Extended weakly nonlinear theory of planar nematic convection
| Autor: | Emmanuel Plaut, Werner Pesch |
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| Rok vydání: | 1999 |
| Předmět: | |
| Zdroj: | Physical Review E. 59:1747-1769 |
| ISSN: | 1095-3787 1063-651X |
| DOI: | 10.1103/physreve.59.1747 |
| Popis: | We study theoretically convection phenomena in a laterally extended planar nematic layer driven by an ac-electric field (electroconvection in the conduction regime) or by a thermal gradient (thermoconvection). We use an order-parameter approach and demonstrate that the sequence of bifurcations found experimentally or in the numerical computations can be recovered, provided a homogeneous twist mode of the director is considered as a new active mode. Thus we elucidate the bifurcation to the new ``abnormal rolls'' [E. Plaut et al., Phys. Rev. Lett. 79, 2367 (1997)]. The coupling between spatial modulations of the twist mode and the mean flow is shown to give an important mechanism for the long-wavelength zig-zag instability. The twist mode is also responsible for the widely observed bimodal instability of rolls. Finally, a Hopf bifurcation in the resulting bimodal structures is found, which consists of director oscillations coupled with a periodic switching between the two roll amplitudes. A systematic investigation of the microscopic mechanisms controlling all these bifurcations is presented. This establishes a close analogy between electroconvection and thermoconvection. Moreover, a ``director--wave-vector frustration'' is found to explain most of the bifurcations. |
| Databáze: | OpenAIRE |
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