An Overview of the Different Particle Size Measurement Techniques
There are many technologies available to determine particle size distribution of materials. Of all the technologies available, laser diffraction has become one of the most widely used and preferred methods. This article will look at some of the more widely accepted techniques used in different industries today.
What is Particle Size?
Particles are three-dimensional objects. In order to provide a complete description of a particle, three parameters are required — length, breadth and height. Thus, it is impossible to describe a particle using a single number that equates to particle size. Therefore, most sizing techniques assume that the material being measured is spherical because a sphere is the only shape that can be described by a single number, its diameter, thus simplifying the way particle size distributions are represented.
Widely Accepted Particle and Surface Measurement Techniques
Adopting different measurement techniques can produce different results when measuring non-spherical particles. That said, any instrument or technique used for particle size analysis needs to generate data in a form that is relevant to the process. The method also needs to be reliable, simple to use and able to generate reproducible data.
- Laser Diffraction
- Dynamic Light Scattering
- Image Analysis
- Acoustic Spectroscopy
Laser diffraction is the one of the most widely used particle sizing techniques and has become the standard method in many industries for characterisation and control. This type of particle size analyser relies on the fact that particles passing through a laser beam will scatter light at an angle that is directly related to their size. When particle size decreases, the observed scattering angle increases logarithmically. Scattering intensity is also subject to particle size, diminishing with particle volume. What this means is that large particles scatter light at narrow angles with high intensity while small particles scatter at wider angles with low intensity.
Laser diffraction has a wide dynamic range, from 0.2 to 2000 microns and is very fast and reliable. It is also very flexible as it can be applied to dry powders, aerosols and emulsions. In addition, laser diffraction does not require calibration but can be easily verified.
Sometimes referred to as Photon Correlation Spectroscopy or Quasi-Elastic Light Scattering, this method is a non-invasive and sensitive technique used for measuring the size of molecules and particles in the submicron region. The results are expressed as particle hydrodynamic diameter. Dynamic light scattering is an accurate, reliable and repeatable technique.
This is a traditional method widely used in the paint and ceramics industries. Equipment as simple as the Andreason pipette or as complex as centrifuges and X-rays can be used in this method. The main advantage of this technique is that it determines particle size as a function of settling viscosity. However, as the density of the material is needed, this method is no good for emulsions where the material does not settle or for dense material that settles too quickly. It is also based on spherical particles, so can give large errors for particles large aspect ratio.
This technology generates data by capturing direct images of each particle, providing users with the ultimate sensitivity and resolution. Image analysis systems are capable of high-resolution sizing ranging from 0.5µm – 1000µm. Subtle differences in particle size and shape can be accurately characterised using this method.
Instead of using light, this technique employs ultrasound for collecting information on the particles that are dispersed in fluid. This can be done because dispersed particles absorb and scatter sound waves similarly to light. Acoustic spectroscopy can be used to measure particle size distribution for any particle in a fluid system and can measure at very high particle concentrations.