Why is particle technology important?  Solid particulates are ubiquitous in both nature and industry.   Natural occurrences include sands on the beach, snow avalanches, ice floes, planetary formation, sand dune formation, pollutants in the atmosphere, etc.  Industrial applications of particulate systems include power generation via coal combustion, synthesis of advanced materials, handling of foodstuffs and pharmaceuticals, fluid catalytic cracking, etc.  Due to this widespread occurrence, processes involving particulates are tightly linked to the national economy.  For example,

• at least 40%, or $61 billion of the value added by the chemical industry is linked to particle technology (Ennis et al., April 1994), and

• 1.3% of U. S. electrical power production is used for the grinding of particles and ores (Ennis et al., April 1994).

Despite this prevalence of particle processes, much room for improvement still exists.  For example,

• 80% of solids processing plants experience solids handling problems such as pipe blockage and erosion, and the performance of such operations is typically 40-50% of design (Knowlton et al., 1994), and

• over 1000 silos, bins and hoppers fail in North America each year (Knowlton et al., 1994).

Current research in particle technology covers a broad range of topics; the specific aim of this research group is to further the understanding of the flow behavior of particulate systems. 

Does particle size matter?  Yes!  Perhaps the most important parameter in characterizing the behavior of a collection of particles is particle size. (Technically speaking, the mass or inertia of the particle relative to the surrounding fluid is the determining parameter. For the purposes of this discussion, though, we will assume that the particles are found in the ambient air and that the particle's size is a good indicator of its mass.) Particle sizes span an enormous range, from the nanoscale to grains of sand to boulders to planets.  Typical sizes for familiar materials are tabulated below, in order of increasing size.

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 will be observed.  For example, a billiard ball placed on a pool table will not be seen to move, even though it is also undergoing a enormous number of collisions with air molecules.  If set to motion by a cue, it will follow a straight-line trajectory until collision with another ball or wall changes its direction, as illustrated in Figure 2.  Hence, as evidenced by these figures, the nature of particle motion very much depends on the size of the particle.

Figure 1:  Path of particle undergoing Brownian motion	Figure 2:  Path of particle undergoing straight-line motion

References:

Ennis, B. J., J. Green and R. Davies, “The Legacy of Neglect in the U.S.,” Chem. Eng.   Prog., 32 (April 1994).

Knowlton, T. M., J. W. Carson, G. E. Klinzing and W.-C. Yang, “The Importance of    Storage, Transfer, and Collection,” Chem. Eng. Prog., 90, 44 (1994).