Collision frequency

Collision frequency describes the rate of collisions between two atomic or molecular species in a given volume, per unit time. In an ideal gas, assuming that the species behave like hard spheres, the collision frequency between entities of species A and species B is
where

Collision in diluted solution

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
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.