Colloidal particle

A colloidal particle is a small dispersed object whose dynamics can be strongly affected by thermal motion and interactions with the surrounding medium. Its behavior bridges molecular and macroscopic descriptions: an optical microscope can track a particle while the surrounding fluid produces unresolved random forces.

For a dilute spherical particle of radius $a$ in a Newtonian fluid of viscosity $\eta$, the Stokes–Einstein relation gives $D=k_{B}T/(6\pi \eta a)$ under its usual continuum assumptions. Larger particles diffuse more slowly. Surface forces, concentration, boundaries and nonspherical geometry can invalidate this simple expression, so the observed motion also probes the particle's environment.