Section 3: Surface Area & Powder Flow Properties

Quantasorb Instrument

The specialized laboratory instrument used to determine particle surface area via the Adsorption Method.

Adsorption Surface Area Method

Measures how much gas or liquid adsorbs as a uniform monolayer on powder particles; greater adsorption signifies a greater exposed surface area.

Fisher Subsieve Sizer

The specialized laboratory instrument used to determine particle surface area via the Air Permeability Method.

Air Permeability Method Principle

Measures how easily air passes through a packed bed of powder, operating on the rule that finer particles provide more resistance to air flow.

Free-Flowing vs Cohesive Powders

Free-flowing powders move smoothly under gravity, while cohesive powders are structurally sticky and resist movement.

Flow Factor: Particle Size

Larger particle sizes exhibit less surface-to-volume friction, resulting in significantly better powder flow.

Flow Factor: Particle Shape

Perfectly spherical particles move over one another with minimal mechanical friction, resulting in much better powder flow.

Flow Factor: Porosity

Lower baseline porosity packs the bed more tightly, resulting in better powder flow.

Flow Factor: Density

Higher particle density allows gravitational forces to overcome cohesive forces easily, resulting in better powder flow.

Flow Factor: Surface Texture

Smoother surface textures prevent mechanical interlocking between particles, resulting in better powder flow.

Angle of Repose (θ) Definition

The maximum angle possible between the surface of a loose pile of powder and the horizontal plane.

Angle of Repose Determination

Determined experimentally using the fixed funnel method and calculated as θ = tan⁻¹(h/r).

Angle of Repose Funnel Variables

The variable h represents the height of the powder pile, while r represents the radius of the pile base.

Excellent Flow Repose Angle

Characterized by an angle of repose strictly less than 30 degrees (< 30°).

Good Flow Repose Angle

Characterized by an angle of repose ranging from 31 to 35 degrees (31° – 35°).

Fair Flow Repose Angle

Characterized by an angle of repose ranging from 36 to 40 degrees (36° – 40°); external aids are not needed.

Passable Flow Repose Angle

Characterized by an angle of repose ranging from 41 to 45 degrees (41° – 45°); the powder may occasionally hang up.

Poor Flow Repose Angle

Characterized by an angle of repose ranging from 46 to 55 degrees (46° – 55°); requires active agitation or vibration.

Very Poor Flow Repose Angle

Characterized by an angle of repose ranging from 56 to 65 degrees (56° – 65°).

Very, Very Poor Flow Repose Angle

Characterized by an extremely steep angle of repose exceeding 66 degrees (> 66°).