Localized Instabilites and Nucleation of Dislocations in Convective Rolls
In fluids driven to convection by an external constraint the occurrence of chaos is usually preceded by well-ordered structures in the flow field. In the case of an anisotropic fluid (a liquid crystal), those structures are arranged in a unique and well-defined sequence [l]. However, as in any other ordered structures, defects often appear spontaneously and introduce some disordering . So far, the nucleation mechanism and the role of the defects are unclear especially in Rayleigh Bénard convection, where two types of defects coexist . In an anisotropic system, only one type of defect occurs and we have also found that the role of this defect is to build a new route to chaos through a dynamical process that presents the essential features of a Martensitic Transformation as in solids . Some theories have recently been developed to describe the complicated patterns of roll structures that include defects at onset of convection in large aspect ratio cells . However, the questions of how a defect is spontaneously produced, and of its precise role in the transition to chaos, are important ones and to our knowledge, have not been theoretically addressed, up to now. Although such defects were always experimentally observed, they would generally be attributed either to preexisting topological (or geometrical) imperfections, or to a spatially localised mechanism of amplification of some fluctuation close to the onset of convection .
KeywordsNematic Liquid Crystal Modulational Instability Roll Diameter Convective Roll Convective Structure
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