The Effect of Particle Shape on Particle Size Analysis

Particle size analysis has a wide variety of applications encompassing virtually all industries. Numerous automated techniques are available for measuring particle size distribution and normally all these techniques report particle size in units of equivalent spherical diameter. This is needed because of the ambiguity of describing the diameter of an irregularly shaped particle and of constraints inherent in the instrument detection system.

A range of measurement methods that report in equivalent spherical diameter often produce different particle size distributions when particles are non-spherical. Understanding what each technique actually measures, how the measurement is performed and how the quantity measured is transformed into equivalent spherical diameters are critical aspects to consider when selecting the right particle sizing technique. To demonstrate this, samples of glass beads, garnet and wollastonite were analyzed mainly due to their shape differences. The analytical techniques employed are static laser light scattering, electrical sensing zone (Coulter Principle), X-ray sedimentation and dynamic image analysis. The impact that particle shape has on the reported particle size is discussed for each of these materials and techniques.

Different Techniques used to Measure Particle Size

The different techniques used to measure particle size are listed below:

  • Laser light scattering
  • Electrical sensing zone
  • Sedimentation
  • Dynamic Imaging Analysis

Laser Light Scattering

The fundamental assumption of laser light scattering is that spherical particles having different sizes scatter light in patterns of intensity versus the scattering angle that are specific to the particle diameter. The scattering patterns are additive when particles of different sizes are involved. The particle sizes reported by this method are the diameters of spheres in an assemblage of spherical particles that produce the same, or most nearly the same, scattering pattern as that detected by the instrument.

Electrical Sensing Zone

The electrical sensing zone method measures the difference of the electrical signal strength end-to-end across an orifice that is filled only with electrolyte and the signal strength when a non-conducting particle is present in the electrolyte as it passes through the orifice. From this measurement, the electrolyte volume displaced by a particle, and therefore the volume of the particle, is calculated. The particle size reported by this technique is essentially the diameter of a sphere, which displaces an identical volume of electrolyte as the detected particle.

Sedimentation

Particle size determination  by sedimentation is calculated by measuring the terminal velocities of particles settling in a fluid medium. The particle size reported by this method is the diameter of a sphere of the same material that settles at the same terminal velocity as the test particle, in the same fluid and under the influence of the same force.

Dynamic Image Analysis

Dynamic image analysis obtains images of particles as they pass through the detection zone. Linear dimensions of the cross-sectional shape of each image are studied using a variety of shape parameters. The particle size reported by this technique is evaluated from these linear dimensions.

Analytical Technique Properties

The table below lists the analytical methods, the fundamental property determined, and the Micromeritics analyzer (each using a different technique)  used to generate the data of this study.

Analytical Technique Basic Measurement Particle Variables Affecting Measurement Micromeritics Analyzer
Laser light scattering Light intensity versus scattering angle Refractive index, shape, orientation Saturn® DigiSizer II
Electrical sensing zone Change in electrical signal across a conducting orifice Porosity, conductivity Elzone® II
Sedimentation Settling velocity Density, shape SediGraph® III
Dynamic image analysis Linear dimensions of projected cross-section of particle Particle orientation Particle Insight

Testing Results

The different materials used include the following:

  • Garnet
  • Wollastonite
  • Glass spheres

The individual test results for each material are discussed below:

Garnet

Garnet crystals are almost cubic shaped. As the cube diagonal is almost 30% longer than a sphere of the same volume, larger particle size is generally reported by methods that take orientation into account, such as laser light scattering and dynamic image analysis.

Wollastonite

Wollastonite particles are rod-shaped. As the apparent dimensions of a rod-shaped particle can vary based on its orientation, the detection techniques that were affected by particle orientation, such as laser light scattering and dynamic image analysis showed larger particle size measurements and broader peaks.

Glass Spheres

Glass spheres produced the most consistent results for the different techniques. Since the particles are spherical, their orientation showed no effect on the measurement. Microscopy of the sample indicated that the glass spheres contained air bubbles in variable sizes, reducing the density of some of the spheres. Due to this, the SediGraph detects some of the particles as being undersized and hence widens the distribution and shifts it to a slightly finer size.

This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.

For more information on this source, please visit Micromeritics Instrument Corporation.

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