Coagulation between fractal aggregates and small particles and fractal properties of marine particles

Abstract

This dissertation includes two parts (designated A and B) that serve two separate but related research purposes. In Part A, coagulation rates between fractal aggregates (200-1000 μm) and small (1.48 μm) particles were studied for collisions induced by differential sedimentation and turbulent fluid shear. The collision frequency functions (beta) between these aggregates and small particles were found to be lower than predicted by a rectilinear collision model but much higher than predicted by a curvilinear collision model for equivalent impermeable spheres. The collision frequencies decreased with the magnitude of aggregate fractal dimensions (D). Based on fractal geometry of aggregates and the comparisons between observed settling velocities and those calculated using Stokes' law, a semi-empirical correlation was derived to describe the permeabilities of settling fractal aggregates. A filtration model was used in conjunction with this fractal permeability correlation to predict capture rates and capture efficiencies of small particles by settling fractal aggregates. In the turbulently sheared fluid, it was demonstrated that the importance of the shear rate (G) on enhancing collision frequencies was dependent on the fractal dimension of aggregates. As D approaches 3, beta became less sensitive to G as predicted by a curvilinear model. It was argued that flow through large pores formed between clusters within fractal aggregates contributed to high aggregate permeabilities and enhanced the coagulation between the aggregates and suspended small particles. In part B, fractal properties of microscopic particles (300 μm) occurring in marine systems were investigated. A new method, called the particle concentration technique (PCT), was developed to calculate the average fractal dimension of all particles in a certain size range by the analysis of particle size distributions in terms of both solid volume and length. During a simulated algae bloom in a mesocosm, as coagulation proceeded the average fractal dimension decreased with time from D = 2.52 to D = 1.68, a value typical of larger marine snow aggregates. Investigations in three eastern Pacific coastal areas suggested that the average fractal dimension indicated the importance of coagulation in determining local particle size distributions. The magnitude of the fractal dimension is likely associated with other factors, such as transparent exopolymer particles (TEP), affecting the coagulation rate of algae during a bloom in seawater.