How to Characterize a Particle

Both the properties and sizes of individual particles are important in many ways, and the science of particles and their properties has several definitions for what a particle is. A general definition might be: a discrete quantity of matter that has an interface with its surrounding matter and environment.

Why do we Measure Particles?

How a particle interacts with its environment is different to the bulk properties of a material. The size, shape and interactions of small particles determine how the bulk material behaves. If you needed an easily flowing bulk material, you might want to make sure that you have manufactured small spherical particles that do not interact with each other. Smoothness of surface is then one characteristic of a particle that you might need to define.

In chemical manufacturing, the particle size of a raw material could determine the rate of reaction. Or the rate a drug is delivered into the body might depend on the surface area of the particles making up the drug. Assume you have a spherical particle of radius a, then:

As the size of the particles get smaller, the surface-area-to-volume ratio gets larger — generally the larger the ratio the quicker a particle can react or dissolve. Finer ground salt dissolves quicker than coarse salt. Hence manufacturing particles of a specific size could be critical, and manufacturers need to know whether they have made particles that meet their specification.

Characterizing Particles

There are several methods that can be used to characterize a particle — depending on the information needed and the size of the particles. Statistical methods play a large part in particle characterization, and the key to obtaining meaningful results is to ensure that the sample used is representative of the whole. Here are the main options:

• Laser Diffraction Particle Sizing

A quick and repeatable technique used to determine a particle’s size, covers several orders of magnitude from tens of nanometers to a few millimeters. Measures the scattering angle of light incident on the particles; larger particles have a smaller scattering angle.

• Dynamic Light Scattering

Measures particles to below one nanometer in size. Measures particles in a fluid or colloid by shining a light through the fluid and measuring the intensity of the light transmitted. Use’s the intensity to determine a particle’s size; the size is the particle’s equivalent hydrodynamic diameter, which includes a fluid shell. Hence, the particle seems larger than if a microscope measured the diameter.

• Imaging

There are several imaging techniques, optical and electron microscopy, automatic and manual handling. They can determine a particle’s size and other characteristics such as shape and surface roughness. An example of imaging analysis is shown in the article: Particle Characterisation with Dynamic Image Analysis Determination of particle size and particle shape with a new measuring technique.