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2.3 Particle Shape

Definition and Types

Particle shape refers to the external appearance of individual particles. They may be:
spherical, cubic, acicular (needle-like), flaky, angular, spongy, or cylindrical.

Shape Descriptors

To quantify shape:

• Aspect ratio: ratio of length to width.
• Convexity: degree to which a shape is outwardly curved.
• Roundness (sphericity): measure of how closely a particle resembles a sphere.

No single descriptor applies universally, as different properties influence behavior in different
systems.

Measurement Methods

• Optical microscopy – provides 2D images for shape and surface observation.
• Scanning Electron Microscopy (SEM) – gives detailed surface topography.
• Transmission Electron Microscopy (TEM) – provides internal and surface

structure.
• X-ray diffraction (XRD) – offers information on crystalline structure, indirectly

reflecting shape.

2.4 Particle Size and Equivalent Diameters

Pharmaceutical powders rarely consist of perfectly spherical particles. Therefore, size must
often be expressed as an equivalent diameter — the diameter of a sphere that exhibits the
same behavior (e.g., sedimentation rate or surface area) as the irregular particle.

Common Equivalent Diameters

Type Definition Example Measurement
Method
Surface diameter (ds) Diameter of a sphere with the same
surface area Air permeation or
adsorption
Volume diameter Diameter of a sphere having the same
(dv) volume Coulter counter

Projected area Diameter of a sphere with the same Optical microscopy
diameter (dp) projected area

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