We have developed a new modulation calorimetry technique using RF-Field heating. This technique eliminates temperature gradients across the sample leading to a higher precision in evaluating the heat capacity compared to the previous techniques. A frequency scan was carried out on a 8CB+aerosil sample showing a wide plateau indicating the region of frequency independent heat capacity. A temperature scan was then performed through the first-order nematic to isotropic and second order smectic-A to nematic transitions and was shown to be consistent with the previous work. The amplitude of the RF heating power applied to the sample depends on the permittivity and the loss factor of the sample. Since the permittivity of a dielectric material has a strong temperature dependence in liquid crystals, new information is obtained. The heat capacity measurements have a relative resolution of better than 0.06%, and the phase shift a resolution of 0.03%, were shown to be significant improvements over traditional heating methods. We then applied this new RF calorimetry on bulk and aerosil 8CB dispersions. For the bulk 8CB, the step-like character of smectic-A to nematic transition, and first order nematic to isotropic transitions indicated the strong dominance of the permittivity and the loss factor of the material. For the 8CB+aerosil samples at different silica density, our data were consistent with the previous work and provides clear evidence for the coupling between the smectic-A and nematic phases. We have undertaken a combined T-dependent optical and calorimetric investigation of CCN47+aerosil samples through the I-N transition over a range of silica densities displaying the double I-N transition peak. This work offers compelling evidence that the I-N transition with weak quenched random disorder proceeds via a two-step process in which random-dilution is followed by random-field interactions on cooling from the isotropic phase, a previously unrecognized phenomena.
Worcester Polytechnic Institute
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Barjami, S. (2005). A New AC-Radio Frequency Heating Calorimetry Technique for Complex Fluids. Retrieved from https://digitalcommons.wpi.edu/etd-dissertations/200
random-dilution, random-field interactions., Radio Frequency Field Heating, modulation calorimetry technique, heat capacity, octylcyanobiphenyl (8CB), liquid crystals, aerosil, nematic, isotropic, smectic A, phase transitions, permittivity, loss factor, dielectric, birefringence, 4' - transbutyl - 4 - cyano - 4 - heptyl - bicyclo, nematic correlation length, turbidity, nematic volume fraction, weak quenched random disorder, Calorimetry, Liquid crystal devices, Dielectrics