Molecules in Flight
Gas molecules are in constant chaotic motion, colliding with each other billions of times per second at atmospheric pressure. The mean free path — the average distance between collisions — is the single most important parameter in vacuum physics. It determines whether gas behaves as a continuous fluid or as a collection of independent ballistic particles, fundamentally changing how vacuum systems must be designed and analyzed.
The Knudsen Number
Martin Knudsen recognized that the ratio of mean free path to system size determines gas behavior. When Kn < 0.01, molecules collide so frequently that the gas acts as a continuum fluid described by Navier-Stokes equations. When Kn > 1, molecules fly freely between wall collisions with no intermolecular scattering — this molecular flow regime has completely different conductance laws.
Pressure Regimes
Vacuum is classified by pressure ranges: rough vacuum (1000–1 Pa), medium vacuum (1–0.001 Pa), high vacuum (10⁻³–10⁻⁷ Pa), and ultra-high vacuum (below 10⁻⁷ Pa). Each regime has characteristic mean free paths ranging from fractions of a millimeter to kilometers, and correspondingly different engineering approaches for pumping, sealing, and measurement.
Simulation Details
This visualization shows molecules as particles moving through a tube. At high pressure, you see dense, collision-rich behavior. As you lower the pressure, collisions become rare and molecules bounce ballistically between walls. The Knudsen number display updates in real time, showing the transition between flow regimes as a visual and quantitative experience.