Step 1: Understanding the Concept:
The stability of a suspension is defined by the rate of sedimentation. This is governed by Stokes' Law.
Step 2: Key Formula or Approach:
The rate of sedimentation (\(v\)) is given by:
\[ v = \frac{d^2 (\rho_1 - \rho_2) g}{18 \eta} \]
where:
\(d\) = diameter of the particle
\(\rho_1, \rho_2\) = densities of the particle and medium
\(\eta\) = viscosity of the medium
Step 3: Detailed Explanation:
1. Particle Size Influence: From the formula, the rate of settling (\(v\)) is proportional to the square of the particle diameter (\(d^2\)).
2. Logic: If the particle size is large, the settling will be very fast. By performing size reduction (microlization or milling) before formulation, we decrease \(d\).
3. Result: A smaller particle size significantly reduces the sedimentation rate, leading to a more physically stable suspension.
Step 4: Final Answer:
Size reduction is the critical unit operation to ensure the particles are small enough to remain suspended for a reasonable amount of time.