JW Lecture 2 - Describing Single Particles and Particle Populations

Question 1

Three most important characteristics of a particle

Answer 1

• Size: affects properties such as surface per volume and the rate at which a particle will settle in a fluid. There are samples with uniform and non-uniform particle sizes
• Shape: regular (e.g spherical or cubical) or irregular (grain, corn flakes)
• Composition: determines properties such as density and conductivity. In many cases, the particle is not completely uniform. Particles might be porous or may be consisting of a consisting of a continuous matrix in which small particles of a second material are distributed.

Question 2

Geometric particle size measures

1. Direct particle size measures

Answer 2

These are either main measures of particles with regular shapes such as diameter and length for cylinder shaped plastic granules or straight fibres, the diameter of nearly sphere shaped droplets or bubbles. It can be also other geometrical measures such as the volume or the surface area.

Question 3

Geometric particle size measures

2. Statistical diameters

Answer 3

Statistical diameters are important for image based size measurement methods. When using these methods the particles are randomly distributed at different angles with respect to the direction from where the measurement is taken. Some common statistical diameters are:

• xMa: Martin’s diameter – length of the line which bisects the particle image. JHW – 2018 8
• xFe: Feret’s diameter – distance between two tangents on opposite side of the particles
• xCmax: Longest chord of the particle projection parallel to the direction in which the measurement is taken.

Question 4

Geometric particle size measures

3. Equivalent diameters

Answer 4

Equivalent diameter is the diameter of a circle or a sphere with the same characteristics than the considered particle. This can be both geometric or physical equivalent diameters d.

A few common ones are

• dSt: “Stokes diameter” – Terminal setting velocity in the Stokes region (see
  dV: Equivalent diameter of a sphere with the same volume
  dS: Equivalent diameter of a sphere with the same surface area
  dP: Equivalent diameter of a circle with the same projected surface area
  dPe: Equivalent diameter of circle with same circumference than particle image
  dW: Equivalent diameter of a sphere with the same terminal settling velocity