Effusion

In physics and chemistry, effusion is the process in which a gas escapes from a container through a hole of diameter considerably smaller than the mean free path of the molecules. Such a hole is often described as a pinhole and the escape of the gas is due to the pressure difference between the container and the exterior. Under these conditions, essentially all molecules which arrive at the hole continue and pass through the hole, since collisions between molecules in the region of the hole are negligible. Conversely, when the diameter is larger than the mean free path of the gas, flow obeys the Sampson flow law. In physics and chemistry, effusion is the process in which a gas escapes from a container through a hole of diameter considerably smaller than the mean free path of the molecules. Such a hole is often described as a pinhole and the escape of the gas is due to the pressure difference between the container and the exterior. Under these conditions, essentially all molecules which arrive at the hole continue and pass through the hole, since collisions between molecules in the region of the hole are negligible. Conversely, when the diameter is larger than the mean free path of the gas, flow obeys the Sampson flow law. In medical terminology, an effusion refers to accumulation of fluid in an anatomic space, usually without loculation. Specific examples include subdural, mastoid, pericardial and pleural effusions. Effusion from an equilibrated container into outside vacuum can be calculated based on kinetic theory. The number of atomic or molecular collisions with a wall of a container per unit area per unit time (impingement rate) is given by: J impingement = P 2 π m k B T {displaystyle {egin{aligned}J_{ ext{impingement}}&={frac {P}{sqrt {2pi mk_{B}T}}}end{aligned}}} If a small area A {displaystyle A} on the container is punched to become a small hole, the effusive flow rate will be: Q effusion = J impingement × A = P A 2 π m k B T = P A N A 2 π M R T ∝ 1 M ( Graham's Law ) {displaystyle {egin{aligned}Q_{ ext{effusion}}&=J_{ ext{impingement}} imes A\&={frac {PA}{sqrt {2pi mk_{B}T}}}\&={frac {PAN_{A}}{sqrt {2pi MRT}}}propto {frac {1}{sqrt {M}}}({ ext{Graham's Law}})end{aligned}}} Where M {displaystyle M} is the molar mass. Note that the outside vacuum has zero pressure, hence P {displaystyle P} is the pressure difference between two sides of the hole.