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Rupture et fusion d'un cristal bidimensionnel

Abstract : Breaking and melting of a two-dimensional crystal : Langmuir monolayers -molecular layers of amphiphilic molecules formed at the air/water interface- provide an ideal model for studying of two-dimensional systems. A first order liquid-solid phase transition has been observed in Langmuir monolayers of NBD-stearic acid. Domains of the solid phase are rectangular crystals, well adapted to micromechanical measurements. In the first part, we show that a crystal bent in the plane of the water under the effect of a bending stress, breaks after a clearly defined delay time. This delay is a function of the strain applied to the crystal. For large deformations, a second mode of breaking appears and coexists with the previous one : a fraction of the rods break instantaneously, while others break after a time-lag. In the second part, we study the melting of the crystals, which coexist with their liquid phase. In our system, melting can be achieved by two different thermodynamic processes (heating and decompression) and by a photochemical process. The latter process is shown to occur through a reversible photochemical reaction with oxygen in the air, leading to freezing-point depression. The three pocesses lead to strictly analogous behaviour, indicating a common mechanism for the initiation of the melting. Our observations show that the interior of the solid melts well before the edges do. The melting of crystals under a mechanical stress is initiated along the line at which the internal stress vanishes. We suggest that this apparently counterintuitive result arises from the migration of defects -probably dislocations- to the region of zero stress, where they accumulate and nucleate the melting. These results support the idea that defects play a crucial role in melting of two-dimensional systems.
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  • HAL Id : tel-00001024, version 1

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Ludovic Pauchard. Rupture et fusion d'un cristal bidimensionnel. Analyse de données, Statistiques et Probabilités [physics.data-an]. Université Pierre et Marie Curie - Paris VI, 1997. Français. ⟨tel-00001024⟩

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