Comparison of particle characterization methods

Sieve Analysis: Committed to tradition

Sieve analysis still is the traditional and most commonly used method for particle size determination. A sieve stack consists of several sieves with increasing aperture size stacked upon each other and the sample is placed on the uppermost sieve.

The stack is clamped to a sieve shaker and set into vibration for usually 5 – 10 minutes. As a result, the particles are distributed to the sieves in the stack (fractions) according to their size.

Ideally, the particles pass the smallest possible sieve aperture with their smallest projection surface. Taking cubic particles as a model, this corresponds to the edge length of the cube. For lenticular particles, the size determined by sieve analysis would be a value between the thickness and the diameter of the lense, as the particle is oriented diagonally towards the sieve aperture (see figure on the right).

Hence, sieve analysis is a technique which measures particles in their preferred orientation with a tendency to determine mostly the particle width.

Sieve analysis is carried out up to a point where the sample mass on the respective sieves no longer changes (= constant mass). Each sieve is weighed, and the volume of each fraction is calculated in percent by weight, providing a mass-related distribution.

The resolution of sieve analysis is limited by the number of obtainable size fractions. A standard sieve stack consists of a maximum of 8 sieves which means that the particle size distribution is based on only 8 data points. Automation of the procedure is hardly possible which makes it rather time-consuming. The processing steps of sieve analysis are initial weighing, 5 – 10 minutes sieving, back weighing, and cleaning of the sieves.

The most common sources of errors are overloading of the sieves (blocking of sieve apertures, too coarse results); old, worn or damaged sieves (too fine results), or errors in data transfer. It should also be taken into account that the aperture sizes of new standard-compliant sieves are also subject to certain tolerances.

The average real aperture size of a 1 mm sieve, for example, is permitted to deviate about ±30 µm, for a 100 µm sieve it is ±5 µm (i.e. the average real aperture size lies between 95 and 105 µm). However, this is just the mean value which implies that some of the apertures can be even larger.

Dynamic Image analysis and Laser Diffraction

With static laser light scattering (SLS) analysis, also called laser diffraction, particle size is measured indirectly by detecting intensity distributions of laser light scattered by particles at different angles. The figure below shows the setup of the Microtrac SYNC, a state-of-the-art laser granulometer with its unique Tri-Laser-Geometry and an additional camera module.

1. Camera, 2. Laser 1, 3. Laser 2, 4. Detector array, 5. Light source DIA, 6. Detector array, 7. Laser 3

This technique is based on the phenomenon that light is scattered by particles and the correlation between intensity distribution and particle size is well-known. Simply put, large particles scatter the light to small angles while small particles produce large angle scattering patterns.

Large particles produce rather sharp intensity distributions with distinctive maxima and minima at defined angles, the light scattering pattern of small particles becomes more and more diffuse and the overall intensity decreases. It is particularly difficult to measure differently sized particles in a polydisperse sample as the individual light scattering signals of the particles superimpose each other.

Static laser light scattering (SLS) is an indirect method which calculates particle size distributions based on super-imposed scattered light patterns caused by a whole collective of particles. Additionally, the optical properties of the material (refractive index) must be known for small particles for the calculation to produces reliable results.

Since the theory of SLS is based on the assumption of spherical particles, shape evaluation is not possible. A disadvantage of the SLS is the relatively low resolution and sensitivity. Oversized grain can also only be detected by modern analyzers from approx. 2 vol%. In order to resolve multi-modal distributions, the size of the two components must differ by at least a factor of 3.

The big advantage of laser diffraction is that it is a fast, established technique offering great flexibility. With a measuring range from a few nanometers to millimeters, the method can be used for most requirements in particle technology. Image analysis cannot be used for particles <1 µm. The analyses with SLS devices are easy to carry out and can be largely automated.

Laser diffraction and dynamic light scattering (DLS)

Dynamic Light Scattering (DLS) is based on the Brownian motion of particles in suspensions. Smaller particles move faster, larger particles move more slowly. The light scattered by these particles contains information about the diffusion speed and thus about the size distribution. The particle size determined is a hydrodynamic diameter. The Stokes-Einstein equation describes the relationship between particle size, diffusion rate, temperature and viscosity:

The hydrodynamic diameter of the DLS is usually somewhat larger than the average particle size determined by static light scattering. DLS is particularly suitable for the analysis of nanoparticles where static light scattering reaches its limits. On the other hand, DLS only works for particle sizes up to 10 µm and is increasingly imprecise above 1 µm. In addition, many DLS analyzers offer the possibility of determining the zeta potential and the molecular weight.

Measuring principle of dynamic light-scattering

1. Detector |  2. Reflected laser beam & scattered light |  3. Sapphire window |  4. Y-beam splitter |  5. GRIN lens |  6. Sample | 7. Laser beam in optical fiber |  8. Laser