effüzyon

Effüzyon, known in English as effusion, is the process by which gas molecules escape from a container through a small opening into a region of lower pressure. The mechanism involves individual molecules crossing the opening rather than bulk gas motion, so the remaining gas inside the container does not rapidly drain.

Two flow regimes are typically distinguished. In the molecular-flow regime, which occurs when the opening is small compared with the mean free path of gas molecules (low pressures, high vacuum), the leak rate is governed mainly by molecular speed and opening area. In this regime, Graham's law describes the relative rates of effusion for different gases: the effusion rate is inversely proportional to the square root of the molar mass, so lighter molecules effuse faster than heavier ones at the same temperature and hole size (r_A/r_B = sqrt(M_B/M_A)). At higher pressures or with larger openings, the process can enter the viscous or transitional regime, where the flow is influenced by gas viscosity and pressure difference rather than molecular speed alone.

History and applications: The concept was studied in the 19th century by Thomas Graham, leading to Graham's law of effusion. Today, effusion is important in vacuum technology, leak testing, gas sampling, and mass-spectrometry calibration, as well as in educational demonstrations such as leaking balloons or pinhole leaks.

Effüzyon differs from diffusion in that effusion requires a physical opening through which molecules pass under a pressure difference, while diffusion is the gradual mixing due to concentration gradients without a defined opening.