Light-induced birefringence in the isotropic phase of a lyotropic liquid crystal

Abstract

In this paper, the light-induced order in the isotropic phase (I) of the KL-DeOH-water mixture is investigated. All mixtures show L1-I-L2 phase transitions being L1 and L2 two optically birefringent phases at low and high temperatures, respectively. The phase transitions were verified by polarized light microscopy technique. In the appropriate conditions of the relative concentration of the mixture and of temperature, an astonishing increase in the optical transmittance of the isotropic mixture can be observed in the presence of an external pulsed laser beam. The best induction effect was obtained with molar relative concentration C = [KL]/[DeOH] = 2.61 at about 2.0 °C from I-L2 phase transition. From each time-domain optical transmittance, the signal was analyzed in the low frequency domain (5 Hz < f < 50 Hz) by means of Discrete Fourier Transform (DFT). DFT analysis revealed two typical frequencies (ν1 and ν2) in the middle of the micellar isotropic phase with pronounced peaks in |F(ω)|. However, for mixtures with C = 2.61 (the lowest concentration), only one well pronounced peak at ν1 = 29.5Hz was found. For C = 2.62, the frequencies were ν1 = 29.5 Hz and ν2 = 36.5 Hz. And, for C = 2.63, ν1 = 36.5 Hz and ν2 = 40.0 Hz. It was verified that for all relative concentrations ν1 and ν2 have no dependence on temperature. These frequencies are being interpreted as an evidence of high correlation between the anisometric micelles (or correlation volumes) of the mixtures.