Thesis (Ph.D.)--Brown University, 2005. / Vita. Thesis advisor: Humphrey Maris. Includes bibliographical references (leaves 88-92). Also available online.

Intermolecular interactions are anisotropic since they depend on the orientations of the interacting molecules. They are responsible for the orientational phase transitions observed in molecular systems. The effects of nonseparable interactions, which depend on both the molecular and intermolecular vector orientations, are investigated using Monte-Carlo simulations. It is found that for systems with quenched positional disorder, the nonseparability of the pair potential leads to orientational frustration. Expanding the pair potential in terms of its rotational invariants allows one to isolate random field terms whose presence are shown to be at the origin of the non-frustrated patches observed in such disordered systems. For liquid crystals, where translational and rotational degrees of freedom are in thermal equilibrium, Monte-Carlo results show that an ordering of the molecular orientations induces an ordering of the nearest-neighbor bond orientations only when the pair potential is nonseparable. These results suggest that it is necessary to reinterpret the mean-field theory of nematic liquid crystals as an approximation which neglects the presence of bond ordering in these systems. As well, they unravel the microscopic origin of the phenomeno-logical coupling between bond and molecular orientations invoked in Ginzburg-Landau theories of liquid crystals. The nonseparability of the intermolecular pair potential leads to a coupling between the nearest-neighbor bond orientational field and the molecular orientational field. It is shown that such coupling introduces new and interesting effects in liquid crystalline systems where interactions between the molecular and nearest-neighbor bond orientations are usually ignored. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Critical velocities have been measured in liquid helium at 1.40 K for counterflow, superflow and counterflow superimposed on superflow. The measurements were made in five pipes having rectangular cross-sections and ranging in size nominally from 0.2 x 1.2 x 10 cm to 1.2 x .1.2 x 10 cm. The turbulence was detected by means of negative ion currents perpendicular to the flow. The results are interpreted in terms of creation of quantized vortex lines. For superflow, the critical velocities were slightly higher but within the experimental range of those measured for counterflow. For the case of superflow-plus-counterflow a second critical point was observed, in addition to the one corresponding to superflow only. Considerable data was also collected regarding the nature of the ionic current attenuation effects for supercritical flow. The delay times between when the flow rate was raised to or above the critical velocity and when the current attenuations were first observed ranged from more than five minutes for the smallest pipe to three-quarters of a minute or less for the largest one. When the pipe width is greater than the ion source width the attenuations may be explained by Vinen's theory of vortex generation and decay but for smaller pipe widths the attenuation buildup and decay times are greatly increased. A method for increasing the sensitivity of the detection method for large pipes is also outlined. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Using EPR, the temperature dependence of the effective order parameter of the molecule vanadyl acetylacetonate in the nematic liquid crystal 4-methoxy benzylidene-n-butyl aniline was studied. In the resulting data, a discontinuity in slope of the curve of effective order parameter versus temperature was found, indicating a change to a more restricted type of motion at lower temperatures. Similar results were obtained from NMR T₁ measurements on the pure liquid crystal. The discontinuity cannot be interpreted solely as a viscosity effect. Two cholesteric liquid crystal mixtures containing vanadyl acetylacetonate were subjected to a high magnetic field (~20 kilogauss). Prom EPR measurements, a 1.75:1 mixture by weight of cholesteryl chloride and cholesteryl myristate was found to orient its helix axis parallel to the field direction. In a.2 percent by weight mixture of cholesteryl chloride in nematic 4-methoxy benzylidene-n-butyl aniline, the molecules tended to arrange their long axes parallel to the field thus making the helix axis perpendicular to the field. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The bend and splay elastic constants of the nematic liquid crystal octylcyanobiphenyl (8CB) are measured as a function of temperature using an electric-field-induced deformation. The capacitance and birefringence of a sample cell, temperature controlled to 0.1mK, were simultaneously measured as a function of applied voltage. The splay constant is determined from the critical voltage at the onset of the deformation (the Freedericksz transition). The bend constant is found by fitting the data above the critical voltage to the theory of Deuling, which is derived in full. The data is also analysed in the high- and low-field limits of the theory. The bend elastic constant displays a pretransitional divergence near the nematic-smectic A phase transition due to smectic fluctuation effects. The divergent part is fit by a power law with a critical exponent of 1.0±0.1. The Deuling theory, which is based on the assumption of linear elasticity, shows systematic deviation from the data at high voltages at all temperatures, with the disagreement increasing rapidly as the smectic phase is approached. This result suggests that the linear elastic theory of Dueling/ fails for large deformations and where smectic fluctuations contribute signifigantly to the bend elasticity. The failure near the smectic transition may be explainable by the quenching of smectic fluctuations by the deformation. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The surface anchoring of the nematic liquid crystal 5CB aligned tangentially on two different glass substrate treatments is studied. These two treatments are obliquely evaporated SiO and buffed poly(vinyl formal) deposited from solution. The latter is believed to be a new treatment and of possible importance to the twisted nematic liquid crystal display industry. Two methods are employed to measure the strength of the surface anchoring potential. The first was originally developed by Yokoyama and van Sprang, and yields the coefficient of the Rapini-Papoular anchoring potential. The second is a new method based on the Frank-Oseen elasticity theory. It is an extension of Deuling's 1972 solution of the splay Freedericksz transition. The second yields the functional form of the anchoring potential. Both methods are based on measurements of the bulk dielectric constant and the effective refractive index of a nematic sample that has undergone a splay Freedericksz transition. The first method requires both these measurements, and the second requires one or the other; both can be employed as a verification of results. The latter method is found to be very sensitive to the value of the dielectric anisotropy; this is believed to be due to not allowing for spatial variations in the scalar order parameter. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

New liquid crystals have been synthesized and their transition phases have been studied. It was found that a family of (-)-2-methyl 2R,3S-epithio-4'-alkoxybiphenyl pentanoates are liquid crystals but a similar system of ( + )-2-methyl 2S,3R-epoxy-4'-alkoxybiphenyl pentanoates are not. Of the liquid crystals, MEAOBP-S-7 exhibits smectic A and B phases; MEAOBP-S-8 and MEAOBP-S-9 show only the smectic B phases; MEAOBP-S-10 shows a monotropic chiral smectic C phase. This distinction is in agreement with the argument that the internal dipolar interaction of the molecule plays an important role in determining liquid crystalline behaviour of this ester system. The reaction rate of the racemization of optically active binaphthyl can be moderately increased in the presence of finely divided titanium dioxide (anatase) powder. This first order catalyzed reaction has a proportional relationship with the concentrations of both the catalyst and the binaphthyl. The catalyzed reaction can be poisoned permanently by the addition of polynuclear aromatic compounds and the poisoning efficiency is dependent of the number of the aromatic rings the poison posesses. By comparison with the catalyzed racemizations of binaphthyl by platinum, nickel and carbon and from the kinetic and mechanistic studies on the titanium dioxide catalyzed racemization, we conclude that (1) this catalytic phenomenon is common among heterogeneous inorganic solids and can be extended to other simple reactions, and that (2) the mechanism of the catalysis probably involves a radical anion intermediate. / Science, Faculty of / Chemistry, Department of / Graduate

Interferometric measurements to determine the refractive indices of the nematic liquid crystals EBBA and BEPC as a function of temperature are described utilizing modified Rayleigh and conoscopic interferometers. Theory is presented relating the refractive indices and density to the orientational order, local field parameter and molecular properties. The results of simple thermal expansivity measurements are also given for EBBA. The Lorentz-Lorenz coefficient for SF₆, and GeH₄, has been determined from refractive index and density measurements. The method utilizes a prism shaped high pressure cell which can be removed from a temperature controlled holder and weighed on a precision balance. The results indicate a variation of 0.5% for SF₆, and 0.8% for GeH₄, over the density range covered. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The tensile strength or the negative pressure required to induce cavitation in a pure liquid has been a puzzling subject. On one hand, the classical nucleation theory has met great success in predicting the nucleation rates of superheated liquids. On the other hand, most of reported experimental values of the tensile strength for different liquids are far below the prediction from the classical nucleation theory. In this study, homogeneous nucleation in liquid nitrogen and its tensile strength have been investigated. In order to carry out the measurement of the tensile strength of liquid nitrogen, different approaches for determining the pressure amplitude were studied carefully. It is shown that Raman-Nath theory, as modified by the introduction of an effective interaction length, can be used to determine the pressure amplitude in the focal plane of a focusing ultrasonic transducer. The results obtained from different diffraction orders are consistent and in good agreement with other approaches including Debye's theory and solving the KZK (Khokhlov-Zabolotskaya-Kuznetsov) equation. The results from experiments in water demonstrated that as long as the nonlinearity is not too large, the experimentally determined pressure follows closely the calculated results using either Debye's theory or the KZK equation. In addition, the light diffraction contains enough information to calculate the second-order harmonic in the sound wave. In principle, it is possible that the contribution to the acoustic wave of the higher than the second-order harmonic can be obtained. The measurement of the tensile strength was carried out in a high pressure stainless steel dewar. A High intensity ultrasonic wave was focused into a small volume of liquid nitrogen in a short time period. A probe laser beam passes through the focal region of a concave spherical transducer with small aperture angle and the transmitted light is detected with a photodiode. When the voltage on the transducer reaches a critical point, nucleation in the focal region occurs and a characteristic signal associated with the nucleation was obtained. At this moment, the pressure amplitude at the focus is calculated based on the acoustic power radiated into the liquid. In the experiment, the electrical signal on the transducer is gated at its resonance frequency with gate widths of 20 ~s to 0.2 ms and temperature range from 77 K to near 100 K. The calculated pressure amplitude is in agreement with the prediction of classical nucleation theory for the nucleation rates from 106 to lOll (bubbles/cm3 sec). This work enhances our understanding of the nucleation process in liquids. It provides the direct experimental support that the validity of the classical nucleation theory can be extended to the region of the negative pressure up to 90 atm. This is only the second cryogenic liquid to reach the tensile strength predicted from the classical nucleation theory.

It is well known that most liquids exhibit a tensile strength which is much smaller in magnitude than the tensile strength predicted by homogeneous nucleation theory. This lack of agreement is usually attributed to the difficulty of preparing liquid samples free from foreign gases which act as heterogeneous nucleation sites. Liquid helium occupies a unique place among liquids for tensile strength measurements because all foreign gases are frozen out at liquid helium temperatures. Furthermore, superfluid 4He should fill all crevices on solid surfaces, eliminating the chance of heterogeneous nucleation on helium vapor pockets. Despite the quantum mechanical nature of liquid helium, Becker-Doring theory of nucleation of the vapor phase from the liquid phase should be valid down to 0.3 K in 4 He, yet previous results have been in stark disagreement with the theory. In this study, a piezoelectric transducer in the form of a hemispherical shell was used to focus high-intensity ultrasound into a small volume of 4He . The transducer was gated at its resonant frequency of 566 kHz with gate widths of less than 1 msec in order to minimize the effects of transducer heating and acoustic streaming. The onset of nucleation was detected from the absorption of acoustic energy and the scattering of laser light from microscopic bubbles. A new theory for the diffraction of light from the focal zone of a spherical converging sound wave was developed to confirm calculations of the acoustic pressure amplitude at the focus of the piezoelectric transducer, calculations which were based on the acoustic power radiated into the liquid and the nonlinear absorption of sound. The experimental results were in agreement with homogeneous nucleation theory for a nucleation rate of approximately 1015 critical size bubbles/sec-cm3. This is only the third liquid for which the theoretical tensile strength has been reached and it confirms homogeneous nucleation theory over a range three times greater than any other experiment. A noticeable decrease in the magnitude of the tensile strength was noted at temperatures near the lambda transition and a hypothesis that bubbles are being nucleated heterogeneously quantized vortices is presented.