Stokes is the most important law for pharmaceutical suspensions.

• A. True
• B. False
• C. Not Sure
• D. Not Applicable

Answer: (A)

Stokes' law describes the settling velocity of small, spherical particles in a viscous liquid under laminar flow. In pharmaceutical suspensions, knowing how fast particles settle is key to achieving physical stability and uniform dosing. The law shows that the rate of sedimentation depends on particle size (bigger particles settle faster), the density difference between the particle and the suspending medium (larger difference increases settling), the viscosity of the medium (higher viscosity slows settling), and gravity. This relationship gives formulators a direct handle: by reducing particle size, increasing the viscosity with a suspending agent, or choosing formulations that minimize density differences, the sedimentation rate can be controlled to keep the suspension uniform for longer.

The standard expression for the settling velocity is v = (2/9) * (r^2 * g * (ρ_p − ρ_f) / μ), where r is the particle radius, g is acceleration due to gravity, ρ_p and ρ_f are the particle and fluid densities, and μ is the fluid viscosity. This equation is most accurate under conditions of small Reynolds number, with spherical particles in a Newtonian fluid and at low concentrations where interactions between particles are negligible.

In practice, while real suspensions may involve non-spherical particles, non-Newtonian fluids, or higher solid contents where hindered settling and flocculation occur, Stokes' law provides the foundational framework for understanding and predicting sedimentation behavior. It underpins decisions about milling to desired particle sizes, selecting appropriate suspending agents, and designing formulations to minimize phase separation, making it the most important guiding principle for pharmaceutical suspensions.