THE EFFECT OF PARTICLE DENSITY ON COLLISIONS BETWEEN SINKING PARTICLES - IMPLICATIONS FOR PARTICLE AGGREGATION IN THE OCEAN

A theory developed by WACHOLDER and SATHER (1974, Journal of Fluid Mec hanics, 65, 417-437) for impermeable spheres moving at low Reynolds nu mbers predicts that the likelihood of collision between two sinking pa rticles of different size may be essentially zero if the larger one se ttles faster but is less dense than the smaller. When the ratios of th e settling velocity and the excess density of the two particles exceed critical values determined by the theory, the trajectory of the small particle is closed in a region of finite size surrounding the large p article. Small particles overtaken by the large one as it sinks are de flected around this region and collisions arc impossible. These findin gs are confirmed by laboratory experiments utilizing model particles s inking in a viscous fluid. A review of measured sinking rates and dens ities of marine particles indicates that a substantial fraction of the particles in the oceanic water column exceed the critical values of s ettling velocity and excess density with respect to other particles wi thin a factor of 10 or so in size. Thus, aggregation by differential s edimentation is probably significant only between very small and very large particles, for which the effect of particle porosity must also b e considered.