8 research outputs found
Mesoscopic theory of the viscoelasticity of polymers
We have advanced our previous static theory of polymer entanglement involving
an extended Cahn-Hilliard functional, to include time-dependent dynamics. We go
beyond the Gaussian approximation, to the one-loop level, to compute the
frequency dependent storage and loss moduli of the system. The three parameters
in our theory are obtained by fitting to available experimental data on
polystyrene melts of various chain lengths. This provides a physical
representation of the parameters in terms of the chain length of the system. We
discuss the importance of the various terms in our energy functional with
respect to their contribution to the viscoelastic response of the polymeric
system.Comment: Submitted to Phys. Rev.
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Smoke clearing by high energy laser beams
We describe the clearing phenomenon that occurs when a continuous wave (CW) high energy laser beam, incident upon a cloud of hygroscopic droplets, vaporizes these droplets. We consider the case when the incident wavelength is greater than the average droplet radius. Williams' model is used to describe the vaporization of a single droplet. The propagation of the laser beam is described by the radiative transfer equation in a slab geometry. The radiative transfer equation is solved using the method of successive orders of scattering
Onset of entanglement
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.Comment: 18 pages, with 4 figure
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A self-similar approach to the explosion of droplets by a high energy laser beam
A model has been constructed in which a small droplet is exploded by the absorption of energy from a high energy laser beam. The beam flux is so high that it is assumed that a plasma is formed. A single-fluid model of a plasma droplet interacting with laser radiation is used. Selfsimilarity is invoked to reduce the spherically symmetric problem involving hydrodynamics and Maxwell's equations to quadrature. It is shown analytically that the model reproduces in qualitative manner certain features observed experimentally
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Induced turbulence in aerosol-loaded atmospheres
This paper considers the effects of a pulse of radiation from a high-energy laser beam on the ambient turbulence that exists in the atmosphere. The atmosphere is considered as a compressible, perfect gas being heated by the high-energy laser pulse. We compute correlation functions of the temperature in the isobaric regime. The two-point correlation function is changed by a multiplicative factor that grows exponentially in time while the pulse is on Empirical formulas permit us to connect temperature fluctuations that we can compute to the refractive index fluctuations of the atmosphere. These self-induced refractive index fluctuations will be useful in studying the propagation characteristics of high energy laser beams through the atmosphere. 9 refs
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Explosive vaporization of small droplets
A model has been created of the explosive vaporization of small droplets by the absorption of energy from a high energy laser beam. The model consists of a polarizable drop of fluid interacting with laser radiation. A criterion for the explosion of the droplet has been introduced. Selfsimilarity is invoked to reduce the spherically symmetric problem involving hydrodynamics and Maxwell's equations to simple quadrature. Experimental evidence in favor of the model is cited
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Extension of the method of the small angle approximation: open detector
We use the radiative transfer equation to study the multiple scattering undergone by a laser beam propagating through a turbid medium. During the propagation, we view the beam as first scattering into a narrow forward cone, and then into a diffuse pattern. To describe this process, we propose a systematic and practical method to combine the small angle approximation with the diffusion approximation. The method works when the scattering cross-section describing scattering from aerosols can be written as the sum of a gaussian sigma/sub s/ to describe scattering into small angles, and a term sigma/sub d/, that can be represented by the first two terms of a Legendre expansion to describe scattering into large/diffuse angles. We use a Green's function formalism to perform partial resummations and set up a hierarchy of approximations in the form of coupled radiative transfer equations to describe the scattering of radiation from small angles into large angles. The adjoint operator formalism then provides a simple way to obtain the net flux received by an open detector at any given point. Our approximations may be described rigorously as a power series expansion in sigma/sup 0//sub d//sigma/sup 0//sub s/, the ratio of the diffusion scattering cross-section to the forward scattering cross-section. Thus our technique works well when small angle scattering dominates
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Scattered radiation field of a laser beam propagating through turbid media
A combination of the small-angle approximation and the diffusion approximation is used to study the propagation of laser beams through dense scattering media. 8 refs., 3 figs