Particle Characterisation
Particle Characterisation with Dynamic Image Analysis Determination of particle size and particle shape with a new measuring technique
Joerg Westermann, Retsch Technology GmbH
Sieve analysis and laser diffraction are long established methods for the determination of particle size distributions. Dynamic Image Analysis (DIA) is another particle analysis technology to measure particles > 1 micron, which has a number of advantages over these methods.
Retsch Technology’s particle analysers CAMSIZER and CAMSIZER XT, which are based on DIA technology, evaluate images of the particles which provide considerably more information on the particles than, for example, a light scattering pattern produced by a laser diffraction particle analyser. These only allow for an indirect measurement of the particle size. The determination of parameters such as the length, width, or sphericity of particles is only possible by using image analysis.
This article deals with the following topics: • A short explanation of the measuring principle of Dynamic Image Analysis
• Demonstration of the advantages of the measuring technique when analysing powders, granules or suspensions with practical application examples • Description of the modular dispersion and sample feeding systems of the CAMSIZER XT
The Principle of Dynamic Image Analysis
The functional principle of Dynamic Image Analysis is similar to that of a microscope: A camera takes enlarged digital pictures of particles which are then analysed by software that calculates the size and shape of every single particle in the picture.
There are two important differences between conventional microscopy and DIA:
Conventional microscopy for particle size analysis is a static method which means that the particles on the object plate don’t move in relation to the optics. The Dynamic Image Analysis technology however, records each particle in movement. A stream of particles is generated, for example by gravity, air pressure or liquids, and passes the camera which takes pictures in a quick succession.
This leads to the second difference. Dynamic Image Analysis typically records more than 10,000 images per minute, whereas static analysis methods only allow for analysis of a few images.
detection effi ciency. The CAMSIZER XT system records more than 275 images per second.
With Dynamic Image Analysis it is possible to analyse statistically relevant numbers of a few million particles very quickly. This method provides reproducible and statistically relevant results not only for the mean particle size but also for small quantities of undersized or oversized particles.
Highest Resolution AND Exact Quantity
A mixture of 4 different monodisperse Latex particles was analysed with the CAMSIZER XT and a laser diffraction system. Thanks to the combination of the two megapixel cameras, the CAMSIZER XT was able to analyse particles smaller than 10 microns with considerably higher resolution than a laser diffraction analyser. Dynamic Image Analysis resolves the single components of 2.5 µm, 5 µm and 12 µm very well within the particle mixture. The mean sizes and proportions of the single components are precisely detected in the mixture. Although laser diffraction systems are able to measure the single components reliably, they fail to provide the total size distribution in high resolution. For example, a double peak of 10 µm and 12 µm cannot be resolved by a laser diffraction analyser.
With Dynamic Image Analysis every single particle is evaluated and assigned to a measuring class according to its size. This results in a high resolution which allows to differentiate between particles that are very close in size. Laser diffraction analysers measure the light scattering signals of the particles, thus determining the particle size indirectly. The size distribution of the particle collective is then back-calculated from the superimposed signals. The resolution provided by this method is substantially lower than that obtained by Dynamic Image Analysis.
Figure 1. Measurement principle Dynamic Image Analysis
Figure 1 shows the basic setup of the optics. A stream of particles moves through the measuring fi eld. The particles are backlit while a camera on the opposite side takes images of the particles’ silhouette. The software evaluates these images and very quickly determines the size distribution of the particles. A few hundred particles per image are evaluated in real time. Modern particle analysers such as Retsch Technology’s CAMSIZER and CAMSIZER XT are equipped with two cameras working simultaneously, with different magnifi cations to cover a wide measuring range: one camera with high magnifi cation is optimised for analysing the small particles, a second camera with a lower magnifi cation but wide fi eld of view allows to simultaneously analyse the larger particles with high
Figure 2. Particle size distribution of a mixture of latex particles of 2.5 µm, 5 µm, 10 µm and 12 µm, measured with the CAMSIZER XT optical particle analyser and a laser diffraction system.
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