11 - Particle capture

Summary

Introduction

In the preceding chapters, fundamental aspects of colloid behavior have been emphasized. Now we are ready to apply this knowledge to processes involving suspensions. Here we investigate the capture of small particles by stationary collector units, one aspect of filtration technology.

Elementary considerations show that a strong attractive force is necessary if freely suspended particles are to come together, because at close separations viscous resistance increases dramatically. Since the interparticle force derives from the combination of electrostatic and dispersion forces, capture is particularly sensitive to the balance between colloidal and hydrodynamic forces. Several mechanisms contribute to particle capture and retention. Inertia is the dominant factor when fast-moving particles impact on a stationary object, whereas geometry and proximity govern the interception of slow-moving particles. The capture of submicron particles is influenced enormously by interparticle forces and Brownian motion. All these aspects are treated here, but technological issues are ignored. For example, a persistent problem encountered in the filtration of small particles is buildup of a deposit. Our treatment deals with the behavior of clean collector units to emphasize basic colloidal phenomena.

Aerosols have received the most study by a wide margin and many comprehensive reviews exist, e.g. Hidy & Brock (1970), Davies (1973), Friedlander (1977), and Kirsch & Stechkina (1978). Ives (1975) and Tien & Payatakes (1979) present broad reviews of liquid filtration; Spielman (1977) concentrates on small-scale processes in liquids.