Basic Guide to Particle Size Analysis-2
Particle Shape
In addition to particle size, the shape of constituent particles can also significantly affect the performance or handling of particulate materials. Currently, in various industries, shape measurements are being conducted alongside particle sizing to better understand products and processes. Some of the areas where shape is important are as follows:
• Reactivity and solubility (e.g., pharmaceutical ingredients)
• Powder flow and handling (e.g., drug delivery systems)
• Characteristics of ceramic sintered body (e.g., ceramic filters)
• Abrasive efficiency (e.g., SiC saws)
• Texture and mouthfeel (e.g., food ingredients)
Especially when aggregates or primary particles are present, the dispersion state of particulate materials can be determined by using shape.
How is shape defined?
Particles are complex 3D objects, and as with particle size measurement, it is necessary to somewhat simplify the description of particles to allow measurement and data analysis. Imaging techniques are most widely used when measuring shape, and data collected by these techniques are 2D projections of particle profiles. Simple geometrical calculations using these 2D projections allow shape parameters to be calculated.
Particle Form
The overall characteristics of particle form can be analyzed using relatively simple parameters like the aspect ratio. Taking the image of the particle below as an example, the aspect ratio can be simplified as follows:
Aspect Ratio = Width/Length
The aspect ratio allows differentiation between particles with regular symmetry like spheres or cubes, and particles with differing dimensions along one axis like needle shapes or ellipsoid particles.
Other shape parameters that can be used to analyze particle form characteristics include elongation and roundness.
Particle Outline
The outline of a particle can provide information on characteristics like surface roughness as well as detecting agglomerated particles.
To calculate particle outline parameters, the concept known as the convex hull perimeter is used. Simply put, this is calculated from an imaginary elastic band stretched around the particle image outline, as illustrated below.
Once the convex hull perimeter is calculated, parameters like convexity or solidity can be defined based on this perimeter.
Here,
• Convexity = Convex hull perimeter/Actual perimeter
• Solidity = Area enclosed by actual perimeter/Area enclosed by convex hull perimeter
Particles with very smooth outlines will have convexity/solidity values close to 1, whereas particles with rough outlines or agglomerated primary particles will have lower convexity/solidity values.
Generic Shape Parameters
Some shape parameters capture variations from both particle form and outline. Monitoring these can be useful when form and outline can both impact the properties of the material being measured. The most commonly used parameter is circularity. Here,
• Circularity* = Perimeter/Circumference of circle with equivalent area
*This is sometimes defined as follows:
Sometimes referred to as HS circularity to avoid confusion with above definition.
Circularity is often used to measure how close a particle is to a perfect sphere, and can also be applied to monitoring characteristics like abrasive particle wear. However, care must be taken in data interpretation as arbitrary deviations can occur due to changes in surface roughness, physical form, or both.
While circularity can be very useful in some applications, it is not suitable for all scenarios. To date, there has been no definition of a universal shape parameter applicable in all cases. In actual practice, careful consideration is needed to determine the most suitable parameter for each specific application.
Zeta Potential
Zeta potential is a measure of electrostatic or charge repulsion or attraction between particles in a liquid suspension.
Zeta potential is one of the fundamental parameters that affect dispersion stability.
Measurement of zeta potential aids in a detailed understanding of the causes of dispersion, aggregation, or flocculation and can be applied to improve formulations of dispersions, emulsions, and suspensions.
The speed at which new formulations are introduced is key to success. Measuring zeta potential is one way to reduce the number of candidate formulations and thus minimize the assessment time and cost by shortening stability testing, as well as improving shelf life.
During water treatment, monitoring dosage using zeta potential measurements can reduce the cost of chemical additives by optimizing dosage control.
Measurement of zeta potential is an important application in a wide range of industrial fields including ceramics, pharmaceuticals, medicine, mineral processing, electronics, and water treatment.
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