# Sedimentation speed

Sedimentation speed is the rate at which a particle sinks or settles within a heterogeneous mixture.

Multi-phase mixtures (solid/liquid, liquid/liquid) can be separated mechanically by the effect of gravitational force (sedimentation). The prerequisite is that the individual components have different densities and are not soluble in one another (for example, water and oil).

## What forces act on the particle?

Particles surrounded by a liquid (like solid matter or fat droplets in water, or water droplets in oil) are subjected to three forces (see figure):

• Force of gravity FG
• Buoyancy FB
• Drag FD

The force of gravity FG (field force) is opposed by the buoyant force FB and the drag force FD. Depending on whether the density of the particle is larger or smaller than that of the surrounding medium, the particle assumes constant velocity Vg of ascent or descent.

## Calculation of sedimentation speed

Under the simplistic assumption of laminar flow, the sedimentation speed can be determined using Stokes Law based on the equilibrium of forces.

In centrifugation or in the centrifugal field, the following formula is obtained for the sedimentation rate:

## Parameters influencing sedimentation rate

This equation can be used to derive the parameters that have a significant influence on the buoyancy or descent velocity:

Diameter "d" of the particle: The particle diameter or particle diameter is entered into the formula as a square value. The larger the diameter (or particle radius), the higher the sedimentation velocity of the particle.

Density difference "Δρ" between particle and liquid: The higher the density difference between the particle and the liquid, the higher the velocity of the particle and the faster the mixture will sediment.

Dynamic viscosity "η" of the mixture: The lower the dynamic viscosity of the product, the higher the settling velocity of the particle.

Source

 Klaus Luckert: Handbuch der mechanischen Fest-Flüssig-Trennung,1. Edition 2004

 Horst Kuchling: Pocketbook of Physics; Published by Fachbuchverlag Leipzig, 16. Edition 1996