The need to determine particle size can arise in many different circumstances and a wide range of measurement techniques have been developed. On of the most popular is Laser Light Scattering (LLS) which has many applications for particles in the size range between about 1 micron and 1 millimetre (1000 micron).

How does it work?

When small particles are illuminated by a beam of light, some of the light is scattered away from its original direction of travel. The angular distribution of the scattered light depends on the size of the particle, with smaller particles scattering more light at higher angles. So by measuring the scattering pattern from a collection of particles, we can calculate the size of the particles and their relative proportions (by volume).

Sample preparation

The tricky bit. To make the measurement we must first present the particles to the light beam in a well dispersed state so that each photon is only scattered once. The simplest way is to disperse the sample (eg. a powder) in a volume of water which is circulated through an optical cell. If water is not suitable (eg. if the sample is water soluble or too hydrophobic), then other fluids can be substituted. Alcohols are commonly used for this purpose. Dry powders can also be dispersed in air. Even where the sample is compatible with water, it may be necessary to use chemical dispersants to improve wetting of the individual particles and prevent aggregation. A range of chemicals are available to suit different sample types. A circulatory pump provides some energy to mix and disperse the material. Most instruments also incorporate ultrasonic transducers to aid dispersion, but these need to to be used with caution as some systems may aggregate under ultrasound.

Optical models
To interpret the scattering pattern in terms of particle sizes, we need a detailed understanding of the way that light interacts with the particles. This is encoded mathematically in the optical models. A simple model is based on the Fraunhofer Theory. This treats each particle as an opaque sphere and assumes that all particles, irrespective of size, scatter with similar efficiency. These are good assumptions for particles which are much larger than the wavelength of light (say >10 micron). For smaller particles, particularly when they are transparent, transmission and refraction of light through the particles becomes important. In this case it is better to use the more complete Mie Theory which takes these phenomena into account. It is necessary to know the refractive indices of the particle and fluid, together with an absorbtion parameter. Values for these factors can be found from tables, though they they may sometimes need to be estimated.

When the size of the particles approaches that of the wavelength of light, the scattering intensity loses its angular dependence, so placing a lower limit on the size of particles which can be determined by LLS. Some instruments incorporate extra detectors at high angles or use other techniques to extend the measuring range. Our LS230 instrument uses a combination of different wavelengths and polarisation (PIDS) to provide extra information, extending the lower limit to 0.04 microns.

At the high end (>1 mm) resolution is limited by the small angle at which scattering occurs and the difficulty of making detectors to measure it. The limit for the LS230 is 2000 microns (2mm).