Flow is a process problem
A high-speed tablet press fills hundreds of dies a minute. Each die must receive almost exactly the same mass of powder, in a fraction of a second, by gravity alone. If the powder hesitates, bridges, or surges, tablet weights wobble and you fail weight variation. So powder flow is not a curiosity — it is the gatekeeper of a manufacturable product, and we test it before committing to a formulation.
Flow is governed by the tug-of-war between gravity, which wants particles to move, and the cohesive and frictional forces between particles, which resist. Coarse, rounded, dense particles win for gravity and flow freely; fine, irregular, cohesive ones lock together. The bench tests below are cheap, fast, and tell you where on that spectrum your powder sits.
The angle of repose
Pour a powder through a funnel onto a flat surface and it builds a cone. The angle between the cone's slope and the bench is the angle of repose. A free-flowing powder spreads into a low, shallow cone; a cohesive one piles up steeply because particles cling rather than slide. As a rough guide, an angle under about 30° signals excellent flow, around 30–40° is passable, and much above 40° warns of trouble.
Bulk, tapped, and what they reveal
Pour a known mass of powder gently into a graduated cylinder and read the volume — mass over that volume is the bulk density. Now tap the cylinder mechanically until the powder stops settling; the smaller settled volume gives the tapped density. The gap between the two is the whole story: a free-flowing powder is already well-packed and barely shrinks, while a cohesive powder traps air, sits loosely, and consolidates a lot when tapped. The empty space between particles is its porosity.
Two ratios turn these densities into a verdict. The Carr's compressibility index is the percentage drop from bulk to tapped volume; the Hausner ratio is the tapped density divided by the bulk density. Both say the same thing differently: the more a powder compresses, the more cohesive and the worse-flowing it is. A Carr's index under ~15% (Hausner under ~1.18) means good flow; above ~25% (Hausner over ~1.35) means poor flow that likely needs a glidant or granulation.
Worked example — Carr's index & Hausner ratio
Sample: 50.0 g of a granule blend in a cylinder
Bulk (poured) volume = 100.0 mL → bulk density = 50/100 = 0.500 g/mL
Tapped (settled) volume = 82.0 mL → tapped density = 50/82 = 0.610 g/mL
Carr's compressibility index (%):
CI = 100 × (tapped − bulk) / tapped
= 100 × (0.610 − 0.500) / 0.610
= 100 × 0.110 / 0.610 ≈ 18.0 %
(Equivalently from volumes: 100 × (100 − 82)/100 = 18 %)
Hausner ratio:
HR = tapped density / bulk density = 0.610 / 0.500 = 1.22
Verdict: CI ≈ 18 %, HR ≈ 1.22 → 'fair' flow.
Likely acceptable on a press with a glidant; borderline for very high speed.