Molecular simulations performed on modern computers provide a powerful tool for the investigation of both static and dynamic properties of liquid crystals. In this thesis several properties of liquid crystal mesogens have been investigated using state-of-the-art Monte Carlo (MC) and molecular dynamics (MD) simulation techniques. The helical twisting power, βm, determines the pitch of the chiral nematic phase produced when a nematic liquid crystal is doped with a low concentration of chiral solute molecules. A new simulation technique that allows the prediction of both the sign and the magnitude of βm is described. The method employs fully atomistic MC simulations of a chiral dopant molecule in the presence of a twisted nematic solvent composed of Gay-Berne particles. Eighteen different chiral dopant molecules were examined and in all cases the results were in good agreement with existing experimental data. The Kirkwood correlation factor, g(_1), has been evaluated for the molecules PCH5, PCH5-C1, me5NF and GGP5C1 using MD simulations in the pre-transitional region of the isotropic phase. The calculations employed an all-atom force field, which was developed specifically for liquid crystal molecules. PCH5 and meSNF were seen to favour anti-parallel dipole alignment whereas, PCH5-C1 and GGP5C1 preferred a parallel arrangement of the molecular dipoles. With the exception of GGP5C1, the simulations gave g(_1) values that were in accordance with existing experimental dielectric measurements. Detailed analysis of the MD trajectories showed that certain molecular pair configurations were preferred in the bulk and indicated which molecular groups were responsible for the stabilization of these configurations. Equilibrium molecular dynamics simulations were carried out in order to evaluate the rotational viscosity coefficient, γ(_1), for a Gay-Berne mesogen using two independent analysis techniques. The methods gave consistent results, which were comparable to experimental data for real mesogens of similar shape and size.

Micron scale robots going inside our body and curing various ailments is a technolog¬ical dream that easily captures our imagination. However, with the advent of novel nanofabrication and nanocharacterization tools there has been a surge in the research in this field over the last decade. In order to achieve locomotion (swim) at these small length scales, special strategies need to be adopted, that is able to overcome the large viscous damping that these microbots have to face while moving in the various bod¬ily fluids. Thus researchers have looked into the swimming strategies found in nature like that of bacteria like E.coli found in our gut or spermatozoa in the reproductive mucus. Biomimetic swimmers that replicate the motion of these small microorganisms hold tremendous promise in a host of biomedical applications like targeted drug delivery, microsurgery, biochemical sensing and disease diagnosis. In one such method of swimming at very low Reynolds numbers, a micron scale helix has been fabricated and rendered magnetic by putting a magnetic material on it. Small rotating magnetic fields could be used then to rotate the helix, which translated as a result of the intrinsic translation rotation coupling in a helix. The present work focussed on the development of such a system of nanopropellers, a few microns in length, the characterization of its dynamics and velocity fluctuations originating from thermal noise. The work has also showed a possible application of the nanopropellers in microrheology where it could be used as a new tool to measure the rheological characteristics of a complex heterogeneous environment with very high spatial and temporal resolutions. A generalized study of the dynamics of these propellers under a rotating field, has showed the existence of a variety of different dynamical configurations. Rigid body dynamics simulations have been carried out to understand the behaviour. Significant amount of insight has been gained by solving the equations of motion of the object analytically and it has helped to obtain a complete understanding, along with providing closed form expressions of the various characteristics frequencies and parameters that has defined the motion. A study of the velocity fluctuations of these chiral nanopropellers has been carried out, where the nearby wall of the microfluidic cell was found to have a dominant effect on the fluctuations. The wall has been found to enhance the average level of fluctuations apart from bringing in significant non Gaussian effects. The experimentally obtained fluctuations has been corroborated by a simulation in which a time evolution study of the governing 3D Langevin dynamics equations has been done. A closer look at the various sources of velocity fluctuations and a causality study thereof has brought out a minimum length scale below which helical propulsion has become impractical to achieve because of the increased effect of the orientational fluctuations of the propeller at those small length scales. An interesting bistable dynamics of the propeller has been observed under certain experimental conditions, in which the propeller randomly switched between the different dynamical states. This defied common sense because of the inherent deterministic nature of the governing Stokes equation. Rigid body dynamics simulations and stability analysis has shown the existence of time scales in which two different dynamical states of the propeller have become stable. Thus the intrinsic dynamics of the system has been found to be the reason behind the bistable behaviour, randomness being brought about by the thermal fluctuations present in the system. Finally, in a novel application of the propellers, they have been demonstrated as a tool for microrheological mapping in a complex fluidic environment. The studies done in this work have helped to develop this method of active microrheology in which the measurement times are orders of magnitude smaller than its existing counterparts.

The genetic algorithm (GA) is used to design helical antennas that provide a significantly larger bandwidth than conventional helices with the same size. Over the bandwidth of operation, the GA-optimized helix offers considerably smaller axial-ratio and slightly higher gain than the conventional helix. Also, the input resistance remains relatively constant over the bandwidth. On the other hand, for nearly the same bandwidth and gain, the GA-optimized helix offers a size reduction of 2:1 relative to the conventional helix. The optimization is achieved by allowing the genetic algorithm to control a polynomial that defines the envelope around which the helix is wrapped. The fitness level is defined as a combination of gain, bandwidth and axial ratio as determined by an analysis of the helix using NEC2. To experimentally verify the optimization results, a prototype 12-turn, two-wavelength high, GA-helix is built and tested on the Virginia Tech outdoor antenna range. Far-field radiation patterns are measured over a wide frequency range. The axial-ratio information is extracted from the measured pattern data. Comparison of measured and NEC-2 computed radiation patterns shows excellent agreement. The agreement between the measured and calculated axial-ratios is reasonable. The prototype GA-helix provides a peak gain of more than 13 dB and an upper-to-lower frequency ratio of 1.89. The 3-dB bandwidth of the antenna is 1.27 GHz (1.435 GHz - 2.705 GHz). Over this bandwidth the computed gain varies less than 3 dB and the axial-ratio remains below 3 dB. / Master of Science

A new variation of the spherical helical antenna made of a wire wound over a hemispherical surface and backed by a conducting ground plane is introduced. A constant spacing is maintained between the turns of winding. The geometry of this antenna is fully described by the number of turns and the radius of hemispherical surface. In addition to the hemispherical geometry, truncated double spherical helices are also examined. Radiation properties of the proposed antennas are studied both theoretically and experimentally. The wire antenna code ESP (electromagnetic surface patch), which is based on the method of moments, is used to obtain simulation results. The results for far-field patterns, gain, axial ratio, bandwidth, and input impedance are presented. Several prototypes of this antenna were constructed and tested using an outdoor antenna range. Far-field patterns were measured over a wide range of frequencies. The measured and calculated radiation patterns are in good agreement. A unique property of the hemispherical helix is its broad half-power beamwidth. Furthermore, this antenna provides circular polarization and relatively high gain over a narrow frequency range. The results of this research show that, for example, a 4.5-turn hemispherical helix with a radius of 0.02 meter designed for operation around 2.8 GHz provides a half-power beamwidth of about 90 degrees, more than 9 dB gain, and less than 3 dB axial ratio over a 300 MHz bandwidth. The input impedance of the antenna is largely resistive and is about 150 ohms in the above frequency range. Compared with a full spherical helix, the hemispherical helix provides comparable radiation characteristics, but occupies only half the volume. The compact size of this antenna makes it attractive to mobile communication applications / Master of Science

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In the current literature, split path gear trains are analyzed for use in helicopter transmissions and marine gearboxes. The goal in these systems is to equalize the torque in each path as much as possible. There are other gear trains where the operator intends to hold the torque split unevenly. This allows for control over the gearbox bearing loading which in turn has a direct effect on bearing stiffness and damping characteristics. Having control over these characteristics is a benefit to a designer or operator concerned with suppressing machine vibration. This thesis presents an analytical method for analyzing the torque in split path gear trains. A computer program was developed that computes the bearing loads in various gearbox arrangements using the torque information gathered by the analytical method. A case study is presented that demonstrates the significance of the analytical method in troubleshooting an industrial gearbox that has excessive vibration. / Master of Science

M.S.

A unique property of spherical and hemispherical helical antennas is that they provide very broad half-power beamwdiths and circular polarization over a narrow bandwidth. In this thesis, new designs for hemispherical helical antennas are introduced that provide significant improvement in bandwidth, while maintaining the directivity and half-power beamwidth of the basic design. In the basic design, a simple wire of circular cross section is wound on the surface of a hemisphere, whereas in the proposed new designs a metallic strip forms the radiating element. Furthermore, the metallic strip may be tapered and tilted relative to the hemispherical surface, allowing wider bandwidth to be achieved. The antenna is fed by a coaxial cable with the inner conductor connected, through a matching section, to the radiating strip and its outer conductor connected to a ground plane. Radiation properties of the proposed hemispherical helical antennas are studied both theoretically and experimentally. A commercial software, based on the method of moments, is used to perform the numerical analysis of these helices. Three-dimensional far-field patterns, axial ratio, directivity, and voltage standing-wave ratio (VSWR) are calculated for several designs. The impacts of tapering as well as tilting of the metallic strip on radiation characteristics are examined. Also, matching of the proposed hemispherical antennas to 50â ¦ transmission lines is addressed. A 4.5-turn hemispherical helix with tapered radiating element and zero degree tilt angle, (metallic strip is perpendicular to the hemisphere axis of symmetry) provides the largest overall bandwidth. A nonlinearly tapered matching section is incorporated into the design in order to reduce the VSWR. For this design, an overall measured bandwidth of about 24% at a center frequency of 3.35 GHz is achieved. Over this bandwidth, the axial ratio remains below 3 dB, the VSWR is less than 2, and the directivity is about 9 ±1 dB. A half- power beamwidth of 70° is also obtained. A prototype of the best design was fabricated and tested using the VT indoor antenna range. Radiation patterns, the scattering parameter S₁₁, and the axial ratio were measured. The measured and simulated results agree reasonably well. In particular, agreements between measured and calculated far-field patterns and VSWR are quite remarkable. This compact, low profile antenna might find useful applications in avionics, global positioning systems (GPS), and high data rate wireless communication systems. / Master of Science