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Theses

Détection optique de films de mouillage de l'hélium liquide sur les métaux alcalins

Abstract : This thesis is an experimental study of the wetting dynamics of a superfluid 4He film on cesium surfaces, passing through the prewetting transition by changing the temperature. To get images of the helium film, we adapted to low temperatures an interferential differential contrast microscope, which gives access to the local gradient of helium coverage. Cesium surfaces are obtained by evaporation on metallic substrates at low temperature. Contact angle were measured and are equivalent to others groups using the same way of making cesium surfaces. Helium film thickness was measured as a function of off-saturation and is compatible with expected values. Depending on the cesium surface deposition, two types of film behaviours during the prewetting transition were observed. In the first kind of surface, the film moves imperceptibly by passing through intermediate states which appear to be a mix between dry and wet surface at a lower scale than resolution. In this case, we only could know the proportion of dry surface ahead of the film edge, averaged on the diffraction spot. The way it behaves with temperature can be interpretated in the random field Ising model at zero temperature without nucleation, with a large parameter of disorder. In the second kind of surface, the correlation length was larger, and the motion of the film was clearly observable. The advancement of the film over the surface takes place at constant temperature by successive thermal activated avalanches. Velocity of the film as a function of temperature and thickness was measured.
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Xavier Muller. Détection optique de films de mouillage de l'hélium liquide sur les métaux alcalins. Matière Condensée [cond-mat]. Université Pierre et Marie Curie - Paris VI, 1999. Français. ⟨tel-00011779⟩

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