Date

January 2011

Document Type

Dissertation

Degree Name

Ph.D.

Institution

Oregon Health & Science University

Abstract

The aim of dynamic light scattering (DLS) is to characterize dynamic processes through the measurement of correlations in temporally varying scattered light. This dissertation deals with the development and application of DLS techniques for monitoring processes that undergo rapid changes in dynamic behavior. Theoretical and technical concepts are examined through the experimental investigation of dental composite polymerization, which is of clinical importance in the field of restorative dentistry. This reaction exhibits changes in dynamic behavior that occur faster than can be resolved with established DLS techniques. A basic overview of DLS is presented through the theory and application of quasi-elastic light scattering (QLS). Due to the stochastic nature of the scattered light signal, thousands of intensity fluctuations must be averaged in order to obtain a statistically reliable measure of the temporal autocorrelation function (ACF). QLS relies on single-detector measurement of dynamically scattered light, and the necessary temporal averaging is time consuming, taking tens of seconds to minutes for a single measurement. Accordingly, QLS is not well suited for studying reactions that vary on the order of seconds or faster. The remainder of this work deals with the development and application of dynamic-speckle-based DLS methods that utilize CCD camera detection. Multi-pixel detection allows for ensemble (spatial) averaging, which enables these methods to achieve faster measurements of dynamic behavior compared to QLS. A sequential speckle correlation (SSC) method was developed and implemented that uses the correlation coefficient between pairs of dynamic speckle patterns to describe reaction dynamics. The temporal resolution of this method compared to QLS is improved by a factor roughly equal to the number of independent speckles included in the region of interest (ROI). For the study of dental composite polymerization, a 64 x 64 pixel ROI was used with a minimum speckle dimension of two pixels, achieving a ~1000 fold increase in temporal resolution (~50 ms). Another multi-pixel DLS method, laser speckle contrast analysis (LSCA), was also implemented. With LSCA, temporal averaging of intensity fluctuations causes a reduction in contrast of the speckle pattern which can be used to characterize the motion of the scattering medium. This method is able to measure spatial variations in reaction dynamics as well as the temporal behavior. LSCA has previously been used to study blood flow velocity, but this work marks its first successful application for studying dental composite polymerization. The reaction kinetics of the photo-activated polymerization of dental resin composite were explored with these DLS methods, for a variety of sample dimensions and curing protocols. Results are presented for samples 0.1-17 mm thick and for curing irradiances between 20 - 320 mW/cm2. Reaction profiles obtained with SSC and LSCA are qualitatively similar to results in the literature obtained using other techniques. Results for polymerization rate as a function of curing irradiance for thin samples using LSCA demonstrate a square root dependence that agrees well with established polymerization theory, as well as with previous results in the literature. Our findings suggest that the multi-pixel DLS methods presented here can be advantageously applied to the study of dental composite polymerization, as well as to other highly scattering dynamic systems.

Identifier

doi:10.6083/M4K64G2C

Division

Div. of Biomedical Engineering

School

School of Medicine

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