Editing Colloid (section)

== Sedimentation velocity ==
[[File:Brownian Motion.gif|thumb|Brownian motion of 350 nm diameter polymer colloidal particles.|268x268px]]
The Earth’s [[gravitational field]] acts upon colloidal particles. Therefore, if the colloidal particles are denser than the medium of suspension, they will [[Sedimentation|sediment]] (fall to the bottom), or if they are less dense, they will [[Creaming (chemistry)|cream]] (float to the top). Larger particles also have a greater tendency to sediment because they have smaller [[Brownian motion]] to counteract this movement.

The sedimentation or creaming velocity is found by equating the [[Stokes' law|Stokes drag force]] with the [[gravitational force]]:

:<math>m_Ag=6\pi \eta rv</math>

where

:<math>m_Ag</math> is the [[Archimedes' principle|Archimedean weight]] of the colloidal particles,

:<math>\eta</math> is the [[viscosity]] of the suspension medium,

:<math>r</math> is the [[radius]] of the colloidal particle,

and <math>v</math> is the sedimentation or creaming velocity.

The mass of the colloidal particle is found using:

:<math>m_A =V(\rho_1 - \rho_2)</math>

where

:<math>V</math> is the volume of the colloidal particle, calculated using the volume of a sphere <math>V = \frac{4}{3}\pi r^3</math>,

and <math>\rho_1-\rho_2</math> is the difference in mass density between the colloidal particle and the suspension medium.

By rearranging, the sedimentation or creaming velocity is:

:<math>v = \frac{m_Ag}{6\pi\eta r}</math>

There is an upper size-limit for the diameter of colloidal particles because particles larger than 1 μm tend to sediment, and thus the substance would no longer be considered a colloidal suspension.<ref name="cosgrove2010">{{Cite book|last=Cosgrove|first=Terence|title=Colloid Science: Principles, Methods and Applications|publisher=[[John Wiley & Sons]]|year=2010|isbn=9781444320183}}</ref>

The colloidal particles are said to be in [[sedimentation equilibrium]] if the rate of sedimentation is equal to the rate of movement from Brownian motion.