Microfluid Mechanics/Intermolecular and Surface Forces

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Fundamental interactions
In particle physics, fundamental interactions (sometimes called interactive forces or fundamental forces) are the ways that elementary particles interact with one another. An interaction is fundamental when it cannot be described in terms of other interactions. The four known fundamental interactions, are: All are non-contact forces. Gravitation is a long range force, as it acts at large distances. Tough electromagnetic forces are short range forces, they might have long range effects: Surface tension and related phenomena are good examples of long range effect electromagnetic forces. Electromagnetic forces (see also Electromagnetism) are causing the intermolecular interactions and, therefore, have to be considered in microfluid mechanics.
 * electromagnetic force
 * strong interaction ("strong nuclear force"),
 * weak interaction ("weak nuclear force") and
 * gravitation.

Common types of intermolecular interactions between atoms, ions and molecules
The interactions between fluid molecules, molecules of particles and molecules of surfaces are mainly due to electromagnetic forces. The bonds generated by these interactions are of two types: As atoms covalently bond (i.e. form chemical bonds), the electron charge distributions of the atoms change completely and merge. Physical bonds occur as a result of small electromagnetic interactions and hold the molecules together in liquids and solids. They can be as strong as covalent bonds. Even the weakest physical binding force is strong enough to hold together all but the smallest atoms and molecules in solids and liquids at STP as well as in colloidal and biological assemblies. These properties, coupled with the long-range nature of physical forces, make them the regulating forces in all phenomena that do not involve chemical reactions.
 * Covalent bonds (chemical bonds)
 * Physical bonds

Unlike gravitational and Coulomb forces, van der Waals forces are not generally pairwise additive: the force between any two molecules is affected by the presence of other molecules nearby, so one cannot simply add all the pair potentials of a molecule to obtain its net interaction energy with all the other molecules. This is because the field emanating from any one molecule reaches a second molecule both directly and by "reflection" from other molecules, since they, too, are polarized by the field. This effect adds an additional contribution to the total van der Waals interaction energy. The nonadditive property of van der Waals forces is more important in the interactions between large particles and surfaces in a medium.

Forces between particles and surfaces
The fundamental forces involved are the same as those already described (i.e., electrostatic, van der Waals, solvation forces). However, they can manifest themselves in quite different ways and lead to qualitatively new features when acting between large particles or extended surfaces. Surfaces referred here are between a solid and a fluid (gas or liquid) or between two fluids (gas-liquid or liquid-liquid). All special cases will be explained when needed in specific chapters of this course. However, a detailed account of these forces can be found in the literature.