Thomas W. Taylor

Application of photon correlation spectroscopy to flowing Brownian motion systems

In this paper we study the application of photon correlation spectroscopy to a system of randomly diffusing particles suspended in a fluid undergoing uniform translational motion relative to the optical scattering volume. To do so we derive theoretical expressions for both the homodyne and heterodyne correlation functions in both the dilute and nondilute particle limits. We then test these results with experiments on a flowing system and find good agreement. We discuss a useful method of analysis and define limits to particle sizing in such a system using this light-scattering technique.

Multiple-scattering suppression by cross correlation

We describe a new method for characterizing particles in turbid media by cross correlating the scattered intensity fluctuations at two nearby points in the far field. The cross-correlation function selectively emphasizes single scattering over multiple scattering. The usual dynamic light-scattering capability of inferring particle size from decay rate is thus extended to samples that are so turbid as to be visually opaque. The method relies on single-scattering speckle being physically larger than multiple-scattering speckle. With a suitable optical geometry to select nearby points in the far field or equivalently slightly different scattering wave vectors (of the same magnitude), the multiple-scattering contribution to the cross-correlation function may be reduced and in some cases rendered insignificant. Experimental results demonstrating the feasibility of this approach are presented.

Soot particle size distribution measurements in a premixed flame using photon correlation spectroscopy

We have measured the first two moments of the soot particle size distribution in premixed methane/oxygen flames using the dynamic light scattering technique, photon correlation spectroscopy. Measurements were performed in a flat flame burner as a function of fuel-oxidizer equivalence ratio and height above burner. We demonstrated the ability of photon correlation spectroscopy to extract two moments, the most probable radius r0, and the geometric width σ of an assumed zeroth-order lognormal distribution. We found that the geometric width σ decreased with increasing height above the burner. Values for number density and total soot volume were also obtained. We discuss benefits of and problems with the measurement technique.

Memory function for a colloidal liquid

The reduced memory function has been measured for a dilute suspension of interacting colloidal particles as the ionic strength of the suspension was slowly decreased. Care was taken to minimize the effects of multiple scattering. In contrast to earlier measurements of Gruner and Lehman, the reduced memory function was found to increase in value with decreasing ionic strength over the experimentally accessible range of k space. The reduced memory function did not exhibit a universal character as the interaction between particles was increased, even to the point where the colloidal liquid was coexisting with the colloidal solid phase. Other features of the reduced memory function are also compared and contrasted with earlier measurements and theoretical models.

Chemical and Optical Probing of Premixed Methane/Oxygen Flames

Abstract We have measured the concentration profiles of the major products in fuel-rich pre mixed CH4/O2 flames. Optical measurements were performed to measure soot number densities, volume fraction and particle size distributions. We have shown that the increase in the soot volume fraction is due to surface growth and that acetylene is the source of the carbon involved in this process. We have also been able to determine the importance of coagulation versus surface growth in the increase of soot particle radius with time in these flames.

Gaussian beam effects on the photon correlation spectrum from a flowing Brownian motion system

The theory of photon correlation spectroscopy for a flowing system of diffusing particles was extended to include the Gaussian shape properties of an incident light beam. We found that the characteristic time of the coherent translational term caused by the bulk flow of the system was independent of the distance between the scattering volume and the focal point of the incident beam. The flow velocity and beam radius at the focus determined the characteristic time of this term. We also studied the effect of defocusing on the amplitude of the incoherent or number density term of the homodyne-detected intensity autocorrelation function. We verified our results experimentally using a flowing dioctylphthalate aerosol.

Determination of the relative number distribution of particle sizes using photon correlation spectroscopy

We investigate the determination of relative number distributions of particle sizes using photon correlation spectroscopy (PCS). To distinguish our work from earlier results, we have studied the extraction of the number distribution, relative number as a function of size, from the intensity distribution, relative scattered intensity vs size, and set limits on its usefulness. Expressions relating the scattered intensity distribution to the number distribution are presented. These results are connected to the output of two common PCS analysis techniques, the method of cumulants, and the inverse Laplace transform. These results are tested, and limits are set by generating a synthetic intensity autocorrelation function from a known number distribution and comparing the recovered number distribution to the input. We find good agreement for distributions narrower than a geometric width of 1.40.

Suppression of Multiple Scattering Using a Single Beam Cross-Correlation Method

We present a simple, single beam, laser light scattering technique which discriminates against multiple scattering in turbid media using cross-correlation of the scattered intensity at slightly different spatial positions. Experimental results obtained at a scattering angle of 90° for colloidal suspensions of various concentrations show that the technique yields information on particle diameter, even for samples which are visually opaque.

Laser light scattering multiple scattering suppression with cross-correlation, and flare rejection with fiber optic homodyning

Laser light scattering is a standard laboratory technique used for particle sizing, critical fluid studies, and many other areas of interest. We describe a method for characterizing particles in both turbid and transparent fluids by crosscorrelating the scattered intensity fluctuations at two nearby points in the far field. This approach is simple, easy to use, and works over a wide range of angles. Using the technique presented we have extended the concentration range of dynamic light scattering measurements to samples which are so turbid as to be visually opaque. Additionally, we are developing fiber optic homodyning as a technique for suppressing the detrimental effects of stray light, which can act as a local oscillator of unknown strength and corrupt otherwise good dynamic light scattering data. This should allow the use of much smaller sample cells, offering an improvement both for high concentrations and protein solutions. It should also allow for the acquisition of good signals from dynamic light scattering at forward and backward scattering angles, where stray light is often severe, and, where using a microscope for example, many lenses are present. By using fiber optics to mix a local oscillator with the scattered light, perfect wavefront matching is achievable. Such mixing efficiency is almost impossible to attain with bulk optics. By using a fiber coupler to add sufficient local oscillator, the beating effect of any stray light can be lowered to the well-characterized asymptotic limit. This technique should also be capable of better particle size resolution when several particle sizes are present. Copyright Q 1999 by William V. Meyer. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. (c)l999 American Institute of Aeronautics & Astronautics