What exactly is particle size and particle shape?

This section describes how particle size and particle shape are calculated and illustrates how image analysis can be used to measure both.

What is particle size?

Describing a 3D particle is often a more complex matter than it first appears. For simplicity it is convenient to describe particle size as one single number. However, unless the particle is a perfect sphere (which is rare in ‘real-world’ samples) there are many ways to describe the size of a particle. This is the basic challenge of particle size analysis – how do we describe a 3-dimensional object with one number only?

Image analysis systems capture a 2-dimensional image of the 3D particle and calculate various particle size and particle shape parameters from this 2D image. One of the principle diameters calculated is CE diameter which is the diameter of a circle with the same area as the 2D image of the particle. Of course different shaped particles will have an influence on this CE diameter but, importantly, it is a single number that gets larger or smaller as the particle does and it is objective and repeatable.

CE diameter
The 3D image of the particle is captured as a 2D image and converted to a circle of equivalent area to the 2D image. The diameter of this circle is then reported as the CE diameter of that particle.

Of course a single value from one particle is unlikely to be the much sought-after “single number for management purposes”. It is unlikely to be statistically significant as the single value depends upon which individual particle is chosen. A number of particles which are representative of your sample as a whole have to be measured and statistical parameters generated.

A more appropriate single characterisation number would be the mean of all the CE diameters for example. Once a distribution is generated all statistical parameters such as mean, median, mode, standard deviation, D10, D90 percentiles etc can be easily calculated.

What is particle shape and why is it important?

If describing a 3D particle size is complex then quantifying particle shape is even more complicated! There are an almost infinite number of ways to describe a complex particle shape and in doing so we seem to be deviating from our stated objective of reducing a sample to one quantifiable number. So why measure particle shape?

Measuring particle size alone is sometimes insufficiently sensitive to identify important but subtle differences between samples. Some batches of samples may differ by such a small amount that this difference is lost during the translation to a circle-equivalent or spherical-equivalent diameter. For example consider the 3 shapes below:

All these 3 shapes have the same area = 4 square units. When they are converted to a circle equivalent diameter they give the same result - a circle equivalent diameter of 2.257 units. This highlights the main disadvantage of measuring particle size only - very different shaped samples could be characterized as identical simply because they have similar projected 2D areas or similar spherical-equivalent volumes. Particle shape often has a significant influence on final product performance parameters such as flowability, abrasive efficiency, bio-availability etc, so some way of characterizing shape is required.

Image analysis systems from Malvern:

Malvern offers three image analysis systems. Both image analysis systems have been developed specifically for routine and repeatable particle size and particle shape analysis. The choice of instruments depends upon the nature of the application and sample form:

	Sysmex FPIA-3000 - for wet samples (suspensions and emulsions). The Sysmex FPIA-3000 is based upon flow cytometry and enables characterization of particle size and particle shape of wet materials in the range 0.8µm to 300µm.
	Morphologi G3 - High sensitivity particle analyzer The Morphologi G3 is based on automated microscopy and enables characterization of particle size, particle shape and particle count in wet and dry materials in the range 0.5µm to 3000µm.