On étudie l'influence sur la transition de Freedericks des conditions d'ancrage : 1) Angle fini d'ancrage ; 2) Energie d'ancrage finie. Cette influence se traduit par une diminution des constantes d'élasticité apparentes
The theory of critical phenomena in the presence of quenched disorder is applied to x-ray and neutron experiments which suggest two different length scales for critical fluctuations.Long-range random strains associated with defects generated near the sample surface can induce crossover to a "disordered" fixed point with different critical exponents, while the bulk retains the ordinary critical behavior. For Ho and Tb, it is proposed that the dominant defects are dislocation dipoles, resulting in a critical exponent for the second correlation length v, . = 1, in reasonable agreement with experiments. PACS numbers: 64.70. -p, 61.10.Dp, 61.12.Bt, 75.40.Cx X-ray and neutron critical scattering experiments in systems undergoing magnetic [1 -3] or structural [4] phase transitions have recently revealed an unexpected feature, which appears to be at odds with the existence, in the critical region, of a single important length scale, given by the correlation length s, and characterized by the critical exponent v, defined by s -t ', where t is the reduced temperature t = ((T -T")/T,~In .contra. st to this basic tenet of the theory of critical phenomena, the wave vector dependence of the scattering intensity is not at all well described by a single Lorentzian with a width proportional to the inverse of the correlation length; a much sharper feature appears near the center of the line, and a reasonable fit is only obtained by a superposition of a broad Lorentzian (referred to as the "broad" component), and a much sharper Lorentzian or Lorentzian squared component (the "sharp" component). For Ho [1] and Tb [2] the broad component has an exponent close to the theoretically predicted one for the magnetic spiral transition [5]. The second, much larger, length associated with the "sharp component" scales with temperature as t ', where the new exponent v, is different from and in most cases larger than the exponent v associated with the first correlation length.Other important results of the experiments led to the conclusion that the sharp feature is related to a layer of the sample close to the surface [2], but with a thickness which can be as large as several p, m, a surprisingly large depth for usual surface phenomena. Also, the dependence of the intensity of the sharp feature on the surface quality and mosaic and its enhancement by polishing and other forms of surface treatment [1] suggest very strongly a defect-related origin. Defects of unspecified nature, and the related long-range strain fields have been mentioned as a possible explanation [1], but no conclusive and quantitative analysis could be arrived at.In the present paper we argue that the emergence of a second length scale is a consequence of the presence v, = 2/a. (2) (If a ) d, the short-range disorder case, analyzed by Grinstein and Luther [6], is applicable instead, and different critical properties are predicted [8].)If this picture is adopted for the near-surface region, and if the scattering probe has a penetration depth allowing one to sample both the superf...
A recently demonstrated technique of aerodynamic levitation has been used to study the resonance frequency of liquid droplets. A thin pressurized gas layer supports the droplet avoiding any contact with the environment. Due to the external gravitational field, the measured frequency is different from the one predicted by the classical models for a spherical drop. An ellipsoidal shape has been introduced to take into account the competition between surface and gravitational energies. Results of this model are in good agreement with experiments in terms of volume, as well as density and surface energy, dependence of the oscillation frequency.
International audienceA recently demonstrated aerodynamic levitation technique is used to perform contactless viscosity measurements. Classical models dealing with free oscillations of droplets without gravity cannot correctly describe the correspondence between damping coefficient and viscosity. An energetic approach taking into account the shape of the drop, and the velocity field of the liquid inside the drop, is introduced, leading to good agreement between experimental measurements and known viscosities of glycerol-water mixtures. Nonlinear effects are also investigated and experimental results are compared with the theory of anharmonic oscillations
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