Powder Particle Size Analysis: Sampling, Sieving, Sedimentation & Light Scattering

What individual and collective powder properties matter?

Individual: size, shape, chemistry, microstructure, density, hardness, surface attributes. Collective: packing, flow, friction, density, and size distribution.

Why can two techniques report different sizes for the same irregular particle?

Each technique responds to a different property, such as volume, projected area, surface area, or another dimension.

What happens to surface-area-to-volume ratio as particle size decreases?

It increases, making surface effects increasingly important.

Particle diameter decreases → what happens to surface energy per unit volume?

It increases approximately with 1/D.

Particle diameter decreases → what happens to agglomeration tendency?

It increases because surface-energy effects become stronger.

Why can agglomeration make measured particle size too large?

A cluster of fine particles may be detected as one larger particle.

Agglomerate vs. aggregate?

Agglomerates are held by weak forces and are relatively easy to disperse; aggregates are strongly bonded.

Why is dispersion especially important for fine powders?

Fine particles readily agglomerate; dispersion separates them so individual particles are measured.

What is the purpose of a dispersant plus ultrasonic agitation/mixing?

Agitation breaks agglomerates; the dispersant helps keep the separated particles apart.

Why can a statistically large sample still be unrepresentative?

It may contain many particles but come from a segregated/nonrepresentative region of the powder lot.

Why is moving-stream sampling preferred?

It samples material passing through the process and reduces bias caused by segregation in a stagnant powder bed.

What is the purpose of a rotary riffler?

To divide powder into representative samples with low sampling error.

What does Dv represent?

Diameter of a sphere having the same volume as the irregular particle.

What does DA represent?

Equivalent diameter based on the particle's projected area.

What does Ds represent?

Equivalent spherical diameter based on surface area.

T/F: Dv, DA, and Ds must be equal for an irregular particle.

FALSE. They represent different equivalent geometric properties and can therefore give different diameters.

What is microscopy especially useful for compared with indirect sizing methods?

Direct observation of particle dimensions and shape.

Main advantages of microscopy?

Direct, accurate size/shape information and the ability to generate a size distribution.

Main disadvantages of microscopy?

Tedious, many particles are needed for statistics, and good dispersion is required.

Why can poor dispersion ruin microscopy results?

Agglomerates can be counted/measured as individual large particles.

What does increasing mesh number do to sieve opening size?

It decreases opening size because there are more wires per linear inch.

What does increasing mesh number generally do to the particle size that passes?

It decreases the particle size that can pass.

What is mesh number?

Number of wires per linear inch of screen.

What does -325 mesh mean?

The powder passes a 325-mesh screen; lecture associates it with particles below about 45 μm.

What does -100/+200 mesh mean?

Passes the 100-mesh screen but is retained on the 200-mesh screen.

What does classification mean?

Separating powder into specified particle-size fractions.

Why is sieve analysis less reliable for irregular particles?

Passage depends on particle orientation and which particle dimension encounters the opening.

Why shouldn't a large powder mass simply be dumped onto a sieve at once?

Crowding/agglomeration can block openings and prevent proper individual-particle classification.

What is the main advantage of sieve analysis?

It is simple, rapid, and widely used.

What is the main limitation of sieve analysis?

It does not properly account for particle shape.

T/F: Higher mesh number means larger screen openings.

FALSE. Higher mesh number means more wires per inch and therefore smaller openings.

How does sedimentation determine particle size?

It relates particle size to terminal settling velocity through a fluid.

What forces act on a particle during sedimentation?

Gravity/weight downward; buoyancy and drag upward.

What happens when terminal velocity is reached?

Forces balance, net force and acceleration become zero, and settling velocity becomes constant.

What flow condition is required for the Stokes sedimentation model?

Laminar/creeping flow; the lecture uses Re < about 0.2.

Particle density increases relative to fluid density → what happens to settling velocity?

It increases because the gravitational driving force becomes larger.

Fluid viscosity increases → what happens to settling velocity?

It decreases because viscous drag increases.

Particle diameter increases → what happens to Stokes settling velocity?

It increases strongly; Stokes settling velocity scales with D².

Why do very small particles cause problems in gravitational sedimentation?

Brownian motion becomes significant and interferes with orderly settling.

Why can very large/heavy particles cause sedimentation errors?

Their Reynolds number may become too high for the Stokes/creeping-flow assumption.

Why does high particle concentration cause sedimentation error?

Particles interfere with each other, causing hindered settling.

Why does internal particle porosity affect sedimentation sizing?

It changes effective particle density and therefore settling velocity.

Why does irregular particle shape affect sedimentation?

It changes drag, orientation, path, and settling velocity.

T/F: Terminal velocity means the particle continues accelerating at a constant rate.

FALSE. Terminal velocity is constant velocity because the net force and acceleration are zero.

Population-based vs. mass-based particle distributions?

Population weights every particle equally; mass-based distributions give much greater weight to larger particles.

Why do large particles dominate a mass-based distribution?

For similar-density particles, mass increases approximately with D³.

How does particle size affect scattering angle in the lecture's light-scattering method?

Larger particles produce smaller scattering angles.

How does particle diameter affect scattering intensity in the lecture?

Scattering intensity increases approximately with D².

Main advantage of light-scattering sizing?

Fast automated analysis of many particles over a large dynamic range.

What is dynamic ratio?

Largest particle size measurable simultaneously divided by the smallest.

How does electrical zone sensing determine particle size?

A particle passing through an aperture displaces electrolyte and changes conductivity; signal magnitude relates to particle volume.

Why must electrical-zone particle concentration be low?

Multiple particles passing the aperture together create measurement errors.

Why can dense particles cause errors in electrical zone sensing?

They can settle in the chamber and distort the measured distribution.

What is the mode of a particle-size distribution?

The particle size at the highest frequency/peak.

What does D10 mean?

10% of the distribution is smaller than that diameter.

What does D50 mean?

50% is smaller; it is the median particle size.

What does D90 mean?

90% of the distribution is smaller than that diameter.

D10, D50, and D90 close together indicate what?

A relatively narrow particle-size distribution.

D10 and D90 far apart indicate what?

A broad particle-size distribution.

Why can a broad PSD improve powder packing?

Small particles can occupy voids between larger particles.

Why might a narrow PSD be desirable for nanoscale powders?

Properties can change strongly with size, so a narrow range gives more uniform behavior.

What is the Andreasen distribution used for?

Optimizing particle-size distribution for high packing density.

What does Dinger-Funk add compared with a simpler packing distribution?

It incorporates both the largest and smallest particle sizes.

Why does particle shape affect powder flow?

Irregular particles have greater mechanical interlocking/contact and friction than smooth spherical particles.

Why does particle shape affect packing?

Spherical/smooth particles generally rearrange efficiently; irregular particles can interlock and leave larger voids.

What is aspect ratio?

Maximum particle dimension divided by minimum particle dimension.

What is the aspect ratio of a sphere?

1.

What happens to aspect ratio as a particle becomes more elongated or flake-like?

It increases.

What are the two major surface-area measurement methods in the slides?

Gas adsorption (BET) and gas permeability.

What does BET stand for?

Brunauer-Emmett-Teller.

What does BET actually measure?

Specific surface area from the amount of gas adsorbed on accessible particle surfaces.

What is the key physical idea behind BET?

Determine the gas quantity corresponding to a monolayer, then convert that coverage to accessible surface area.

Particle size decreases → what happens to specific surface area?

It increases for the same material and similar particle shape.

Why can two powders with the same measured mean size have different BET areas?

They may differ in shape, roughness, porosity, agglomeration, or accessible internal surface.

Why can BET give a larger surface area than gas permeability?

BET can access internal/pore surfaces that do not contribute equally to resistance through the packed bed.

How does gas permeability determine surface area?

It infers surface area from resistance to viscous gas flow through a packed porous powder bed.

What does Darcy's law describe in this chapter?

Viscous fluid/gas flow through a porous powder bed.

T/F: BET determines surface area by measuring settling velocity.

FALSE. BET uses gas adsorption; settling velocity is used in sedimentation sizing.

What happens to powder flow as interparticle friction increases?

Flow becomes worse/slower.

What happens to packing as interparticle friction increases?

Packing efficiency generally decreases.

What particle features increase interparticle friction?

Greater surface area, roughness, and unfavorable surface chemistry.

What is apparent density?

Density of loose powder without agitation.

What is tap density?

Density after tapping/vibration rearranges particles without external pressure.

What is theoretical density?

Density of fully dense material with no porosity.

Why is tap density normally greater than apparent density?

Vibration allows particles to rearrange and reduce void space.

What does Hausner ratio compare?

Tap density divided by apparent density.

Hausner ratio closer to 1 generally suggests what?

More spherical/lower-friction/better-flowing powder.

A larger Hausner ratio generally suggests what?

Greater irregularity/friction and poorer flow.

Why can measured particle density be below handbook theoretical density?

Closed internal porosity can reduce the effective measured particle density.

What is a pycnometer used for?

Measuring particle/material density using pressure-volume/gas-displacement behavior.

What are the four mechanical comminution mechanisms?

Impact, attrition, shearing, and compression.

Impact vs. attrition?

Impact is a rapid blow causing cracking; attrition is size reduction by rubbing.

Shearing vs. compression?

Shearing uses cutting/cleavage; compression crushes material.

Why are brittle materials suited to grinding/milling?

Cracks readily nucleate and propagate, repeatedly fracturing the particles.

Main advantage of producing powder from machining scrap?

It reuses material that would otherwise be waste.

Main disadvantages of machining powder?

Slow/inefficient, coarse irregular powder, contamination risk, and poor control of powder characteristics.

Why is milling energetically inefficient?

Only a small fraction of energy creates new surface; most is lost to deformation, motion, impact/friction, and heat.

Approximately what efficiency does the lecture give for jar milling?

About 0.1%.

Even high-energy milling has approximately what maximum efficiency in the lecture?

Less than about 1%.

What happens to rewelding tendency as milling produces finer particles?

It increases because smaller particles have higher surface activity.