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The Hertz–Knudsen equation describes the non-dissociative adsorption of a gas molecule on a surface by expressing the variation of the number of molecules impacting on the surfaces per unit of time as a function of the pressure of the gas and other parameters which characterise both the gas phase molecule and the surface: [1] [2]
This equation implies that if one makes a log–log plot of adsorption data, the data will fit a straight line. The Freundlich isotherm has two parameters, while Langmuir's equations has only one: as a result, it often fits the data on rough surfaces better than the Langmuir isotherm.
In fluid dynamics, the Knudsen equation is used to describe how gas flows through a tube in free molecular flow. When the mean free path of the molecules in the gas is larger than or equal to the diameter of the tube, the molecules will interact more often with the walls of the tube than with each other. For typical tube dimensions, this occurs ...
The langmuir (symbol: L) is a unit of exposure (or dosage) to a surface (e.g. of a crystal) and is used in ultra-high vacuum (UHV) surface physics to study the adsorption of gases. It is a practical unit, and is not dimensionally homogeneous , and so is used only in this field.
The Langmuir-Rideal mechanism is often, incorrectly, attributed to Dan Eley as the Eley-Rideal mechanism. [5] The actual Eley-Rideal mechanism, studied in the thesis of Dan Eley and proposed by Eric Rideal in 1939, was the reaction between a chemisorbed and a physisorbed molecule. [ 6 ]
The Knudsen number is a dimensionless number defined as =, where = mean free path [L 1], = representative physical length scale [L 1].. The representative length scale considered, , may correspond to various physical traits of a system, but most commonly relates to a gap length over which thermal transport or mass transport occurs through a gas phase.
In supersonic and hypersonic flows rarefaction is characterized by Tsien's parameter, which is equivalent to the product of Knudsen number and Mach number (KnM) or M /Re, where Re is the Reynolds number. [4] [5] In these rarefied flows, the Navier-Stokes equations can be inaccurate. The DSMC method has been extended to model continuum flows (Kn ...
Desorption is the physical process where adsorbed atoms or molecules are released from a surface into the surrounding vacuum or fluid. This occurs when a molecule gains enough energy to overcome the activation barrier and the binding energy that keep it attached to the surface.