Size measurement Flashcards
Challenges in measuring particle size
1) Cannot describe a 3D object with just one number
- Assumes particle is a sphere, which may not be true
2) Different sizing methods may give different size information
Overall: Sizing methods involve a lot of derivation & estimation
- Data interpretation and validation of accuracy & reliability of results can be difficult
Equivalent sphere theory
Determines diameter of a sphere that has the same weight as a particle of a particular weight
- Can be used to gauge if particle becomes larger/smaller
Weight of sphere = 4/3 π r^3 p
- r = Radius of sphere
- p = Density
List of particle sizing methods & particle size that can be measured
1) Scales (vernier, micrometer): > 5 mm (larger particles)
2) Sieves: > 10 µm
3) Microscopy (image analysis): 5 µm - 5 mm
4) Laser diffraction: 5 µm - 5 mm
5) Laser scattering: 0.001 µm - 5 µm
6) Sedimentation: 0.05 µm – 150 µm
7) Scanning electron microscope (SEM): nm - µm
8) Atomic force microscope (AFM): nm - µm
Microscopy - How it works
Particle size depends on what is measured:
1) Maximum particle length as diameter
2) Minimum diameter
3) Other diameters (e.g. Feret’s diameter)
4) Minimum length
5) Volume
6) Surface area
Microscopy - Types of diameter measurements
1) Feret’s diameter
- Greatest distance possible between any 2 points along boundary of region of interest OR
- Distance between 2 vertical lines tangent to ends of particle
2) Martin’s diameter
- Length of horizontal line that appears to divide particle in half
Microscopy - No. of images to examine
National Bureau of Standards (NBS) recommendation: Min 10,000 images for statistical validity
Microscopy - Advantages
1) Direct visual examination
- Can differentiate between individual particles & agglomerates
2) Relatively cheap
Microscopy - Disadvantages
1) Operator dependent; Manual method
- Prone to bias
- May have non-representation –> fewer particles examined (but impact might not be as large if only smaller particles missed)
2) Sample preparation requires skill & can be laborious & slow
3) Only provides 2D image
- Particles tend to be oriented in position with lowest centre of gravity –> tend to measure maximum diameter of particle
Sieves - How it works
Longer measurement time –> smaller particle size
Sieves - Advantages
1) Economical & robust technique for larger particles
Sieves - Disadvantages
1) Low resolution method
2) Usually only 4-8 size fractions provided
Sieves - Application
1) May be used for quality control
Air jet sieve - How it works
1) Weighed material placed on sieve of certain aperture size (may be microsieve)
2) Jet of air blown upwards –> particles blown as well
- Creates pressure in chamber
- Separates particles
3) As air escapes –> particles pulled out with air
- Only particles of size ≤ aperture size may be pulled out
4) Remaining particles weighed
5) Calculate cumulative weight %
6) Repeat with sieves of other aperture size
Air jet sieve - Determining particle size
Plot graph of cumulative weight % against sieve equivalent diameter
Mass median diameter = D50 = Sieve equivalent diameter when cumulative weight % = 50%
Span (Size distribution) = (D90 - D10) / D50
Air jet sieve - Application
Fine particles < 200 µm (35 - 200 µm)
- Such particles tend to form agglomerates that are difficult to break down and sieve with normal sieve
