Particle Properties

Question 1

What is the simplest particle and why?

Answer 1

A sphere, as it looks the same from all angles and will respond the same way in a fluid regardless of it’s orientation.

Question 2

What types of sphere are used to represent particles (2)?

Answer 2

• Spheres with the same volume

- Spheres with the same surface area

Question 3

How do you determine how spherical a particle is?

Answer 3

Sphericity = Surface area of a sphere with the same volume as the particle / Surface area of the particle

Sphericity<1 but the closer to 1 it is the more spherical the particle is

Question 4

What is the volume of a sphere?

Answer 4

V = 4/3 pi r^3

Question 5

What is particle size distribution?

Answer 5

Not all of the particles in a population will be the same size. PSD is a way of indication what size the particles are and in what proportion they make up the population/

Question 6

What is cumulative distribution?

Answer 6

Expresses the percentage of particles of that size and smaller. Values of D20, D50 and D80 express the size of mesh that 20%, 50% and 80% of the population would pass through. This succinctly describes the distribution and how the population will act.

Question 7

What is a normal distribution?

Answer 7

• Mode/Peak is in the middle

- Bell curve is symmetrical either side of the peak

Question 8

What is a positively skewed distribution?

Answer 8

• Mode/peak is to the left/more small particles

- Elongated tail

Question 9

What is a negatively skewed distribution?

Answer 9

• Mode/Peak is to the right/ more course particles

- Elongated head

Question 10

What is a bimodal distribution and why would you get one?

Answer 10

• Two modes/peaks

- Two populations have been mixed, one which is much coarser and one which is finer.

Question 11

How would you define a particles shape?

Answer 11

Use imaging techniques i.e. print a grid over a picture of the particle and fill in any squares that contain particle.

Question 12

What is aspect ratio?

Answer 12

Aspect ratio = width/length
Used to determine if a particle has a regular symmetry (cubes, spheres) or a different ratio along one axis (rectangles, cylinders)

Question 13

What are the 5 ways to determine PSD?

Answer 13

• Sieve
• Elutriator
• Laser Diffraction
• Dynamic Light Scattering
• X-ray

Question 14

How to measure PSD using a sieve? Pros/Cons?

Answer 14

• Use multiple sieves with known mesh sizes
• Best for coarse particles i.e d>50 micrometers
• Fine particles don’t sieve well

Question 15

How to measure PSD using an Elutriator? Pros/Cons?

Answer 15

• Population is put in the T shaped Elutriator. Fluid is then blown through the Elutriator at a know velocity. Small particles are blown up through the Elutriator, then settle at the top as the expansion of the chamber causes the fluid velocity to drop. Successively larger fluid velocities can then be used to separate off larger particles into fractions.
• Used for particle of d<1 micrometer.

Question 16

How do you measure PSD using Laser Scattering? Pros/Cons?

Answer 16

• A laser is shone at the particles that are suspended in a fluid. The amount the laser is scattered by the particles is then used to determine their size, assuming that they are spherical.
• A fast method that is used to measure particles 10 nanometers

Question 17

How do you measure PSD by DLS? Pros/Cons?

Answer 17

• Basically laser scattering but it is the speed that the laser is scattered/changes angle that is analysed (giving the Brownian motion). Smaller particles are displaced further by the solvent they are suspended in, and so cause a much faster scattering.
• Non invasive and can measure particles d<1nm
• DLS produces an intensity weighted PSD and so the presence of oversized particles can dominate the results.

Question 18

What is the Bragg equation?

Answer 18

n.lambda = 2d sin(theta)

n = the order of reflection
lambda = the x-ray wave length
d = the lattice spacing
theta = Bragg angle

Question 19

What is the Scherrer Equation?

Answer 19

d = K.lambda/beta.cos(theta)

d = the crystallite size
K = the Scherrer constant
lambda = the X-ray wavelength
beta = the full width at half maximum width (on graph in radians)
theta = the bragg angle