An effusive limit in ultra-low

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

fluid flow is the limit at which a

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

of certain

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

is able to expand into a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

such as a molecular beam line.Osborn D., Zou P. Johnsen H., ''et al.'' (2008)
"The multiplexed chemical kinetic photoionization mass spectrometer: A new approach to isomer-resolved chemical kinetics "
'' AIP Review of Scientific Instruments'', 79(28) . Retrieved on 31 January 2017

References

Fluid dynamics {{fluiddynamics-stub