WHAT DO WE MEAN BY PARTICLE SIZE?

Why is it that the simplest questions are often the hardest to answer?

If we have a sphere, then it's size is easily defined by the diameter (or radius, if you prefer it). Given this single number we can unambiguously calculate the volume, surface area, circumference - or whatever else we may wish to know about the particle. A cube also can be characterised by a single dimension - the length of a side, though the relationships to volume, etc. will be different.

A rod can be characterised by two dimensions - diameter and length. A rectangular prism needs three - length, breadth and height.

In the real world we meet particles of all shapes. Some are spheres, cubes or rods, but most are just irregular lumps. Even the more regular shaped particles may have rough surfaces, which will affect their properties. So how are we going to describe our 'particle size'?

Practically, we cannot measure all of the dimensions of an irregular particle. Even if we put it under a microscope, we only see a 2 dimensional projection of the object which will vary, depending on how the particle is lying. And if we could do a complete measurement, what would we do with all those numbers?

In most cases what we want is a single number, which gives a useful measure of the size of the particle - like the diameter of the sphere. Which is why most particle sizes are reported as the 'diameter of an equivalent sphere'. What does that mean?

 

 

There are many ways to measure particle size:

Using microscopy we can directly measure a diameter (several diameters if the particle is irregular), a circumference or a projected area
Using sieves we can find the size of the hole (usually square) through which the particle will pass; this actually corresponds to the second largest dimension of a three dimensional object (think of a pencil passing through a hole).
Using sedimentation we can measure the rate at which particles settle in a fluid, which we can relate to a size.
Using zone sensing (Coulter method) we can directly count and measure the volume of individual particles.
Using light scattering we measure the characteristic scattering pattern.
And there are many more.


In each case we are measuring some property of a collection of test particles and relating that to the behaviour expected of a collection of spheres of a known size. The results will depend on the characteristics of the material (size, shape, density, etc.) and on the method being used.

In general, one cannot expect to get the same result using different methods - because we are looking at different properties. The important thing is to choose a method that gives results that are appropriate for the application. It is particularly important to use a counting technique if you are interested in numbers of particles.

In most cases, people want to know about relationships: Is this one bigger than that? Is the mean particle size changing? Here, reproducibility is the key factor - and Laser Light Scattering is particularly good at producing reproducible results.

At Microns to Measure, we use Laser Light Scattering (LLS) as our primary technique for particle size analysis because it is fast, reproducible and applicable to a wide range of problems.
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