in silica aerogels, where the adsorbed amount is easily and precisely controlled by varying the temperature of a gas reservoir between 80 K and 180 K. We present validation experiments and a first application to aerogels. This device is well adapted to study hysteresis, relaxation, and metastable states in the adsorption and desorption of fluids in porous media.

We present the first measurements of the thermal conductivity of spin-polarized normal liquid 3He. Using the rapid melting technique to produce nuclear polarizations up to 0.7, and a vibrating wire both as a heater and a thermometer, we show that, unlike the viscosity, the conductivity increases much less than predicted for s-wave scattering. We suggest that this might be due to a small probability for head-on collisions between quasiparticles.

We present results on helium co-current two-phase flow experiments at high vapor velocity obtained with the use of the new CEA/SBT 400 W/1.8 K refrigerator [1]. For vapor velocities larger than typically 4 m/s, a mist of droplets develops from the bulk liquid interface accompanied by an increase in heat transfer at the wall. Experiments were conducted in a 10 m long, 40 mm I.D. straight pipe, both in helium II and in helium I to compare these two situations. The respective roles of vapor density, vapor velocity and liquid level on atomization were systematically investigated. Light scattering experiments were performed to measure sizes, velocities and interfacial areas of droplets in a complete cross section. In-house-made heat transfer sensors located in the mist allowed us to deduce an upper value of the extra cooling power of the dispersed phase. The practical interest of atomized flow for cooling large cryogenic facilities is discussed by considering the balance between increase in heat transfer and pressure drops it induces.

Previous experiments performed on Hell co-current two-phase flow at CEA-Grenoble have shown the existence of a transition from stratified two-phase flow to droplet mist flow at high vapour velocities. The realisation of a new refrigerator/liquefier able to produce up to 20 g/s of single phase superfluid helium at 1.8 K (instead of 7 g/s previously) was achieved. Benefit was taken of the necessary junction between the existent test line and the refrigerator to introduce some new instrumentation [1]. Results of preliminary experiments performed on this new configuration are given. The response of liquid level and vapour density on droplet flow is presented. First results on pressure drop obtained for a total mass flow rate of 15 g/s are also presented. Finally, the use of various capacitive level gauges glued at different azimutal positions along the inner pipe give access to the perimeter wetted by a continuous thin liquid film.

The importance of spin fluctuations on the physical properties of liquid 3 He can be probed by studying the polarization dependence of its magnetic susceptibility and viscosity. We discuss the present state of two such experiments. Using the rapid melting technique, to strongly polarize liquid 3 He, we have measured the viscosity enhancement induced by the polarization up to unprecedented large polarizations at a low temperature (80 mK). At a polarization of 60 %, the enhancement reaches a factor 2.5, comparable to that of the inverse susceptibility at the same polarization.

We use optical methods to study condensation of 4He into a 95% porosity silica aerogel at temperatures below the bulk critical point. Simugtaneous pressure and optical measurements are performed along isotherms, as the cell is very slowly filled or emptied. We find that the pressure presents a quasiplateau below the bulk saturation pressure Psat over a finite range of densities inside the aerogel. In this range, strong light scattering is observed, which shows that the helium density fluctuates on a microscopic scale. Quantitative analysis shows that the helium density is correlated over distances somewhat larger than the gel correlation length. We discuss our resugts in terms of two possible scenarios, capillary condensation and liquid-gas transition.

We present preliminary measurements of the thermal conductivity of spin-polarized normal liquid He-3. Our experimental apparatus allows good thermal homogeneity of the 3 He as well as high sensitivity to the 3 He conductivity. Initial measurements show the conductivity changes by < 10% for polarizations up to 70%. (C) 2002 Elsevier Science B.V. All rights reserved.

Our recent measurements of the viscosity of spin polarized liquid 3 He revealed that, below 100mK, the relative enhancement of the viscosity is pressure independent and quadratic in polarization m up to m= 70 %. This simple behavior is remarkably close to that expected for a gas (i.e. isotropic scattering). For the dense liquid 3 He, the so-called s-p approximation is believed to give a more realistic description of scattering. Using the formalism developed by Anderson, Pethick and Quader, we have computed the expected polarization dependence in this case, assuming polarization independent collision probabilities. Surprisingly, this leads to an initial linear decrease of the viscosity with polarization, which is not observed experimentally. This may imply that the present description of transport in liquid 3 He is not complete.

We describe the optical techniques we used to detect droplets in the Hell two phase flow of the Cryoloop experiment. These include quantitative light scattering, imaging, and laser phase sensitive anemometry and granulometry (PDPA). We demonstrate that droplets appear for vapor velocities larger than 5 m/s, and that they progressively invade the entire pipe cross section as the vapor velocity is increased. Estimates are given for the droplet size and density.

We report on the performances of a capacitive sensor designed to probe the wetting by a liquid film in a pipe-flow experiment. Two 0.2 μm thick inter-digitised gold electrodes deposited on a single thin kapton foil constitute a soft capacitor that can conveniently fit onto the pipe walls without disturbing the flow. Selectivity on the minimum thickness of the film to be observed is achieved by properly choosing the distance between the two electrodes, and we discuss the case of a 100 μm gap capacitor. Some results in the specific situation of a cryogenic two-phase helium flow experiment for which a typical resolution of 0.2% in wetted perimeter is obtained are discussed.

The enhanced effective mass of liquid 3 He has been alternatively ascribed to the large spin fluctuations in this system (nearly ferromagnetic model) or to local correlations effects (nearly localized model). We recently measured that polarizing liquid 3 He decreases its specific heat. We present our experiment, and discuss its implications in this theoretical context.