Abstract:
Using a continuum description, we account for fluctuations in the ionic solvent surrounding a Gaussian, charged chain and derive an effective short-ranged potential between the charges on the chain. This potential is repulsive at short separations and attractive at longer distances. The chemical potential can be derived from this potential. When the chemical potential is positive, it leads to a melt-like state. For a vanishingly low concentration of segments, this state exhibits scaling behavior for long chains. The Flory exponent characterizing the radius of gyration for long chains is calculated to be approximately 0.63, close to the classical value obtained for second order phase transitions. For short chains, the radius of gyration varies linearly with $N$, the chain length, and is sensitive to the parameters in the interaction potential. The linear dependence on the chain length $N$ indicates a stiff behavior. The chemical potential associated with this interaction changes sign, when the screening length in the ionic solvent exceeds a critical value. This leads to condensation when the chemical potential is negative. In this state, it is shown using the mean-field approximation that spherical and toroidal condensed shapes can be obtained. The thickness of the toroidal polyelectrolyte is studied as a function of the parameters of the model, such as the ionic screening length. The predictions of this theory should be amenable to experimental verification.

Abstract:
A non-equilibrium plasma was studied using classical electrodynamic field theory. Non-linear interaction terms contribute to a finite lifetime for the dressed electrodynamic field. The lifetime exhibits a $\sim n^{-1} T_{e}^{3/2} T_{i}^{-2}T_{r}^{1/2}$ dependence, where $n$ is the number density, $T_{e}$ is the electron temperature, $T_{i}$ is the ion temperature, and $T_{r}$ is the temperature of the radiation field. The resulting width of the plasmon resonance is shown to decrease as equilibrium is approached. Dynamic screening leads to opaqueness of the plasma for low energy electromagnetic radiation. This leads to a quadratic correction to the quartic Stefan-Boltzmann law. We also briefly discuss the effect of dynamic screening on fusion rates. Solitonic solutions to our non-linear wave equation allow localization of positive charges, which may enhance fusion rates.

Abstract:
We develop a series expansion of the plasma screening length away from the classical limit in powers of $\hbar^{2}$. It is shown that the leading order quantum correction increases the screening length in solar conditions by approximately 2% while it decreases the fusion rate by approximately $ 0.34%$. We also calculate the next higher order quantum correction which turns out to be approximately 0.05%.

Abstract:
We have formulated a theory of self-assembly based on the notion of local gauge invariance at the mesoscale. Local gauge invariance at the mesoscale generates the required long- range entropic forces responsible for self-assembly in binary systems. Our theory was applied to study the onset of mesostructure formation above a critical temperature in estane, a diblock copolymer. We used diagrammatic methods to transcend the Gaussian approximation and obtain a correlation length zeta ~ (c-c*)^-gamma, where c* is the minimum concentration below which self-assembly is impossible, c is the current concentration, and gamma was found numerically to be close to 2/3. The renormalized diffusion constant vanishes as the c* is approached, indicating the occurrence of critical slowing down, while the correlation function remains finite at the transition point.

Abstract:
We have mapped the physics of a system of semi-flexible inextensible polymers onto a complex Ginzburg-Landau field theory using techniques of functional integration. It is shown in the limit of low number density of monomers in a melt of semi-flexible, inextensible polymers, kept apart by an excluded volume interaction, the radius of gyration scales as N^{nu}, where N is the chain length, and nu = 1, in contrast to the value of nu ~ 0.6 for flexible polymer melts. Using Renormalization Group arguments, we show that the system exhibits an infra-red stable fixed point which can be identified as the transition to the entangled state. Experiments to test these calculations are suggested.

Abstract:
We have mapped the physics of polymer melts onto a time-dependent Landau-Ginzburg $|\psi|^4$ field theory using techniques of functional integration. Time in the theory is simply a label for the location of a given monomer along the extent of a flexible chain. With this model, one can show that the limit of infinitesimal concentration of a polymer melt corresponds to a {\em dynamic} critical phenomenon. The transition to the entangled state is also shown to be a critical point. For larger concentrations, when the role of fluctuations is reduced, a mean field approximation is justifiably employed to show the existence of tube-like structures reminiscent of Edwards' model.

Abstract:
We consider the case when a supercritical fluid emerges at sonic speed from a small orifice in a high pressure chamber. The subsequent expansion causes a pressure drop and the fluid then enters a regime where its equation of state in $P-V$ space becomes concave towards the origin. This is the signal for an expansion shock to occur in a non-ideal fluid. This paper provides the details of an analytic calculation of the shape and location of this expansion shock using Whitham's front-tracking method. Dependence of the shape of the front on various operating conditions was calculated for the particular case of supercritical carbon dioxide. The results shed light on the rapid expansion of supercritical solutions (RESS), a process which is used in many manufacturing technologies.

Abstract:
We have developed a theory of polymer entanglement using an extended Cahn-Hilliard functional, with two extra terms. One is a nonlocal attractive term, operating over mesoscales, which is interpreted as giving rise to entanglement, and the other a local repulsive term indicative of excluded volume interactions. This functional can be derived using notions from gauge theory. We go beyond the Gaussian approximation, to the one-loop level, to show that the system exhibits a crossover to a state of entanglement as the average chain length between points of entanglement decreases. This crossover is marked by critical slowing down, as the effective diffusion constant goes to zero. We have also computed the tensile modulus of the system, and we find a corresponding crossover to a regime of high modulus. The single parameter in our theory is obtained by fitting to available experimental data on polystyrene melts of various chain lengths. Extrapolation of this fit yields a model for the cross-over to entanglement. The need for additional experiments detailing the cross-over to the entangled state is pointed out.

Abstract:
We have developed a theory of polymer entanglement using an extended Cahn-Hilliard functional, with two extra terms. One is a nonlocal attractive term, operating over mesoscales, which is interpreted as giving rise to entanglement, and the other a local repulsive term indicative of excluded volume interactions. We show how such a functional can be derived using notions from gauge theory. We go beyond the Gaussian approximation, to the one-loop level, to show that the system exhibits a crossover to a state of entanglement as the average chain length between points of entanglement decreases. This crossover is marked by critical slowing down, as the effective diffusion constant goes to zero. We have also computed the tensile modulus of the system, and we find a corresponding crossover to a regime of high modulus.