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We report an experimental study of a "gas" of inelastically colliding particles, excited by vibrations in low gravity. In the case of a dilute granular medium, we observe a spatially homogeneous gaslike regime, the pressure of which scales like the 3͞2 power of the vibration velocity. When the density of the medium is increased, the spatially homogeneous fluidized state is no longer stable but displays the formation of a motionless dense cluster surrounded by low particle density regions. 81.70.Ha, 83.10.Pp, 83.70.Fn Vibrated granular media display striking fluidlike properties: convection and heaping [1,2], period doubling instabilities [3], and parametric extended [4] or localized [5] surface waves. When the vibration is strong enough, the granular medium undergoes a transition to a fluidized state. It looks like a gas of particles that can be described using kinetic theory [6]. The "granular temperature," i.e., the mean kinetic energy per particle, is determined by the balance between the power input due to the external vibration and dissipation by inelastic collisions. Fluidization by vibrations has been studied experimentally [7,8] and numerically [8,9], but no agreement has been found so far for the dependence of the granular temperature on the amplitude and the frequency of external vibrations [10][11][12].One of the most interesting properties of such "granular gases" is the tendency to form clusters. Although this has probably been known since the early observation of planetary rings [13], there exist only a few recent laboratory experiments. One experiment, with a horizontally shaken two-dimensional layer of particles, displayed a cluster formation, but the coherent friction force acting on all the particles was far from being negligible [14]. We performed a similar three-dimensional experiment in the laboratory and observed clustering, but we could not rule out the possibility of a resonance mechanism between the time of flight under gravity and the excitation frequency [15]. Various cluster types in granular flows have also been observed numerically [16]. The mechanisms of cluster formation are an active subject of research that still deserves more study because of its relevance to technical, astrophysical [17], or geophysical [18] applications of granular media. At a more fundamental level, it is of a primary interest to understand the new qualitative behaviors due to inelasticity of collisions, i.e., nonconservation of energy, in kinetic theory.In this Letter, we report a study of the kinetic regimes of a granular medium, fluidized by vibrating its container in a low gravity environment. The motivation for low gravity is to achieve an experimental situation in which inelastic collisions are the only interaction mechanism. The aim of the experiment is to observe new phenomena which result from the inelasticity of the collisions and are thus absent in a usual gas. In the dilute case, we show that the pressure of a granular gas scales like the 3͞2 power of the vibration velocity. When the density o...

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Boiling crisis experiments are carried out in the vicinity of the liquid-gas critical point of H2. A magnetic gravity compensation set-up is used to enable nucleate boiling at near critical pressure. The measurements of the critical heat flux that defines the threshold for the boiling crisis are carried out as a function of the distance from the critical point. The obtained power law behavior and the boiling crisis dynamics agree with the predictions of the vapor recoil mechanism and disagree with the classical vapor column mechanism.

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We study the response of a fluid in near-critical conditions to a heat pulse, in the absence of gravity effects. The fluid under investigation is CO 2 at critical density. It is enclosed in a thermostated sample cell. We apply a theory that accounts for hydrodynamics and a real equation of state. Comparison with experiments performed under reduced gravity on board the MIR orbital station show quantitative agreement and demonstrate that the dynamics of relaxation is ruled by two typical times, a diffusion time t D and a time t c associated to adiabatic heat transport, the so-called ''Piston effect'' ͑PE͒. Three regions are observed in the fluid. First, a hot boundary layer, developing at the heat source, which shows large coupled density-temperature inhomogeneities. This part relaxes by a diffusive process, whose density and temperature relaxations are slowed down close to the critical point. Second, the bulk fluid, which remains uniform in temperature and density and whose dynamics is accelerated near the critical point and governed by the PE time. At the thermostated walls a slightly cooler boundary layer forms that cools the bulk also by a PE mechanism. The final equilibration in temperature and density of the fluid is governed by the diffusion time t D , which corresponds to the slowest mechanism. Comparison with a one-dimensional model for temperature relaxation is performed showing good agreement with experimental temperature measurements. A brief comparison is given with the situation in the presence of gravity.

We give simple expressions for the mean of the max and min bounds of the critical-to-classical crossover functions, previously calculated [Bagnuls and Bervillier, Phys. Rev. E 65, 066132 (2002)] within the massive renormalization scheme of the Phi(d)4(n) model in three dimensions (d = 3) and scalar order parameter (n = 1) of the Ising-like universality class. The mean functions are determined relying on the properties of the theoretical functions in the two limiting three-dimensional (3D) Ising-like and mean-field-like descriptions close to the Wilson-Fisher fixed point and to the Gaussian fixed point, respectively. Such descriptions correspond to the preasymptotic domains near each fixed point where a Wegner expansion restricted to two terms (leading and first confluent terms) is valid. The Ising-like preasymptotic domain description includes the correlations between parameters due to the error-bar determination of the exponents and amplitude combinations very close to the Wilson-Fisher fixed point. Adding the equivalent description of the mean field preasymptotic domain close to the Gaussian fixed point leads to define each mean crossover function with three calculated parameters. Fixing a unique value of one parameter whatever the selected mean crossover function, we use this parameter as a relative sensor to estimate the dominant nature, either (Ising-like) critical, or (mean-field-like) classical, of the crossover behavior. Finally, we obtain an explicit criterion to measure the extension of the Ising-like preasymptotic domain which can then permit to coherently account for measurements performed in systems where the asymptotical approach to the critical point remains finite, using a well-controlled number of system-dependent parameters (like in the subclass of one-component fluids).

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