This is an important property of scanning electron microscopy in which gas molecules do not interfere with the electron beam. At ultrahigh vacuum or in relatively small geometries, however, the Knudsen number becomes very large. Examples are everyday phenomena under standard conditions, such as the abovementioned viscosity effects, as well as high-pressure reaction engineering 12. This so-called continuum regime is present in many technical applications. Under elevated pressure conditions or in larger geometries, the Knudsen number is close to zero. #Argon molar mass freeThe ratio between the mean free path and the characteristic length of the geometry in question is represented by the dimensionless Knudsen number Kn 11. In contrast, the smaller the surrounding geometry is, the more likely a collision is between gas molecules and the surface. The lower the pressure is, the less likely a collision is between two molecules within a given time. The frequency of intermolecular collisions is determined by the mean free path of a molecule. The movement of a gas molecule is strongly affected by its interaction with its environment involving collisions with other gas molecules or the surrounding geometry. The kinetic theory of gases can be utilized to describe the behaviour of a gas 10. These diameters are typically utilized to calculate the mean free path λ 9, which plays a crucial role in gas kinetics: Measured viscosity, usually obtained at normal pressure, can therefore be used to estimate molecular diameters. Viscosity is also an effect resulting from the molecular diameter of a gas. The drag on wind turbines is mainly a result of gas viscosity. Here, the kinetic diameter of a molecule is frequently used, which is obtained from molecular sieving experiments 4. The diffusion coefficients of molecules can be estimated based on their diameter. Transport processes such as the absorption of oxygen in the lungs or gas transport through a membrane are strongly influenced by diffusion effects. This diameter is also applied in the van der Waals equation of state to account for the nonideal behaviour of gases under high pressure 7, 8. The van der Waals diameter is obtained by crystallographic experiments such as X-ray diffraction, zero point density data 5 or the molar volume of solids 6. The van der Waals diameter is a prominent measure and is typically used to calculate the properties of condensed matter 5. As a result, different operationally defined diameters are used for various scenarios. It is, therefore, impossible to state one definite diameter of a molecule. Its volume is made up of electron orbitals representing the probability of finding an electron at a specific position. However, a molecule is neither a rigid object nor has, in most cases, a spherical shape. For unification, the size is often generalized as the diameter of an equivalent sphere. In many natural processes and technical applications, the size of a gas molecule is a key property that influences the rate at which lungs can absorb oxygen 1, affects chemical reactions 2, defines the drag on wind turbines 3 and limits diffusion across membranes 4, mainly because the size strongly affects the mobility of a molecule.
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