Particle size

This article is about general definitions related to particle size. For other uses, see Particle size (disambiguation).

Particle size is a notion introduced for comparing dimensions of solid particles (flecks), liquid particles (droplets), or gaseous particles (bubbles). The notion of particle size applies to colloidal particles, particles in ecology, particles present in granular material (whether airborne or not), and particles that form a granular material (see also grain size).

Expressions for particle size

The particle size of a spherical object can be unambiguously and quantitatively defined by its diameter. However, a typical material object is likely to be irregular in shape and non-spherical. The above quantitative definition of particle size cannot be applied to non-spherical particles. There are several ways of extending the above quantitative definition to apply to non-spherical particles. Existing definitions are based on replacing a given particle with an imaginary sphere that has one of the properties identical with the particle.

Volume-based particle size

Volume-based particle size equals the diameter of the sphere that has the same volume as a given particle:

$D = 2 \sqrt[3] {\frac{3V}{4\pi}}$

where

$D$: diameter of representative sphere

$V$: volume of particle

Weight-based particle size

Weight-based particle size equals the diameter of the sphere that has the same weight as a given particle.

$D = 2 \sqrt[3] {\frac{3W}{4\pi dg}}$

where

$D$: diameter of representative sphere

$W$: weight of particle

$d$: density of particle

$g$: gravitational constant

Area-based particle size

Area-based particle size equals the diameter of the sphere that has the same surface area as a given particle.

$D = 2 \sqrt[2] {\frac{A}{4\pi}}$

where

$D$: diameter of representative sphere

$A$: surface area of particle

Aerodynamic particle size

Hydrodynamic or aerodynamic particle size equals the diameter of the sphere that has the same drag coefficient as a given particle.

Another complexity in defining particle size appears for particles with sizes below a micrometre. When a particle becomes that small, the thickness of the interface layer becomes comparable with the particle size. As a result, the position of the particle surface becomes uncertain. There is a convention for placing this imaginary surface at a certain position suggested by Gibbs and presented in many books on interface and colloid science.

Definition of the particle size for an ensemble (collection) of particles presents another problem. Real systems are practically always polydisperse, which means that the particles in an ensemble have different sizes. The notion of particle size distribution reflects this polydispersity. There is often a need for a certain average particle size for the ensemble of particles. There are several different ways of defining such a particle size.

International standards

There is an international standard on presenting various characteristic particle sizes. This set of various average sizes includes median size, geometric mean size, average size.

Measuring particle size

There are several methods for measuring particle size and particle size distribution. Some of them are based on light, other on ultrasound, or electric field, or gravity, or centrifugation.

See also

• Dynamic light scattering
• Micromeritics

References

8.ISO Standard 14644-1 Classification Airborne Particles Cleanliness