CHARACTERIZATION OF PARTICLES BY MODULATED DYNAMIC LIGHT-SCATTERING .2. EXPERIMENT

In an earlier paper we described the theoretical foundations of modulated dynamic light scattering (MDLS) and indicated how these methods can be implemented in the characterization of particles. In this paper we illustrate the range of application of MDLS methods by describing eight types of MDLS experiments we have performed. In addition to illustrating the range of application of MDLS methods, the results of these experiments validate our theoretical predictions. The experiments were done with an MDLS apparatus in which the requisite modulation is produced by frequency-biasing two intersecting laser beams with acousto-optic modulators. Measurements were made by introducing a single sample particle into the intersection volume of the two beams and processing the scattered light signal from the particle. Data from these experiments typically provide detailed records of the translational and/or rotational Brownian motion for one or more single particles suspended in a fluid medium. Dynamic particle properties are inferred by comparing measured results with theoretical results based on models of Brownian motion. Described MDLS method experiments include (a) simultaneous measurement of the translational diffusion coefficient and velocity component of particles suspended in air; (b) simultaneous measurement of the translational and rotational Brownian motion of hematite ellipsoids and spindles with submicrometer dimensions suspended in water; (c) determination of the diameters and slip correction factors of micrometer-sized spheres suspended in air; (d) determination of the precision of the present MDLS apparatus for finding sphere diameters; (e) precise measurement of sphere diameters; (f) simultaneous measurement of the mass and friction coefficient of a particle suspended in air and verification of predicted effects of particle inertia on Brownian translation and rotation; (g) measurement of translational Brownian displacement distributions; and (h) determination of precision vs measurement time for an MDLS method.