mean free time

{{Short description|Fluid property}}

Molecules in a fluid constantly collide with each other. The mean free time for a molecule in a fluid is the average time between collisions. The mean free path of the molecule is the product of the average speed and the mean free time.{{Cite web|title=The Feynman Lectures on Physics Vol. I Ch. 43: Diffusion|url=https://feynmanlectures.caltech.edu/I_43.html|access-date=2021-02-04|website=feynmanlectures.caltech.edu}} These concepts are used in the kinetic theory of gases to compute transport coefficients such as the viscosity.{{cite web |title=The Kinetic Theory of Gases |url=https://www.hunter.cuny.edu/physics/courses/physics110/repository/files/section51/15TheKineticTheoryofGasesRev2.pdf |website=Department of Physics & Astronomy Hunter College |access-date=16 May 2024}}

In a gas the mean free path may be much larger than the average distance between molecules. In a liquid these two lengths may be very similar.

Scattering is a random process. It is often modeled as a Poisson process, in which the probability of a collision in a small time interval $dt$ is $dt\; /\; \backslash tau$. For a Poisson process like this, the average time since the last collision, the average time until the next collision and the average time between collisions are all equal to $\backslash tau$.