If only particle sizing were easy to do! Very few materials are monodisperse systems in which all particles have the same size. Most of the samples to examine by particle analysis in the laboratory and in research are polydisperse systems, so the particles they contain are differently sized. However, because particle size can have a decisive effect on the properties of products, performing particle size analysis and determining the particle size distribution is important.
The particles contained in polydisperse systems are not all identical but differently sized and shaped. Particle size distribution measures particles’ equivalent diameter to assign them to certain sizes (or size ranges), and the results are shown as percentages in each particle class.
- Particle size distribution in sieve analysis
- Particle size distribution in laser diffraction
- Particle size distribution in dynamic image analysis (DIA)
- Particle size distribution in dynamic light scattering (DLS)
How is the particle size distribution illustrated?
The quantitative proportion of each particle class in the disperse phase represents the distribution of particle sizes. Quantification can be performed by weighing (mass or volume) or counting. Depending on which quantitation method is used, the graphical or tabular representation of a sample can look very different.
Particle size distribution in sieve analysis
The data is represented according to the mass-based distribution of particles. Summing up the quantities in the various particle size fractions (starting with the smallest fraction) yields the so-called "cumulative curve". Sieve analysis is suitable for particle sizes from approximately 5 µm to 125 mm.
Particle size distribution in laser diffraction
The data is represented according to the volume-based distribution. This is difficult for the analyst because in polydisperse samples the scattering angles of the differently sized particles overlap. Therefore, the distribution must be calculated from a superimposed scattered light pattern. Laser diffraction can be used for suspensions, emulsions and mixtures with particle sizes from approximately 0.01 µm to 8,000 µm.
Particle size distribution in dynamic image analysis
Dynamic image analysis can be used for dry, non-agglomerating bulk solids with particle sizes upwards of approximately 1 µm. Imaging is according to the count-based distribution. As high-tech equipment is required, this method is quite cost-intensive.
Particle size distribution in dynamic light scattering
For suspensions and emulsions, dynamic light scattering can be used for particle sizes from 0.3 nm to 10,000 nm. The data is represented according to the intensity-based distribution. As scattering intensity strongly decreases with particle size, the contribution of large particles to the total scattering intensity is over-represented.
Depending on the requirements of the sample and on the sizes of the particles, different particle analyzers are needed. Different particle analyzers to determine the particle size distribution yields different values for the same sample; these values, however, can be easily converted using mathematical models.
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