Sedimentation Particle-Size Analysis

For particle characterization, one of the most important measurements is the determination of the distribution of particle size in a sample. While this may seem to be a simple measurement, the reality is often complicated by a number of factors, particularly if the particle size becomes very small. At least a half dozen different techniques are used for particle sizing, ranging from acoustic analysis to chromatographic methods. For many applications, the particles of interest are truly nanoscale, making them smaller than wavelengths of light.

This does not mean that light cannot be used to measure the particles; in fact, the most common methods of particle sizing rely on diffraction and scattering of laser light by the sample. However, they also rely on mathematical theories and formulas to model the diffraction and scattering data in order to determine the particle size distributions. This is not as much of a drawback as it might seem, because all of the methods rely on certain assumptions and models of nanoscale particles’ behavior. Another method is the use of sedimentation phenomena.

The basic mathematical theory of sedimentation is related to Stokes’s Law, which governs the behavior of spherical particles suspended in a liquid. Stokes’s Law relates the terminal velocity of a particle to its diameter and the properties of the liquid in which it is suspended, such as viscosity and density. All things being equal, larger particles settle more rapidly than smaller particles.

In a dispersion containing particles of different sizes, the differential sedimentation rate provides a method for determining the distribution of particle sizes present in the sample. As they settle, particles are detected by a sensor; typical methods include X-ray attenuation and light obscuration. These measurements offer a basis for determining the density of particles of a given size at a particular moment. As the sedimentation process continues, the sensor essentially scans through the total size range present in the sample, building up the complete distribution.

While some instruments use gravity alone to generate sedimentation, other instruments use centrifugation to help speed the process and alter the dynamic range of the measurement technique. Sedimentation instruments are used with a wide variety of samples: not only powdered materials, but also various emulsions. Generally speaking, these instruments are used in non–life science applications. For example, there are significant applications in the food industry.

There are only a handful of vendors of sedimentation particle-sizing instruments, with Micromeritics and Brookhaven being the most significant. Other competitors in this market are CPS Instruments and LUM GmbH. The total market for sedimentation systems was about $7 million in 2006.

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