Rayleigh scattering of a new lyotropic nematic liquid crystal system: crossover of propagative and diffusive behaviour

In this paper we present to our knowledge the first light-scattering measur ements on a rather new lyotropic system potassium laurate, decylammonium ch loride, H2O. Relative concentrations were chosen in order to obtain the dis cotic ND phase over a large temperature range. Measurements involve digital recording of the autocorrelation function resulting from self-beating of t he depolarized scattered light. They were carried out at a fixed temperatur e in the ND phase range, by varying the scattering angle. A strong tendency to oscillations, superimposed on the usual relaxation signal, was found. T his behaviour is quite surprising, as special precautions had been taken to minimize any eventual propagative signal. Such precautions have been teste d previously, while investigating the older system potassium laurate, decan ol, D2O. Nevertheless, we have found that in the new lyotropic system, a pr opagative component may occur in a significant way, or even become dominant for some particular situations. Actually, the characterization of such sit uations is the main purpose of the present paper. Based upon a detailed ana lysis of photocorrelation data, it will be shown that the observed behaviou r is consistent with the expected wave vector dependences of propagative an d diffusive modes. Finally, a discussion is given on possible causes for th e tendency to instability of lyotropic nematic liquid crystals.