The gaseous molecules present in our atmosphere tend to travel at tremendous speed because they have higher energies compared to their liquid and solid equivalents. For instance, a single molecule in the air around you can be as fast as 500 km/h. The reason that you do not feel them is that they are too small. At room temperature, they experience billions of collisions with other gaseous molecules. The average distance traveled by a moving particle (such as an atom, a molecule, a photon) between successive collisions that modify its direction, energy, or other particle properties is described by the term ” mean free path.” When a sample of gas is introduced to one part of a closed container, its molecules very quickly disperse throughout the container. This process is known as diffusion. The dispersion takes place to achieve thermal equilibrium by facilitating the movement of particles from a higher concentration to a lower concentration. Now, let’s say we prick a hole in the container with a diameter considerably smaller than the mean free path of the gas molecules. The process by which this gas will escape the container through this “pinhole” is known as effusion. Under such conditions, essentially all molecules that arrive at the hole can pass through it, as the collisions between molecules in this region are negligible. It is important to notice that in the case of diffusion, the movement of molecules was aided by the concentration gradient (the difference between the concentration), whereas in the case of effusion, the movement is facilitated by the pressure difference. The rate of effusion is defined as the number of molecules that diffuses through the hole in a unit of time. At constant pressure and temperature, the effusion rate is inversely proportional to the square root of the molecular weight. Gases with a lower molecular weight effuse more rapidly than gases with a higher molecular weight, so that the number of lighter molecules passing through the hole per unit time is greater. The ratio of the rates of effusion of two gases at the same temperature and pressure is given by the inverse ratio of the square roots of the masses of the gas particles.
\frac {{r}_{1}} {{r}_{2}}= \sqrt\frac {{M}_{2}} {{M}_{1}}
where {r}_{1} and {r}_{2} represent the rate of effusion of gas 1 and gas 2, respectively, and {M}_{1} and {M}_{2} represent the molar masses of the gases. If the molecular weight of one gas is four times more than that of another, it would diffuse through a porous plug or escape through a small pinhole in a vessel at a rate that is one-half the rate of the molecules of the other gas (heavier gases diffuse more slowly). Moreover, in medical terminology, effusion refers to the unusual accumulation of fluid within the anatomic spaces. Let’s discuss a few examples of effusion in daily life.
1. A Deflating Balloon
Have you ever noticed why a fully inflated balloon gets deflated over time even after getting perfectly sealed/tied at its end? Well, it’s possible only because of the process of effusion that causes the balloon to get deflated.
2. Tires
Have you ever wondered what causes your vehicle’s tire to lose air and sometimes making it turn flat even when it has not been driven for a long time? Well, it happens because of the phenomenon of effusion as in a properly sealed tire, air pressure drops over time through effusion.
3. Knudsen Effusion Cell
The Knudsen effusion cell is a device to measure the vapor pressures of a solid with very low vapor pressure. Such a solid forms a vapor at low pressure by sublimation. A typical Knudsen cell contains a crucible (made of pyrolytic boron nitride, quartz, tungsten, or graphite), heating filaments (often made of metal tantalum), a water cooling system, heat shields, and an orifice shutter. The material to be deposited is heated to provide a suitable vapor pressure in an isothermal enclosure. Molecular effusion from an aperture at the end of the cell gives rise to a cosine intensity distribution. The vapor slowly effuses through a pinhole, and the loss of mass is proportional to the vapor pressure and can be used to determine this pressure.
4. Pleural Effusion
Pleural effusion sometimes referred to as “water on the lungs,” is a medical condition in which an unusual amount of fluid gets build-up between the layers of the pleura outside the lungs. The pleura are thin membranes that line the lungs and the inside of the chest cavity and act to lubricate and facilitate breathing. Normally, a small amount of water is present in the pleura but based on the increase in amount of water, pleural effusion can be classified into two categories. This can be exudative, meaning there is a high protein count (>3g/dL), or transudative, meaning there is a relatively lower protein count (<3g/dL). Exudative causes are related to inflammation. The inflammation results in protein leaking out of the tissues into the pleural space, whereas transudative effusion corresponds to fluid moving across into the pleural space.
5. The Separation of Isotopes
Gaseous diffusion uses semi-permeable membranes to separate enriched uranium
Natural uranium is composed mainly of two isotopes – U-235 and U-238. The proportion of the two is 0.72% and 99.28%, respectively. However, U-235 is the only valuable resource for nuclear energy projects as it is the only nuclide existing in nature (in any appreciable amount) that is fissile with thermal neutrons. Although U-235 and U-238 are chemically identical, they differ slightly in their physical properties, most importantly mass. This small mass difference allows the isotopes to be separated and makes it possible to increase the percentage of U-235 in uranium. The process is known as uranium enrichment. One of the many techniques involved in uranium enrichment is effusion, but effusion usually takes place for fluids, and since naturally occurring uranium is usually in solid form, it is reacted with fluorine and converted into uranium hexafluoride, a compound that becomes gaseous when heated to over 80 degrees Celsius. Gaseous uranium hexafluoride (UF6) is used as the feed in the isotopic separation chamber and passed through the semipermeable membrane. The process relies on the fact that uranium 235 can pass through a porous membrane slightly more quickly than its heavier rival isotope U-238. Once the gas passes through a mesh of membranes, it can be enriched to the required 3 to 5% level in uranium-235. Throughout the Cold War, gaseous effusion played a major role as a uranium enrichment technique, and by 2008, it accounted for about 33% of enriched uranium production.