Particle size analysis, particle size measurement, or simply particle sizing, is the collective name of the technical procedures, or
laboratory techniques which determines the
size range, and/or the average, or
mean size of the particles in a
powder
A powder is a dry solid composed of many very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms ''powder'' and ''granular'' are sometimes used to distinguish se ...
or liquid
sample.
Particle size analysis is part of
particle science, and it is generally carried out in
particle technology Particle technology is the science and technology of handling and processing particles and powders. It encompasses the production, handling, modification, and use of a wide variety of particulate materials, both wet and dry. Particle handling may in ...
laboratories.
The particle size measurement is typically achieved by means of devices, called Particle Size Analyzers (PSA), which are based on different technologies, such as
high definition image processing, analysis of
Brownian motion
Brownian motion is the random motion of particles suspended in a medium (a liquid or a gas). The traditional mathematical formulation of Brownian motion is that of the Wiener process, which is often called Brownian motion, even in mathematical ...
, gravitational settling of the particle and light scattering (
Rayleigh Rayleigh may refer to:
Science
*Rayleigh scattering
*Rayleigh–Jeans law
*Rayleigh waves
*Rayleigh (unit), a unit of photon flux named after the 4th Baron Rayleigh
*Rayl, rayl or Rayleigh, two units of specific acoustic impedance and characte ...
and
Mie scattering) of the particles.
The particle size can have considerable importance in a number of industries including the chemical, food, mining, forestry, agriculture, cosmetics, pharmaceutical, energy, and aggregate industries.
Particle size analysis based on light scattering
Particle size analysis based on light scattering has widespread application in many fields, as it allows relatively easy optical characterization of samples enabling improved quality control of products in many industries including pharmaceutical, food, cosmetic, and polymer production. Recent years have seen many advancements in light scattering technologies for particle characterization.
For particles in the lower nanometer to lower micrometer range,
dynamic light scattering
Dynamic light scattering (DLS) is a technique in physics that can be used to determine the size distribution profile of small particles in suspension or polymers in solution. In the scope of DLS, temporal fluctuations are usually analyzed using ...
(DLS) has now become an industry standard technique. It is also by far the most widely used light scattering technique for particle characterization in the academic world.
This method analyzes the fluctuations of scattered light by particles in suspension when illuminated with a laser to determine the velocity of the Brownian motion, which can then be used to obtain the hydrodynamic size of particles using the Stokes-Einstein relationship. DLS is a fast and non-invasive technique, which is also precise and highly repeatable. Furthermore, since the technique is based on the measurement of light scattering as a function of time, the technique is considered absolute and the DLS instruments do not require calibration.
Amongst its disadvantages is the fact that it does not properly resolve highly polydisperse samples, while the presence of large particles can affect size accuracy. Other scattering techniques have emerged, such as
nanoparticle tracking analysis (NTA), which tracks individual particle movement through scattering using image recording. NTA also measures the hydrodynamic size of particles from the diffusion coefficient but is capable of overcoming some of the limitations posed by DLS. The next generation of NTA technology is called interferometric nanoparticle tracking analysis (iNTA) and is based on the
interferometric scattering microscopy (iSCAT). In contrast to NTA, iNTA has a superior size resolution and gives access to the effective refractive index of the particles.
While the above-mentioned techniques are best suited for measuring particles typically in the submicron region, particle size analyzers (PSAs) based on
static light scattering
Static light scattering is a technique in physical chemistry that measures the intensity of the scattered light to obtain the average molecular weight ''Mw'' of a macromolecule like a polymer or a protein in solution. Measurement of the scattering ...
or
laser diffraction (LD) have become the most popular and widely used instruments for measuring particles from hundreds of nanometers to several millimeters. Similar scattering theory is also utilized in systems based on non-electromagnetic wave propagation, such as ultrasonic analyzers. In LD PSAs, a laser beam is used to irradiate a dilute suspension of particles. The light scattered by the particles in the forward direction is focused by a lens onto a large array of concentric photodetector rings. The smaller the particle is, the larger the scattering angle of the laser beam is. Thus, by measuring the angle-dependent scattered intensity, one can infer the particle size distribution using
Fraunhofer or
Mie scattering models. In the latter case, prior knowledge of the refractive index of the particle being measured as well as the dispersant is required.
Commercial LD PSAs have gained popularity due to their broad dynamic range, rapid measurement, high reproducibility and the capability to perform online measurements. However, these devices are generally large in size (~700 × 300 × 450 mm), heavy (~30 kg) and expensive (in the 50–200 K€ range). On the one hand, the large size of common devices is due to the large distance needed between the sample and the detectors to provide the desired angular resolution. Furthermore, their high price is mainly due to the use of expensive laser sources and a large number of detectors, i.e., one sensor for each scattering angle to be monitored. Some commercial devices contain up to twenty sensors. This complexity of commercial LD PSAs, together with the fact that they often require maintenance and highly trained personnel, make them impractical in the majority of online industrial applications, which require the installation of probes in processing environments, often at multiple locations. An alternative method for PSD is cuvette-based SPR technique, that simultaneously measures the particle size ranging 10 nm-10 μm and concentration in a standard spectrophotometer. The optical filter inserted in the cuvette consists of nano-photonic crystals with very high angular resolution, which enables the analysis of PSD by automatically quantifying
Mie scattering
In electromagnetism, the Mie solution to Maxwell's equations (also known as the Lorenz–Mie solution, the Lorenz–Mie–Debye solution or Mie scattering) describes the scattering of an electromagnetic plane wave by a homogeneous sphere. The sol ...
and
Rayleigh scattering
Rayleigh scattering ( ) is the scattering or deflection of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scat ...
.
The application of LD PSAs is also normally restricted to dilute suspensions. This is because the optical models used to estimate the particle size distribution (PSD) are based on a single scattering approximation. In practice, most industrial processes require measuring concentrated suspensions, where multiple scattering becomes a prominent effect. Multiple scattering in dense media leads to an underestimation of the particle size since the light scattered by the particles encounters diffraction points multiple times before reaching the detector, which in turn increases the apparent scattering angle. To overcome this issue, LD PSAs require appropriate sampling and dilution systems, which increase capital investments and operational costs. Another approach is to apply multiple scattering correction models together with the optical models to compute the PSD. A large number of algorithms for multiple scattering correction can be found in the literature.
[Wei, Y. H., Shen, J. Q. & Yu, H. T. Numerical calculation of multiple scattering with the layer model. Particuology 7, 76–82 (2009).] However, these algorithms typically require implementing a complex correction, which increases the computation time and is often not suitable for online measurements.
An alternative approach to compute the PSD without the use of optical models and complex correction factors is to apply machine learning (ML) techniques.
[Hussain, R., Noyan, M. A., Woyessa, G. et al. An ultra-compact particle size analyser using a CMOS image sensor and machine learning. Light Sci Appl 9, 21 (2020). https://doi.org/10.1038/s41377-020-0255-6]
Microfluidic diffusional sizing
Microfluidic diffusional sizing (MDS) is a method of particle size analysis dependent on the
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
of particles within a
laminar flow
Laminar flow () is the property of fluid particles in fluid dynamics to follow smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral m ...
. The method has found applications in
proteomics
Proteomics is the large-scale study of proteins. Proteins are vital macromolecules of all living organisms, with many functions such as the formation of structural fibers of muscle tissue, enzymatic digestion of food, or synthesis and replicatio ...
and related fields where nano-sized particles may vary in size depending on their environment.
Paints and Coatings
Typically,
paints and coatings are subjected to multiple rounds of particle size analysis, as the particle size of the individual components influences parameters as diverse as hint strength, hiding power, gloss, viscosity, stability and weather resistance.
Mining and Building Materials
The size of materials being processed in an operation is very important. Having oversize material being conveyed will cause damage to equipment and slow down production. Particle-size analysis also helps the effectiveness of
SAG Mills when crushing material.
In the building industry, the particle size can directly affect the strength of the final material, as it observed for
cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
. Two of the most used techniques used for the particle size characterization of minerals are sieving and laser diffraction. These techniques are faster and cheaper compared to image-based techniques.
Food and Beverages Industry
The optimization of the particle size distribution facilitates the pumping, mixing and transportation of foodstuff. Particle size analysis is usually done with any milled food, such as coffee, flour, cocoa powder. It is especially helpful with chocolate quality to ensure there is a consistent taste and feeling when eaten. Furthermore, in the case of food
emulsion
An emulsion is a mixture of two or more liquids that are normally Miscibility, immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloi ...
s, particle size analysis is relevant to predict stability and shelf-life, and optimize homogenization.
Agriculture
The gradation of soils, or
soil texture
Soil texture is a soil classification, classification instrument used both in the field and laboratory to determine soil classes based on their physical texture. Soil texture can be determined using qualitative methods such as texture by feel, and ...
, affects water and nutrient holding and drainage capabilities. For sand-based soils, particle size can be the dominant characteristic affecting soil performances and hence crop. Sieving has long been the technique of choice for soil texture analysis, although
laser diffraction instruments are increasingly used as they considerably speed up the analytical process, and provide highly reproducible results.
Particle size analysis in the agriculture industry is paramount because unwanted materials will contaminate products if they are not detected. By having an
automated particle size analyzer, companies can closely monitor their processes.
Forestry
Wood particles used to make various types of products rely on particle-size analysis to maintain high quality standards. By doing so, companies reduce waste and become more productive.
Aggregates
Having properly sized particles allow aggregate companies to create long-lasting roads and other products. Particle size analysis is also routinely conducted on
bitumen
Bitumen ( , ) is an immensely viscosity, viscous constituent of petroleum. Depending on its exact composition, it can be a sticky, black liquid or an apparently solid mass that behaves as a liquid over very large time scales. In American Engl ...
emusions to predict their stability and their behavior.
Biology
Particle size analyzers are used also in biology to measure
protein aggregation
In molecular biology, protein aggregation is a phenomenon in which intrinsically disordered proteins, intrinsically-disordered or mis-folded proteins aggregate (i.e., accumulate and clump together) either intra- or extracellularly. Protein aggre ...
.

DLS is a particularly appreciated technique for the characterization of nanoparticles designed for drug delivery, such as vaccines. DLS instruments are for instance part of the quality control process for
mRNA vaccine
An mRNA vaccine is a type of vaccine that uses a copy of a molecule called messenger RNA (mRNA) to produce an immune response. The vaccine delivers molecules of antigen-encoding mRNA into cells, which use the designed mRNA as a blueprint to b ...
s formulated in
lipid nanoparticle carriers.
Selecting the most appropriate technique for size analysis
There is a large number of methods for the determination of particle size, and it is important to acknowledge that these different methods are not expected to give identical results. The size of a particle depends on the method used for its measurement, and it is important to choose the method that is most relevant to the application.
The "See also" section covers many of these techniques. In most of them, the particle size is inferred from a measurement of, for example: light scattering; electrical resistance; particle motion, rather than a direct measurement of particle diameter. This enables rapid measurement of a particle size distribution by an instrument, but does require some form of calibration or assumptions regarding the nature of the particles. Most often this includes the assumption of spherical particles, thus giving a result which is an
equivalent spherical diameter
The equivalent spherical diameter of an irregularly shaped object is the diameter of a sphere of equivalent geometric, optical, electrical, aerodynamic or hydrodynamic behavior to that of the particle under investigation.
The particle size of a pe ...
. Thus, it is usual for measured particle size distributions to be different when comparing the results between different equipment. The most appropriate method to use is normally the one where the method is aligned to the end use of the data.
For example, to choose whether a chemical compound should be measured by
dynamic light scattering
Dynamic light scattering (DLS) is a technique in physics that can be used to determine the size distribution profile of small particles in suspension or polymers in solution. In the scope of DLS, temporal fluctuations are usually analyzed using ...
or
laser diffraction, one generally considers the expected size range, the sample type (liquid or solid), the amount of sample available, the chemical stability, as well its application field. If designing a sedimentation vessel, then a sedimentation technique for sizing is most relevant. However, this approach is often not possible, and an alternative technique must be used. An online Expert system to assist in the selection (and elimination) of particle size analysis equipment has been developed.
Expert system for equipment selection
/ref>
See also
* Sieving
* Sieve analysis
A sieve analysis (or gradation test) is a practice or procedure used in geology, civil engineering, and chemical engineering to assess the particle size distribution (also called ''gradation'') of a granular material by allowing the material to p ...
* Laser diffraction analysis
* Sedimentation
Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to th ...
* Elutriation
* Microscope
A microscope () is a laboratory equipment, laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic ...
counting
* Coulter counter
A Coulter counter is an apparatus for counting and sizing particles suspended in electrolytes. The Coulter counter is the commercial term for the technique known as resistive pulse sensing or electrical zone sensing. The apparatus is based on t ...
* Dynamic light scattering
Dynamic light scattering (DLS) is a technique in physics that can be used to determine the size distribution profile of small particles in suspension or polymers in solution. In the scope of DLS, temporal fluctuations are usually analyzed using ...
* Imaging Particle Analysis
*Aerosol mass spectrometry
Aerosol mass spectrometry is the application of mass spectrometry to the analysis of the composition of aerosol particles. Aerosol particles are defined as solid and liquid particles suspended in a gas (air), with size range of 3 nm to 100 ...
* Obscuration SPOS
References
{{Authority control
Particulates
Analytical chemistry
Laboratory techniques