TABLE OF CONTENTS An understanding of particle transport or movement from one point to another in a gas is basic to the design of gas cleaning equipment and aerosol sampling instruments. The scavenging of particulate matter from the atmosphere by dry and wet deposition processes is also determined by particle transport processes.
The classical problems of particle transport were studied by well-known physicists in the late nineteenth and early twentieth centuries. Stokes, Einstein, and Millikan investigated the motion of small spherical particles under applied forces primarily because of application to (at that time) unsolved problems in physics. They derived relatively simple relationships for spherical particles that can be considered as ideal cases; irregular particles are usually discussed in terms of their deviations from spherical behavior.
In this chapter, we consider Brownian diffusion, sedimentation, migration in an electric field, and thermophoresis. The last term refers to particle movement produced by a temperature gradient in the gas. We consider also the London van der Waals forces that are important when a particle approaches a surface. The analysis is limited to particle transport in stationary that is, nonflowing gases. Transport in flow systems is discussed in the chapters which follow.
The rate of transport of particles across a surface at a point, expressed as number per unit time per unit area, is called the flux at the point. Common dimensions for the flux are particles/cm 2 sec. Expressions for the diffusion flux and diffusion coefficient that apply to submicron particles are derived from first principles in...
