Particle Characterisation Particle Size and Shape Analysis – A Challenging Test
By Dr Graham Rideal, CEO, Whitehouse Scientific Ltd, Whitchurch Road, Waverton, Chester, CH3 7PB Tel: +44 (0) 1244 332626, Fax: +44 (0) 1244 335098,
info@whitehousescientific.com,
www.whitehousescientific.com
The trouble with particle size analysis is that size is often quoted as a single number, such as, a 500 micron powder. The only particles that fulfil this criterion are those that are perfectly spherical in shape and exactly the same size. A simple analogy would be taking a ball bearing from a store bin. One could safely assume that, within the specified tolerance, all the ball bearings would be spherical in shape and of the same size.
In particle technology however, such a situation is rarely seen. Most powders are irregular in shape and have a wide distribution in sizes. This leads to a number of problems, not just in describing the size and shape distribution of a collection of particles, but in their use and downstream application.
The Challenge Test
A good case in point is the Challenge Test method of measuring the cut point of a filter [1]. Here the filter is challenged by particles having an average size close to the expected maximum pore size of the filter. The particle carrier may be air or liquid. The particle size is measured before and after passing the filter, from which the filter cut point is determined.
The first question is, out of the myriad of particle sizing techniques, which is the most appropriate for the job?
The most ubiquitous method in particle size analysis, the laser diffraction method, is not suitable in this case because, although it is highly accurate and reproducible, the resolution at the top end of the distribution is not sufficiently high enough to detect the comparatively small number of particles that pass the largest pores in the filter. Furthermore, internal modelling algorithms can often smooth out fine structural detail.
The other really important issue when particle sizing is used in Challenge Testing is how the concentration of particles at the different sizes is measured. Many techniques from sieving to laser diffraction measurement give results based on a weight or volume basis. The averaging process can give significantly different results (Figure 1).
sufficiently robust statistical data. However, in today’s technology of ultrahigh speed cameras and computers, it is possible to count hundreds of thousands or even millions of particles very rapidly.
The influence
of particle shape Particle shape becomes important in Challenge Testing for two reasons: Firstly, a particle will pass a filter depending on its width, not its equivalent spherical diameter (Figure 2) and secondly, the size of the particles passing the filter is strongly dependent on the method of measurement (Figure 3).
In the Challenge Test method therefore, the particle sizing technique must be able to discriminate between particle width and equivalent spherical diameter in order to provide accurate data. Alternatively, the challenge test particles should be perfectly spherical in shape, which is not always possible to achieve.
Size Distribution and Resolution
Some of the earliest challenge test particles were Arizona Test Dusts [4]. These are still popular today and have both irregular shapes and broad particle size distributions. They therefore suffer from a number of disadvantages including ambiguous sizing, orientation dependency and, because of their broad size distribution, poor resolution (Figure 4).
Figure 2. Using ESD rather than particle width seriously overestimates a filter aperture size
In order to further increase the accuracy of the challenge test, narrow distribution microspheres are to be preferred. However, very narrow particle size distributions also have their limitations:
Figure 1. Averaging particle concentration by number or volume gives significantly different results
As the performance of a filter is based on the number of pores available, a number averaging of the particles is the most appropriate for the Challenge Test method. Particle counting methods include Electrical Zone Sensing, such as the Coulter [2] and Elzone [3] counters and Optical Zone Sensing, which use white light or laser light blocking techniques.
Microscopy is perhaps the most obvious, and certainly the oldest, method of particle counting and has a distinct advantage over all the other methods in that it can measure the shape of a particle as well as its size. Hitherto, microscopy has been more of a qualitative rather than a quantitative tool in that counting sufficient particles has been too labour intensive to provide
Figure 3. Particle size of irregular particles depends on the method used
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36
