Particle sizes analysis methods
Why quality control in manufacturing and research is often based on particle size analysis
Particle sizes make a difference. You’re skeptical? Surely you’ve experienced that longing moment when a sugary piece of creamy milk chocolate melts on your tongue? Well, it melts only if the particle size of the sugar crystals and cocoa powder contained in the cocoa butter does not exceed a certain level so they do not interfere on the tongue.
Particle sizes play an important role in everyday life, in numerous applications and in many industries. This is because particle size has a decisive effect on the properties of products. How quickly drug substances are released in the body, how well fertilizers dissolve in soil or how evenly paint distributes on a wall – all this depends on particle sizes. This makes particle size analysis an important method to analyze powders, granulates and other bulk materials, but also dispersions, for example suspensions (solid-liquid mixtures) or emulsions (liquid-liquid mixtures).
In particular for quality control, particle size analysis is a standard method to test the quality of a product. We’ll explain which measurement methods you should know about.
Contents
- Sieve analysis
- Image analysis – either static (SIA) or dynamic (DIA)
- Static light scattering (SLS) or laser diffraction
- Dynamic light scattering (DLS)
On your methods, get set, go – a closer look at particle size analysis
Particle size is a term used to compare the dimensions of solid particles (grains), liquid particles (droplets) or gaseous particles (bubbles). In reality, particles are often very complex in shape. For measurement purposes, particle size analysis assumes them to occur in simple, standardized shapes: a sphere or a cuboid.
There are technologies and measuring instruments to analyze a wide spectrum of sizes, ranging from nanoparticles to pebbles. These methods can be based, for example, on light, ultrasound, the electric field, gravity or centrifugation. Depending on the size range or the material properties of the particles, each method is more or less optimally suitable, and not all methods cope with every task. As a user, be prepared that both sample preparation and evaluation can be challenging.
Sieve analysis: the traditional method
Sieve analysis is, at least at first glance, the simplest method for particle size analysis. This method is used to determine the grain size distribution of organic and inorganic bulk materials (powders, granules, etc.) and is suitable for particle sizes ranging from several micrometers to several millimeters or even centimeters.
Sieve analysis is a basic, cost-effective particle size analysis method. In a sieve tower mounted onto a sieve machine, test sieves are arranged one above the other, and the mesh size decreases from top to bottom. The movement of the sieve machine (vibrating, shaking, tapping, etc.) causes the bulk material to pass through the different screens (dry screening) and to separate according to particle size.
Areas of application
Sieve analysis is used in the food, pharmaceutical and chemical industries for production and quality control of powdered and granular bulk materials. Particle size matters for the properties of many of these industries’ products, for example the strength of concrete, the taste of chocolate, the dissolution behavior of tablets or the flowability and dissolution behavior of washing powders.
Disadvantages
It involves many manual tasks, which are all potential sources of error. Nor should the time required for initial weighing, sieving, backweighing, calculating the result and cleaning the sieves be underestimated. Thankfully there are alternative techniques for particle size analysis.
Image analysis – either static (SIA) or dynamic (DIA)
Two different image analysis methods are available to characterize particles by image analysis: static image analysis (SIA) and dynamic image analysis (DIA).
Static image analysis
For static image analysis, the sample to be measured is placed on a slide and evaluated under a microscope. This method, however, is usually only applied for very small sample quantities.
Disadvantage: The quality and optical resolution of the individual images, although very high, is not sufficient to obtain a statistically relevant overview of the entire sample. This method is also very time-consuming and delivers only a few individual images.
Dynamic image analysis
In contrast, dynamic image analysis for particle sizes above 1 µm provides fast and precise data for the particle size analysis of powders, granules and pellets, but also for particles in a suspension. A camera captures digital images of a particle stream (either in free fall, in a jet of compressed air or in a liquid stream), which are then used by dedicated software to record the length and width of each individual particle, as well as to map its shape. This method is used particularly for quality control in the pharmaceutical industry, but is also becoming established in more and more industries and laboratories for routine determination of particle size and shape.
Disadvantage: Appropriate algorithms are often needed to reliably correlate DIA data and sieve data (which is actually more of an additional expense or effort than a disadvantage).
Static light scattering (SLS) or laser diffraction
Many quality control laboratories consider laser diffraction a fast and robust method for particle size analysis. The main reasons why laser diffraction is so popular are its enormous flexibility – it can be automated – and its wide range of applications. In addition, this technique is easy and fast to learn.
This well-established method is suitable for determining particle sizes of between 10 nm and 4 mm. In principle, it therefore covers a very wide spectrum of applications. The measurement principle of static light scattering is based on the angle of laser light diffraction at the particles. Both wet and dry samples can be analyzed in a matter of seconds.
Disadvantage: Since static light scattering (SLS) measures particle sizes only indirectly by detecting intensity distributions of laser light scattered at different angles by the particles, laser diffraction is unsuitable for very small particles. Tiny particles lead to large scattering angles, and the scattered light pattern becomes more diffuse.
Dynamic light scattering (DLS)
In contrast, dynamic light scattering works with particle sizes in the nanometer range. Suspensions and emulsions can be analyzed with this method. Dynamic light scattering is unsuitable for dry measurements.
DLS is based on the principle that smaller particles move faster in a liquid than larger particles. Due to the so-called Brownian motion of the particles, the scattered light intensity is subject to fluctuations over time, becoming more or less pronounced, and this depends on the size of the particles. This allows the hydrodynamic diameter to be determined as the particle size. DLS can be used to measure both highly diluted and highly concentrated samples.
Disadvantage: Sample preparation plays a very important role in dynamic light scattering. Above all, contamination (dust, etc.) must be avoided in order to get reliable results.
A brief summary of the application areas for particle size analysis
Used for the analysis of product properties, quality assurance of products, process optimization or to monitor clean room facilities or the natural environment, particle analysis is an important method in a wide variety of areas.
To make sure you find the perfect instrument for your own particle size analysis application, take a look at the world's largest market overview of particle analyzers.
