Critical properties of the uniaxial–biaxial transition in micellar nematic phases

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Abstract
The critical properties of the second order uniaxial–biaxial nematic transition (NL–Nbx) of the potassium laurate/1‐decanol/D2O system were studied on surface and magnetic field aligned films of 0.1 and 0.5 mm thickness. The biaxial order was measured as a function of temperature and field. We find deviations from mean field behavior in a 20 mK range below the transition NL and in a temperature range of at least the same extent above the transition Nbx. The values obtained for the critical exponents of order parameter (β) and susceptibility (γ) in the uniaxial range are in good agreement with the values calculated for the xy model. A reliable value for the susceptibility exponent in the biaxial phase could not be obtained because the field range for which the effect is proportional to H2 is very small and the low field limit of the susceptibility could not be determined. From high field measurements we obtain a value of 4.0 as the lower limit for the exponent of the critical isotherm (δ). The result confirms that the transition follows the xy model. Drifts of the transition temperatures complicated the experiments. They are probably due to a partial esterification of potassium laurate.