One of the most common techniques for particle size determination is sieving. Sieving is a simple method. It is low cost and several samples from the original specimen can be prepared for several uses. However, sieving is time consuming and results can be obtained only for a very limited number of particle sizes.
Results from sieving typically vary due to several factors such as the method of moving the sieve, the period of operation, the number of particles on the sieve and some physical properties such as the shape or the stickiness of the sample. Furthermore, the actual size of the mesh gaps of the sieves can have large variations from the nominal size. Due to these limitations, this technique is being widely replaced by light scattering methods, especially for sizing particles smaller than a few millimetres.
Figure 1. Static Laser Scattering Equipment.
Technique for Measuring Particle Size
Static laser light scattering can be used to measure particle size ranging from approximately 10-20 nm up to a few millimetres. When particles are illuminated by a laser beam, light scattering is observed and their size can be determined from the angular intensity distribution. The physical theories that support this calculation are the Fraunhofer theory for rather large particles and the Mie theory which applies both to large and small particles.
Particles are defined “small” when their diameter is not larger than the wavelength of the illuminating laser light. Typically, Laser Particle Sizers use laser light with a wavelength between 500 and 700 nm. Therefore, the transition between the Fraunhofer and the Mie limit takes place in the region 0.5-1 μm. For the sake of completeness, it must be said that the Mie and Fraunhofer limits may not only depend on the particle size, but also on the sample material and the specific application.
The Need for Dispersion
It is possible that particles are found in the form of agglomerates. Agglomerates need to be dispersed and the clusters need to be separated. There, in most cases, wet dispersion is used.
During the wet dispersion, the sample powder or suspension is added to a closed circuit filled with an appropriate liquid. This mixture is pumped continuously through a measuring cell where the laser beam can illuminate the particle ensemble. During the pumping in the measuring circuit, ultrasound is applied to the system enabling destruction of the agglomerates. Single, separated particles are produced. The quantity of material added to the measuring circuit must be controlled carefully since multiple scattering processes may alter the result of the measurement.
Multiple scattering refers to the fact that the light initially scattered by a particle is then scattered on a second particle before leaving the measurement cell. To ensure that the correct amount of material is used, the beam obscuration is observed while feeding the sample material to the system. The beam obscuration provides the percentage of light that is scattered away from its original path. A value of 10-15% has proven to be a good value for the beam obscuration ensuring a reliable measurement. Figure 2 shows the volume of sample material needed to obtain 10% or 20% of beam obscuration in a FRITSCH ANALYSETTE 22 NeXT Nano Laser Particle Sizer as a function of the particle size.
Figure 2. Calculated total volume of sample material needed to get a beam obscuration of 10 and 20%. The calculation was performed using the Mie theory with optical parameters of Alumina. For a particle size smaller than about 1 μm the required amount of sample material would be significantly influenced by the refractive index of the material.
Large particles require a much larger amount of material than small particles. For a particle smaller than 0.5 μm in size, the amount of material required increases again. The exact value depends not only on the particle size, but also on the refractive index of the material, which is not indicated in Figure 2. For most samples, wet dispersion represents the ideal method of preparation for particle size measurement.
This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.
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