Dynamics in supercooled liquids and in the isotropic phase of liquid crystals: A comparison

A comparison is made of the dynamics observed over wide ranges of time and temperature between five supercooled liquids and four isotropic phase liquid crystals that have been previously studied separately. Optical-heterodyne-detected optical Kerr effect (OHD-OKE) measurements were employed to obtain the orientational relaxation dynamics over time scales from sub-ps to tens of ns. For the supercooled liquids, the temperatures range from above the melting point down to similar toT(c), the mode coupling theory critical temperature. For the liquid crystals, the temperatures range from well above the isotropic-to-nematic phase transition temperature T-NI down to similar toT(NI). For time scales longer than those dominated by intramolecular vibrational dynamics (greater than or similar to1 ps), the fundamental details of the dynamics are identical. All nine liquids exhibit decays of the OHD-OKE signal that begin (>1 ps) with a temperature-independent power law t(-z), where z is somewhat less than or equal to 1. The power law decay is followed in both the supercooled liquids and liquid crystals by a crossover region, modeled as a second power law. The longest time scale decay for all nine liquids is exponential. In supercooled liquids, the exponential decay is the alpha relaxation (complete structural relaxation). In liquid crystals, the exponential decay is the Landau-de Gennes decay (relaxation of pseudonematic domains). As T-c (supercooled liquids) and T-NI (liquid crystals) are approached from above, the time range over which the "intermediate" power law can be observed increases, until near T-c and T-NI, the power law can be observed from >1 ps to many ns. The data for all nine liquids are described accurately by the same functional form and exhibit a scaling relation in common. The nature of the dynamics in the liquid crystals is understood in terms of pseudonematic domains that have a correlation length xi, which increases as T-NI is approached. It is conjectured that the similarities between the liquid crystal data and supercooled liquid data are produced by the same underlying physical features: that is, like liquid crystals, supercooled liquid dynamics is a result of structural domains even at relatively high temperature. (C) 2003 American Institute of Physics.