Fine particles play a crucial role in defining the properties of both natural and synthetic materials, significantly influencing processes such as adsorption, dissolution, and reaction rates.
In many cases, the impact of fine particles is determined by properties such as size, shape, surface area, porosity, and agglomeration. Controlling these characteristics is essential to achieving desired outcomes, and precise measurement is a key aspect of effective regulation.
Various methods are available to measure particle dimensions, each with its own advantages and limitations. Selecting a method that is inappropriate for the specific application can significantly compromise the quality of the measurement.
The majority of industrial particles fall within a similar size range, as illustrated below.
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Various approaches serve different purposes when compared by the size range they cover.
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Choosing the right method for particle size analysis requires a clear understanding of the distinctions between available approaches to ensure the chosen technique aligns with the application’s specific requirements.
Additional factors related to customer samples must also be taken into account. For instance, if particle counts or concentrations are important, specific methods such as dynamic image analysis, light blockage, or electrical sensing zone (the Coulter Principle) may be more suitable.
Similarly, the medium in which the particles are suspended—whether wet/organic or dry—can influence the choice of technique. These considerations highlight the diverse reasons for selecting among the many particle analysis methods available.
It is important to note that different measurement approaches can yield slightly varied results, even when analyzing the same sample. This variation often arises due to factors such as particle shape, which can influence the interpretation of size data. While these differences may seem significant, it is essential to recognize that no single method is inherently "right" or "wrong." Instead, each approach provides unique insights based on its principles of measurement.
To illustrate these differences, an experiment was conducted several years ago in collaboration with Vision Analytical, Micromeritics Instrument Corporation, and MVA Scientific Consultants.
The study analyzed three distinct sample types, each chosen for its unique shape characteristics:
The first was a glass bead, which is normally very round and measures around 50 μm. The second was a garnet sample. Garnet particles are frequently employed as abrasives to remove material. They have irregular shapes and measured 50 μm using a size-only method. The last was Wollastonite, which is mined and has numerous industrial uses. Unlike glass beads and garnets, Wollastonite is long and shaped like a needle.
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Several methods were employed to analyze the three distinct particle types.
Laser light scattering or laser diffraction is a widely used technique that indirectly measures particle size by calculating the angular light intensity scattered from the particles. When particles are present, the scattering patterns are mathematically analyzed to estimate their size. While effective for many applications, the method assumes particles are spherical, which may introduce inaccuracies for irregularly shaped particles.
Sedimentation was also utilized. Based on Stokes' law, sedimentation calculates particle velocity to determine particle size. As particles settle in a medium, their velocity is proportional to their size. However, this is another indirect method, and the results assume all particles are round.
Another approach employed was the electrical sensing zone (or Coulter Principle). This technique involves suspending particles in a conductive liquid and passing them through a small aperture. The method counts particles and measures the voltage change caused by each particle's passage, which is proportional to its volume. Similar to sedimentation and laser diffraction, it assumes particles are spherical.
Dynamic image analysis was utilized last—this technique measures particles by capturing images as they move across a detection zone. Size and shape are determined from the 2D projected image of each particle. As a direct measurement technique, it provides more comprehensive data, offering insights into both the size and shape of particles.
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Results
Glass Sphere Results
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For glass beads, which are nearly perfectly spherical, similar results were anticipated across the various measurement techniques. Since most methods relate particle size to an equivalent sphere, the median particle size (D50) values are comparable across all approaches.
An analysis of glass spheres using the four particle size distribution methods revealed that, while the D50 values are closely aligned, the breadth of the particle size distribution varied between methods. It is important to note that all these methods assume the particles are round.
Garnet Results
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Garnet particles are cube-shaped, not spherical. The diagonal distance between opposite corners is roughly 30 % longer in cube-shaped samples than in an equal-volume sphere.
As a result, techniques such as light scattering and image analysis, which are less constrained by spherical assumptions, yield larger particle size results compared to methods that rely on the spherical diameter.
Analysis of the garnet sample using the four particle size distribution techniques shows less agreement in the D50 values. Due to the irregular shape of the particles, the statistical histograms are wider. As mentioned earlier, all measurements assume the particles are round.
Wollastonite Results
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The flow and shape effects that caused small variations in garnet particle sizes are more pronounced with rod-shaped Wollastonite. The light-scattering method measures the largest particle dimension, while the electrical sensing zone and sedimentation techniques provide a particle diameter based on spherical assumptions.
Light scattering also has a thicker flow path than the image analyzer, meaning some particles may align with their smaller dimension facing the detection system.
Analysis of the Wollastonite samples using four particle size distribution methods shows minimal agreement in the D50 values. These methods, which assume the fiber-like particles are “round,” highlight the lack of consistency and greater variation in the size distribution histograms.
Technical Reasons for Differences
Technical differences between techniques become more pronounced as particle shapes deviate from round. Additional observations are listed below.
Number-based methods, such as dynamic image analysis and electrical sensing zones, can provide both volume-weighted and number-weighted size distributions.
When comparing these number-based methods with others that primarily report volume-weighted statistics, it is important to choose the same type of weighted statistic for a more accurate and meaningful comparison.
Each approach varies in how it measures particles, which impacts reported particle size:
- Sedimentation: The velocity of particles falling in a liquid is based on the drag on the particle’s surface. Material properties like skeletal density can affect the particle size in sedimentation. Heavy particles will sediment more quickly.
- Laser diffraction: The correct refractive index of the sample and the medium in which it is suspended is crucial. Enter this data accurately to guarantee satisfactory size results. Real and imaginary refractive indexes may be required, depending on the size and optical mathematical model utilized.
- Electrical sensing zone: Measures the displaced particle volume. While no prior knowledge of particle properties is required, it is crucial to recognize that round and irregular particles will have different volume displacements.
- Dynamic image analysis: Creates measurements from 2D projected cross-section regions of images of particles. No specific optical properties are needed for calculations.
Conclusion
While size results may vary between techniques, especially for irregular particles, none of the outcomes are inherently incorrect.
The results demonstrate that, even in the absence of complete agreement, there is a clear correlation. By understanding the shape properties of their particles, users can more effectively interpret the outcomes from any measurement method.
This information has been sourced, reviewed and adapted from materials provided by Vision Analytical Inc.
For more information on this source, please visit Vision Analytical Inc.