In kinetic theory a liquid is viewed as an incompressible substance in which the atoms are free to move. I would like to know how to find an expression for the molecular flux in a liquid. By molecular flux I mean the collisions per unit area, where collision refers not to the collision between individual molecules but to the collision with a general surface. I hope that was understandable, but just to make sure I will elaborate. I want to find the rate at which a molecule hits a cluster of n molecules of area A, so to do that I will somehow need to estimate what area is swept over by the molecules in the liquid per unit time.