To slow sedimentation in suspensions, which approach is commonly used?

• A. Increasing viscosity
• B. Decreasing viscosity
• C. Increasing density
• D. Increasing temperature

Answer: (A)

Sedimentation is governed by how easily a particle can move through the liquid. According to Stokes’ law, the settling velocity is inversely proportional to the viscosity of the medium and directly proportional to the particle size and the density difference between the particle and the liquid. Increasing viscosity adds resistance to motion, reducing the terminal velocity and slowing sedimentation. In practice, thickening the suspension with polymers or other agents keeps particles suspended longer and improves uniformity. Conversely, decreasing viscosity would let particles settle faster, increasing temperature usually lowers viscosity (also speeding sedimentation), and increasing density would raise the driving force for settling. So boosting viscosity is the effective approach to slow sedimentation.