text: In the case of equal-size particles at a concentration  in a solution of viscosity  , an expression for collision frequency  where  is the volume in question, and  is the number of collisions per second, can be written as:
  Where:
  *  is the Boltzmann constant
  *  is the absolute temperature (unit K)
  *  is the viscosity of the solution (pascal seconds)
  *  is the concentration of particles per cm
  Here the frequency is independent of particle size, a result noted as counter-intuitive. For particles of different size, more elaborate expressions can be derived for estimating .