How does particle size affect the motion of a particle?
First consider a solid particle at rest in a vacuum, with no external forces acting on it. Newton s first law of motion indicates that the particle will remain at rest. If the particle is now surrounded by a gaseous medium such as air, the particle may or may not be noticeably affected depending on its size. In particular, because the air is composed of molecules in constant motion, the solid particle surrounded by air is constantly being bombarded by air molecules. For example, a 0.1 mm particle suspended in air at standard temperature and pressure experiences 1014 collisions with air molecules in a mere second! (Note that the ratio of particle diameter to molecular diameter is only 260.) For very small particles (e.g., tobacco smoke particles), this constant bombardment can give rise to Brownian motion, as shown qualitatively in Figure 1. This wiggling motion is the cumulative effect of the billions and billions of collisions. For very large particles, however, no perceptible change
