Angle of Repose, Powder Flowability & Optimum Glidant Concentration

Vocabulary flashcards covering core terms, mathematical formulas, physical mechanisms, and experimental parameters for determining powder flowability and angle of repose.

Angle of Repose ($\theta$)

The maximum angle possible between the surface of a freely formed, static pile of powder and the horizontal plane, determined by pile geometry using $\tan(\theta) = \frac{h}{r}$.

Flow Rate (or Flow Velocity)

A dynamic measure of powder movement defined as the mass or volume discharging through an orifice per unit time (such as $\text{g/s}$ or $\text{mL/s}$).

Glidant

A fine excipient added at a low level (such as $0\,\%\text{ w/w}$ to $1\,\%\text{ w/w}$) to improve flowability by coating larger particles to decrease surface roughness, friction, and interparticle contact.

Excess Glidant

A concentration of glidant added beyond the optimum level that no longer helps and can worsen flowability (increasing $\tan(\theta)$) by adding cohesion or filling voids.

mathematical steps

The mathematical relationship $\tan(\theta) = \frac{2}{r}$, derived by fixing the funnel tip height at $h = 2\,\text{cm}$ above the base and dividing by the mean radius $r$.

Mean Base Diameter ($\bar{d}$)

The average of three diameter measurements taken across the base of the conical powder pile through its center, calculated as $\bar{d} = \frac{d_1 + d_2 + d_3}{3}$.

Optimum Glidant Concentration

The glidant concentration in a test series that produces the lowest $\tan(\theta)$ (or lowest angle of repose $\theta$), signifying the best flowability for that specific formulation and method.

Coefficient of Friction ($\mu$) Approximation

The idealized approximation $\tan(\theta) \approx \mu$, which applies strictly to dry, free-flowing non-cohesive grains where pile slope is dictated solely by interparticle friction.

Forces Hindering Powder Flow

Mechanical friction between rough or irregular particles, cohesion resulting from van der Waals forces, liquid/moisture bridging, and electrostatic attraction.

Limitations of the Angle of Repose

Methodological drawbacks including method and apparatus dependency, operator sensitivity, poor suitability for cohesive powders that form irregular piles, environmental effects (humidity/static), and failure to predict dynamic orifice discharge on its own.