What is particle size analysis?

Particle size analysis is used to characterise the size distribution of particles in a given sample. Particle size analysis can be applied to solid materials, suspensions, emulsions and even aerosols. There are many different methods employed to measure particle size. Some particle sizing methods can be used for a wide range of samples, but some can only be used for specific applications. It is quite important to select the most suitable method for different samples as different methods can produce quite different results for the same material.

Who uses particle size analysis?

Particle size analysis is a very important test and is used for quality control in many different industries. In just about every industry where milling or grinding is used, particle size is a critical factor in determining the efficiency of manufacturing processes and performance of the final product. Some industries and product types where particle sizing is used includes:

• Pharmaceuticals
• Building materials
• Paints and coatings
• Food and beverages
• Aerosols

Equivalent sphere theory

One basic problem in particle size analysis is characterizing particles using just one number. Most particle sizing techniques aim report particle size distributions on a two dimensional graph (ie. particle size on the x-axis and quantity of material on the y-axis). However, the difficulty with this is that there is only one shape that can be described by a single unique number, and that is the sphere. Only a sphere measures the same across every dimension. If we say we have a 100 micron sphere, this describes it exactly. We cannot say the same for a cube, where the 100 micron may describe the length of one edge, or even a diagonal transect.

For this reason, all particle sizing techniques measure a one dimensional property of a particle and relate this to the size of an “equivalent sphere”. One example is to measure the surface area of a particle and then report the size of sphere which has the same surface area. Probably the most common method is to measure the “volume” of each particle in a sample and report the size of a sphere which has the same volume as the particles being measured (this is what is done in Laser Diffraction methods).

Particle Sizing by laser diffraction

Laser diffraction has become one of the most commonly used particle sizing methods, especially for particles in the range of 0.5 to 1000 microns. It works on the principle that when a beam of light (a laser) is scattered by a group of particles, the angle of light scattering is inversely proportional to particle size (ie. the smaller the particle size, the larger the angle of light scattering). Laser diffraction has become very popular because it can be applied to many different sample types, including dry powders, suspensions, emulsions and even aerosols. It is also a very fast, reliable and reproducible technique and can measure over a very wide size range.

Other methods

There are many other methods for analysing particle size, other than laser diffraction. Sieving is one of the oldest particle sizing methods and is still widely used for relatively large particles (ie. > 1mm). When measuring very small particles (ie. < 0.5um), Dynamic Light Scattering is by far the easiest methods to use. And if you need to measure morphological properties of particles, (ie. shape as well as size), then image analysis methods are the only way to gain the extra information.

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The ability to measure the size of very fine particles is an integral part of many industries today. For every material that goes through a grinding or milling process, the final particle size is usually the primary factor that governs product performance or process efficiency. This is why particle sizing has become essential in areas such as the pharmaceutical industry, foods and beverages, cement and building materials and for manufacturers of chemical compounds for industrial and residential uses.

One of the most popular and widely used particle sizing technologies available today is Laser Diffraction. Laser Diffraction has become so popular because it offers a very wide dynamic range; it can be applied to liquid suspensions, dry substances and aerosols and provides a very rapid and reliable measurement process.

The goal of all particle sizing technologies is to provide a systematic and reliable measurement for a range of different sized particles. However it is important to understand that there is a wide range of particle sizing technologies available and no one technology is suitable for every job. Each measurement technology has its own advantages and each is often best suited to specific industries or applications.

It is also important to understand that different measurement technologies can often give different particle results for the same sample. This is because each measurement technique measures a different aspect of the same material.

Sphere Approximations

When reporting particle size, we tend to display a graph showing “particle size” on one axis and “percent of material” on the other axis. However, it is very difficult to describe a multi-dimensional particle using one dimension. In fact there is only one shape that can be described by one dimension, and that is the diameter of a sphere.
For this reason, all particle sizing techniques measure some property of material and then relate this to an “equivalent sphere”.

Some common particle sizing techniques and there reporting methods are as follows:

• Sedimentation techniques – measures the rate at which particles settle in a liquid column and reports the size of a sphere with the same settling rate.
• Sieve technique – measures the mass of material retained on a series of screens and reports the amount of material between spherical hole sizes.
• Aerodynamic sizing technique – measures the behaviour of particles in an airstream and reports the size of a sphere that has the same behaviour.
• Laser diffraction – measures light scattering from a group of particles and reports the size of a sphere that produces the same scattering.

So, with so many different measurement techniques available. The question is: which techniques are the best?

Getting the Answer as Right as Possible

Like most questions in the world of science, the answer has multiple parts. In short, you need to choose the most suitable measurement technology for your product and your process. However, if you had to pick a single technology that can universally applied then it would have to be Laser Diffraction.

Laser diffraction is the best general technique as it can be used with a very wide range of particle sizes and also a very wide range of sample types. Laser diffraction works very well for sprays, dry powders, suspensions and emulsions. The results reported are also displayed in terms of a “volume” distribution, which is the most appropriate description for bulk material properties.

Apart from Laser Diffraction, another technique that is gaining popularity is Image Analysis. Image Analysis is also considered a particle sizing technique, but it does offer one significant difference. It is the only methodology that provides any information on particle “shape”! If particle shape is known to have an influence on product performance, then image analysis may be the most appropriate option.