PARTICLE SIZE ANALYZERS
Particle size distribution plays a critical role in various applications involving powders or dispersions, including construction materials, pharmaceuticals, ceramics, colored pigments, fertilizers, and emulsions. As the range of applications widens, the requirements for measuring methods in terms of size range, measurement time, and reproducibility also increase.
It is particularly challenging to measure particles close to the measurement range limits and simultaneously detect particle sizes of both small (nanometer range) and large particles (millimeter range) for polymodally or broadly distributed samples. However, modern laser diffraction particle size analyzers like the Bettersizer S3 Plus address these challenges through innovative optical system design that detects backscattered light of very small particles and captures large particles with an integrated high-speed CCD camera, achieving a combination of laser diffraction technology and image analysis.
Measuring Method
The laser diffraction method of particle sizing involves the interaction of laser (monochromatic and coherent light) with the particles that need to be measured for their size. The diffraction of light waves by the particles follows a distinct pattern depending on their size: larger particles scatter more light in the forward direction. For particles smaller than 100 nm, the scattering intensity is almost the same in all directions.
The intensity of scattering is determined by fixed detectors based on the angle. Cutting-edge laser diffraction systems like the Bettersizer S3 Plus laser diffraction particle size analyzer ensure the measurement of scattering intensities across a continuous angular range of 0.02 – 165°, covering the forward, side, and backward directions. This is made possible through the innovative Dual Lens and Oblique Incidence (DLOI) optical system: Fourier lenses (collective lens) are placed between the laser and particles, as well as between particles and detectors. The particles interact with the light within a parallel laser beam. This offers the advantage of detecting scattered light at very large angles (in the backward scattering direction), allowing for precise detection and measurement of even very small particles. DLOI technology can also help to avoid the issues associated with traditional measuring setups. As a result, there is no need to select appropriate lenses for the specific particle size measurement range before the measurement (unlike with Fourier optics), and measurement inaccuracies arising from varying particle-to-detector distances when particles do not all lie in one plane are also eliminated (compared to inverse Fourier optics).
The measured scattering spectra are used to determine the particle size distribution by applying either the FRAUNHOFER or MIE theory. According to the FRAUNHOFER theory, the scattering pattern is attributed to thin, opaque, and spherical particles, and diffraction only occurs at the edges. Consequently, no additional optical input constants of the material are needed for this calculation. In contrast, the MIE theory is based on the concept of virtually translucent and spherical particles, which means that light permeates the matter and scatters elastically at the particle's atoms. Knowledge of the complex refractive index of both the particles and the liquid is required for this theory, which is suitable for particles of all sizes.
The figure displays a sample of a volume-based particle size distribution for a calcium carbonate powder, which was measured using a Bettersizer S3 Plus. The chart depicts the cumulative throughput curve in blue and the corresponding histogram represented by black bars.
