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Suspensions: Settling, Caking, and Controlled Flocculation

Solid drug in a liquid will settle — the only question is whether it redisperses with a shake or sets into a hard cake. Stokes’ law tells you how fast it falls; controlled flocculation tells you how it lands.

Why suspensions settle at all

A suspension is solid drug particles dispersed through a liquid, usually because the drug is too insoluble to make a solution or tastes better as undissolved crystals. Because the particles are coarse (>1 µm), gravity wins: they sediment. The rate is captured by Stokes’ law — settling speeds up with bigger particles and a larger density gap between drug and liquid, and slows down as the liquid gets more viscous.

Stokes' law (terminal settling velocity):
  v = d^2 (rho_s - rho_l) g / (18 eta)

Worked example — drug particle in a syrup vehicle:
  d       = 10 micrometres = 10e-6 m   (particle diameter)
  rho_s   = 1500 kg/m^3                (drug density)
  rho_l   = 1100 kg/m^3                (vehicle density)
  g       = 9.81 m/s^2
  eta     = 0.020 Pa.s                 (viscous syrup, ~20 cP)

  v = (10e-6)^2 x (1500 - 1100) x 9.81 / (18 x 0.020)
    = (1.0e-10) x 400 x 9.81 / 0.36
    = 3.924e-7 / 0.36
    = 1.09e-6 m/s   ~ about 3.9 mm per hour

Halve the particle size (d -> 5 um): v drops by 4x to ~1.0 mm/h.
Triple the viscosity (eta -> 0.060): v drops by 3x.
Lesson: smaller particles + thicker vehicle = slower settling.
Stokes’ law lets you estimate and slow settling by tuning particle size and viscosity.

The real enemy is caking, not settling

Some settling is acceptable in an oral suspension — patients expect to shake the bottle. The disaster is caking: particles packing into a dense, hard sediment that no amount of shaking will redisperse, leaving the drug stuck at the bottom and the dose wrong. Whether you get a loose, fluffy sediment or a hard cake depends on how the particles interact as they land.

In a deflocculated system each particle stays separate (high zeta potential, strong repulsion). They settle slowly, one by one, and pack tightly into a hard cake — looks good in the short term, fails badly later. In a flocculated system particles loosely associate into open, fluffy aggregates (flocs). Flocs settle faster, leaving a clearer top layer, but they form a high, loose, easily redispersed sediment.

Controlled flocculation: the practical recipe

The pharmaceutical answer is controlled flocculation: deliberately let particles floc just enough to avoid caking, then thicken the vehicle so the loose flocs barely settle at all. This is the rare case where you want a *lower* zeta potential, not higher.

  1. Wet the powder properly first — use a wetting agent or surfactant so hydrophobic crystals don’t float as clumps.
  2. Add a flocculating electrolyte or charged polymer to lower zeta potential toward zero, encouraging loose flocs.
  3. Thicken the continuous phase with a suspending agent to raise viscosity and slow sedimentation.
  4. Confirm the sediment redisperses with a few gentle shakes — that, not zero settling, is the acceptance test.