TRANSLATIONAL BROWNIAN DIFFUSION-COEFFICIENT OF LARGE (MULTIPARTICLE) SUSPENDED AGGREGATES

Aggregates (composed of large numbers of ''primary'' particles) are produced in many engineering environments. One convenient characterization is the fractal dimension D-f, the exponent describing how the number of primary particles in each aggregate scales with radial distance from its center of mass. By viewing each ensemble of aggregates of fixed size N as a radially nonuniform but spherically symmetric ''porous solid'' body, we describe a finite-analytic, pseudocontinuum prediction of the drag, and corresponding translational Brownian diffusivity, for a fractal aggregate containing N (>>1) primary particles in the near-continuum (Kn << 1) regime. While Stokes' equation is used to define the creeping Newtonian flow outside the aggregate, Brinkman's equation is used inside, with suitable matching conditions imposed at R(max) = [(D-f + 2)/D-f](1/2)R(gyr), where R(gyr) is the familiar gyration radius. A rational/accurate correlation technique is developed to rapidly estimate drag for an aggregate with any self-consistent combination of N, D-f, and Kn(2R1). Our numerical results/rational correlations allow prediction of aggregate deposition rates via the mechanisms of Brownian diffusion and/or inertial impaction, modeling sol reaction engineering systems involving aggregate Brownian coagulation and in interpreting dynamic light scattering measurements on aggregate populations.