Beyond the sieve: why sieves and laser diffraction disagree on non-spherical particles

Introduction

Sieving is a long-established technique for particle size analysis, widely used across industries such as building materials, pharmaceuticals, and additive manufacturing. While effective, sieving can be labor-intensive and prone to errors stemming from factors like human handling, sampling, mesh degradation, and blockages. Additionally, sieve methods can require substantial sample quantities, which can be restrictive in certain applications. 

Laser diffraction offers an attractive alternative to sieving for several reasons. It is a standardized and widely adopted technique in many industries, delivering rapid measurements with smaller sample requirements. Laser diffraction also provides highly repeatable and reproducible results, making it a preferred method for routine particle size analysis adopted by many industries that have transitioned away from sieving.

However, differences in measurement principles between sieving and laser diffraction can lead to variations in the particle size distributions the techniques measure. This article explores the origins of these discrepancies and the Mastersizer 3000+ software features that support the transfer of sieve-based specifications to laser diffraction workflows.

Different definitions of size and differences in measurement techniques

Sieve-based particle size distributions describe the mass distribution of a sample across size classes that are defined by sieve mesh diameters. These classes are typically logarithmically spaced, with the number of classes influenced by the material's size range, ISO standards (e.g., ISO 2391, ISO 3310), or industry-specific standards (e.g., USP 786). For spherical particles, the retaining and passing mesh diameters accurately represent particle size. Non-spherical particles, however, typically pass through a sieve mesh via their smallest or intermediate caliper diameter, though the repeatability of this may depend on the method and intensity of agitation used (e.g., manual shaking, mechanical sieving, wet or dry sieving, see Fig. 1). 

Laser diffraction works on the principle of volume equivalent spheres to report particle size as it is a volume weighted technique. Therein lies the initial challenge to overcome if comparing or transferring methods from sieving to laser diffraction. Sieving measures the dimension of a particle that allows it to pass through the mesh. This dimension is not equal to the volume-equivalent spherical diameter, except when measuring perfectly spherical particles. 

The mismatch between sieving and laser diffraction is demonstrated by analyzing the same nominal sieve fraction for volcanic ash samples with different shaped particles using the Mastersizer 3000+ (Fig. 2). The laser diffraction Particle Size Distributions (PSDs) report a large variation in the Dv10 and Dv90 percentiles across the different samples even though, according to sieving, they contain the same sized particles (Table 1). This variation relates to the shape of the particles as confirmed using the Hydro Insight, our in-line Dynamic Imaging accessory for Mastersizer 3000(+). Dispersion images of these volcanic ash samples (Fig. 3) coupled with quantitative shape parameter data from the Hydro Insight (Fig. 4) reveals the range of particle shapes present, from flat plates (Campanian) to elongated rods (Mazama) and blocky grains (Mount St Helens).

Integrating sieve-based methods with laser diffraction

The samples shown above are material produced by explosive volcanic eruptions known as “volcanic ash”. It consists of <2 mm fragments of volcanic rock that can vary significantly in size and shape based on the explosivity of the eruption and the type of volcano that formed it. The size and shape of this material is a key input for models that predict the atmospheric spread of this material after an eruption, which is important as volcanic ash poses a hazard to human health and infrastructure.

As with many industries, historically sieving has been used to measure the size of volcanic ash. Researchers are finding out, however, that due to the diverse shapes of the particles and the practical challenges of sieving materials with a high proportion of fine particles (<50 µm), sieve data alone often fails to provide sufficient insight into the material's characteristics.  Now techniques such as laser diffraction and dynamic imaging are being incorporated into workflows used to characterize the size of volcanic ash.

Whilst laser diffraction is an attractive option for volcanologists, like many industries, researchers still need to be able to incorporate some sieve-based data for material that is above the upper size range of laser diffraction instruments (>3.5 mm). Similarly, researchers also want to be able to compare size data across different techniques especially as sieving is still a widely used method.

The Mastersizer 3000+ software, Mastersizer Xplorer, has several software tools to support instrument users comparing laser diffraction PSDs with sieve-based results, as well as features to support method and specification transfer. These features include (1) result extension, (2) result emulation, and (3) custom user sizes which are available in the measurement and SOP settings (Fig. 5). The result extension option extends laser diffraction results with coarse sieve data (e.g., >2 mm). Result emulation supports method transfer by adjusting for shape-related differences that arise between sieving and laser diffraction PSDs. Custom user sizes allow data visualization using sieve-based size classes, including the φ (phi) or Krumbein scale. More details are available in the webinar linked below.

Final thoughts - explore more and get in touch

In this article, we've explored the limitations of sieve-based methods for non-spherical particle size characterization and demonstrated how laser diffraction and image analysis can explain discrepancies in the two sizing techniques and give more insight into our particles. By understanding the differences between sieving and laser diffraction, you can make more informed decisions in your particle characterization processes.

For those interested in diving deeper into this topic, we recommend the following resources:

• How to replicate a sieve analysis method on the Mastersizer 3000 – recorded webinar
• Top 10 reasons to migrate from sieving to laser diffraction for routine particle size measurements - whitepaper
• Five reasons to add the Hydro Insight to your Mastersizer 3000 - whitepaper

We encourage you to get in touch if you have any questions regarding method transfer from sieving, laser diffraction methods or any general inquiries. Our team is here to support you and provide the information you need to optimize your particle characterization techniques.

You can contact us via the support portal or visit our knowledge center for more details.