Diffusing wave spectroscopy.
Item
-
Title
-
Diffusing wave spectroscopy.
-
Identifier
-
AAI9224870
-
identifier
-
9224870
-
Creator
-
Zhu, Jixiang.
-
Contributor
-
Adviser: David A. Weitz
-
Date
-
1992
-
Language
-
English
-
Publisher
-
City University of New York.
-
Subject
-
Physics, Optics
-
Abstract
-
We present the development of Diffusing-Wave Spectroscopy (DWS) and the applications of DWS to study concentrated colloidal suspensions. DWS is an extension of the traditional photon correlation spectroscopy to the multiple scattering limit. It is a unique tool for studying the dynamics and the structures of optically opaque materials and thus it opens up for study a new class of systems. An important feature is that DWS probes the motion of scatterers at very short length scale.;To utilize DWS, one must account for the effects of internal reflection which results from the index of refraction mismatch at the boundaries between the scattering medium and its surroundings. We develop a model which accounts for the internal reflection by amending the boundary conditions for the diffusion equation for the propagating light. We show that static angular correlation functions can be used to measure the effect of internal reflection. After doing a series of experiments, we find that the theory is in excellent agreement with the experiments.;As an important application of DWS we study the effect of hydrodynamic interactions on the short time non-diffusive motion of Brownian particles suspended in a fluid. We measure the mean square displacement, {dollar}\langle\Delta r\sp2(\tau)\rangle{dollar}, for concentrated suspensions at times short to observe the retarded nature of hydrodynamic interactions. A power-law decay of the velocity autocorrelation function, {dollar}R(\tau){dollar} {dollar}\sim{dollar} {dollar}\tau\sp{lcub}3/2{rcub}{dollar}, is observed for all concentrations. A remarkable scaling behavior of the time dependent diffusion coefficient, {dollar}D\sb{lcub}s{rcub}(\tau)=\langle\Delta r\sp2(\tau)\rangle/6\tau{dollar}, with particle volume fraction is observed: if {dollar}D\sb{lcub}s{rcub}(\tau){dollar} is scaled by its asymptotic value and if time is scale by a viscous time constant inversely proportional to the shear viscosity of the suspension, all the data fall onto a single universal curve.;Using DWS in backscattering, we find that light backscattered from an optically dense random medium exhibits a pronounced polarization dependence. An unexpected memory of the incident circular polarization of multiply scattered light arises because the wave's helicity is randomized less rapidly than is its direction. We develop a simple model to account for the observed polarization dependence of the intensity and temporal correlation of the intensity fluctuations of backscattered light.
-
Type
-
dissertation
-
Source
-
PQT Legacy CUNY.xlsx
-
degree
-
Ph.D.
