Granular material is a collective term for aggregations of solid particles including powder materials. Generally, granular materials larger than 1 mm are called grains and those smaller than 1 mm are called powders. Its shape and volume can be freely changed. Therefore, it can be said that powders are solids with liquid and gas properties. This section provides an overview of powders, explains their properties, and introduces examples of their observation and measurement using digital microscopes.

Observation and Measurement of Powders Using Digital Microscopes

Particle Size Distribution of Powders

The particle diameter of a powder varies and the degree of variation is called the particle size distribution.
Particle size distributions are usually expressed graphically with the particle diameter [μm] on the horizontal axis and the frequency [%] on the vertical axis.

Particle size distribution and typical powder examples

Normal distribution

Typical example: Powdered activated carbon

Large particle diameter and small particle diameter (two peaks)

Typical example: Powders that clump easily

Even diameter

Typical example: Toners

Large particle diameter

Typical example: Sand, sugar, salt

Small particle diameter

Typical example: Titanium oxide, pigments, dyes

Fluidisation of Powders

Powders that exhibit a normal particle size distribution are likely to retain air and thus are easy to fluidise. On the other hand, powders composed of particles with similar diameters are hard to fluidise because air passes between the particles.

Powders that are easy to fluidise
Powders that are difficult to fluidise

Fluidity of Powders

The angle between the horizontal plane and the sloping plane is called the angle of repose. Powders with a high fluidity have a small angle of repose, while those with a low fluidity have a large angle of repose.

Powders with a high fluidity often have large particles, such as sand. These powders spread on the horizontal plane and slump when piled in a conical shape.

A: Angle of repose

Powders with a low fluidity, such as wheat flour, do no spread on the horizontal plane, maintaining their shapes when piled in a conical shape. Powders with a low fluidity are hard to discharge from hoppers and silos, and may form bridges or ratholes.

A: Angle of repose
  1. A: Bridge
  2. B: Rathole

Observation and Measurement Examples of Powders Using Digital Microscopes

These are the latest examples of observation and measurement of powders using KEYENCE’s VHX Series 4K Digital Microscope.

Observation of crude drug powder
500×, ring illumination + with/without depth composition
500×, ring illumination + depth composition, with depth composition
The depth composition function allows for observation of all particle surfaces in focus.
Automatic area measurement of powder
ZS-200, 1000×, ring illumination, before measurement
ZS-200, 1000×, ring illumination, after automatic area measurement
The automatic area measurement function makes analysis efficient.
Observation of powder
VH-Z20, 20×, ring illumination
Observation of powder
VHX-E300, 200×, ring illumination
Observation of finely ground powder
VH-Z100, 1000×, ring illumination
Observation of powder used for drug manufacturing
VHX-E500, 500×, transmitted illumination