Laser Diffraction: A Firm Foundation for Particle Characterization

For manufacturers of solid and/or particulate products, particle size is frequently a critical parameter, and is one of the principal variables routinely used to characterize their materials. Finished product performance and in-process behavior often correlate with particle size and/or size distribution, creating a universal requirement for pertinent information. Laser diffraction provides a robust particle sizing solution for many applications, and has become the technique of choice in many industrial sectors.

The last decade or so has seen the development of laser diffraction analyzers that are increasingly flexible, easier to use, and highly automated. Many users rely heavily on such systems—in the laboratory for development and QC, and in production facilities for routine process monitoring. The commercial availability of laser diffraction technology for off-, at-, in-, or online implementation enables its efficient use throughout the product lifecycle.

Understanding the importance of laser diffraction relies, in the first instance, on recognizing the relevance of particle size data. This article discusses how this vital technique continues to evolve to meet industrial requirements.

Why measure particle size?

Looking at the applications for laser diffraction in different manufacturing environments, there is clear commonality in the reasons for measuring particle size. Some of the most important aspects of performance influenced or controlled by particle size include:

Those designing fuel injection systems and producing cement exemplify a group of laser diffraction users who control particle size to meet a required rate of reaction. Engine designers control the size to which fuel is atomized to modify both the rate and efficiency of combustion; cement manufacturers mill to a size specification to control the rate of hydration of the product during use, since this influences developed strength. In either case, finer particles/ droplets present a larger specific surface area and therefore react more quickly than those having a coarser distribution.

In the pharmaceutical industry, the closely related issue of dissolution rate is of prime importance because of its impact on bioavailability. That said, this is a sector that routinely manipulates particle size toward a number of goals: to achieve a stable (nonsettling) suspension for oral dosage, for example, or a high respirable fraction for pulmonary delivery. Detergent and agrochemical producers share the requirement to tailor dissolution rate, while ease of inhalation is frequently an issue for manufacturers trying to reduce the risk of harm, and many measure particle size to confirm that a product cannot be easily inhaled.

Packing density, which is a function of particle size and distribution, is closely controlled in many industries to achieve superior product quality. Those producing components via mold-filling processes, for example, from metal or ceramic powders, manipulate particle size distribution to achieve an efficient fill with little voidage, thereby ensuring a strong, flaw-free product postmelting/sintering. Powder coating manufacturers similarly optimize size distribution, aiming for a closely packed layer of particles that will melt efficiently at lower temperatures.

Particle packing is also an issue in the production of suspensions with desirable rheological behavior. For a given volume fraction of suspended solids, increasing polydispersity (the breadth of the particle size distribution) reduces viscosity—an effect that can be exploited to increase the solids loading in, for example, a paint, or ceramic suspension, without compromising viscosity. The tendency of a suspension to settle, or an emulsion to coalesce, can also be modified through efficient particle size control—smaller particles, or droplets being less prone to either phenomenon.

Finally, although many control particle size for technical reasons, for others it is customer perception that is the driving force. With consumer products, food being the prime example, particle size can influence our enjoyment or perception of quality, both of which are valuable commodities. The particle size of coffee, for example, impacts the flavor released during brewing, while a fine particle size in chocolate imparts a smooth mouth feel that is superior to a grainy finish.

Introducing laser diffraction

The preceding exploration of the links between particle size and performance is far from exhaustive, and not all of these requirements are met by laser diffraction alone. The technique does, however, have several key attributes that confer substantial appeal:

  • Flexibility (ability to measure emulsions, suspensions, dry powders, and sprays)
  • Broad measurement range
  • Speed of data acquisition
  • Automated measurement
  • Nondestructiveness.

Figure 1- Examples of diffraction patterns caused by light scattering from a particulate sample.

Laser diffraction is an ensemble technique, generating a single result for the whole sample rather than building up a size distribution from measurements of individual particles. A sample passing through a collimated laser beam scatters light over a range of angles (see Figure 1). Large particles result in a high scattering intensity at relatively narrow angles to the incident beam, while smaller particles produce a lower intensity signal at much wider angles. Laser diffraction analyzers determine particle size from the recorded angular dependence of the intensity of light scattered by a sample, using an appropriate theory of light behavior. The Mie theory is typically applied, as recommended in the latest version of ISO 133201 (the ISO standard for laser diffraction), for all particles in the size range over which laser diffraction is used: 0.1–3000 μm.

The nondestructive nature of laser diffraction is an inherent advantage of the technique. Furthermore, because particle size measurement by laser diffraction relies on the laws of light behavior, calibration requirements are minimal. The measurement range over which the method is applicable is well matched to many manufacturing applications, and measurement times are short. Over and above these attractions, however, the adoption of laser diffraction has been driven by instrument manufacturers harnessing the technique’s intrinsic advantages in reliable, easy-to-use systems. Developments over the last 10–15 years have been pivotal.

Evolution of a favored particle sizing technique

The practical realization of reliable systems that can deliver across the full measurement range accessible by laser diffraction has required significant advances in optical hardware. Early systems often employed different lenses to measure over relatively narrow size ranges. With such systems, the associated switching of lenses and accompanying realignment of the instrument limit flexibility and/or productivity and are especially disadvantageous when analyzing samples across a very broad particle size distribution. Today, improvements in optical design have largely eliminated the need for multilens systems, although they remain in use: Their functionality is less of an issue for anyone routinely measuring closely similar samples within a well defined size range.

At the same time, optical improvements have delivered significantly enhanced resolution. In simple terms, the precision of particle size measurement relies not only on detecting particles at either extreme of the distribution, but also on accurately resolving the population into size fractions. This is an area of performance in which even the most modern laser diffraction analyzers vary considerably. Poor resolving capability compromises the generation of a reliable particle size distribution, and the ability to accurately quantify, for example, the amount of fines or coarse particles present in a sample.