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Abstract
The sample is a nematic liquid crystal (PAA) in a Hele-Shaw cell. This cell is submitted to a vertical temperature difference ΔT and a magnetic field H. The temperature difference destabilizes the system, producing the well-known Rayleigh-Bénard rolls. The magnetic field stabilizes the system. The combination of both produces a phase diagram. T. Riste et al. showed that, for moderate temperature difference and fields, periodic fluctuations of the neutron scattered intensity appear. Moreover the squared frequencies are linear versus ΔT over adjacent ΔT intervals (frequency quantification). The aim of this experiment was to determine the origin of these fluctuations by simultaneous neutron scattering time series measurements and optic observation thanks to a transparent cell.
In our first experiment with a cell similar to Riste's we have been able to reproduce the first part of his observations; our conclusion is that the fluctuations do not arise from the rolls themselves but from oscillators which periodically perturb the rolls structure.