Intermolecular frictional force

Single-stage simulations reveal that intermolecular friction forces do not lead to reverse diffusion effects, and thus the molar fluxes calculated with the effective diffusion approach differ only slightly from those obtained via the Maxwell-Stefan equations without the consideration of generalized driving forces. This result is as expected for dilute solutions and allows one to reduce model complexity for the process studied (143). 

Acceleration force = converted momentum change partial pressure gradient + external forces - thermal diffusion force + shearing force + intermolecular friction force... 

In addition to the intramolecular forces, the chain is subjected to a Brownian force g n,t) due to random collisions with other chains, and an intermolecular frictional force, x(n, t) where < is an atomic friction coefficient. Defining the following Fourier transforms... 

However, as RH further increases, tablet strength decreases for most tested tablets, and it was suggested that condensed water on the solid surface at high RH weakens intermolecular attraction forces between particles in the tablets and the further softened particles and solid bonds cause the tensile strength to decrease. " In addition, at high RH, water may form multilayers on the solid surface, which can act as a lubricant and reduce the frictional forces between particles, thus decreasing tensile strength as well. Ultimately, the effects of adsorbed moisture on particle surfaces are very complex, affected by many factors, especially the properties of the tablet excipients. 

Viscosity is the property of a fluid which characterizes its resistance to flow. It is often measured by timing the flow of a liquid through a cylindrical tube under the influence of gravity. In order to understand the definition of the viscosity, consider a fluid flowing between two large plane parallel plates (fig. 6.1). The velocity of the fluid in the direction of the flow, varies with position. It is at its maximum midway between the plates and decreases to zero between each plate on the basis of experimental observation. Now imagine that the fluid is made up of horizontal layers which are parallel to the plates. The movement of one layer with respect to another is retarded by a frictional force which is related to the fluid s viscosity. The origin of this friction is clearly intermolecular forces. 

In the case of the translational frictional forces opposing translational motion, there are two extremes known as the stick boundary condition (strong intermolecular interactions) and the slip boundary condition (negligible intermolecular interactions) that are characterised by the following two basic equations ((7.2) and (7.3) respectively) ... 

From their results on molecular reorientation in pure and mixed tin tetrahalides. Sharp and Tolan [94] discuss the origin of the intermolecular friction. Since the rate of reorientation varies in a regular way with the composition of the molecule it could be argued that molecular shape or dipolar interactions are relatively unimportant and that instead intermolecular friction is closely related to the magnitudes of dispersion forces. 

The Lubrication Theory. The lubrication theory is based on the assiunp-tion that the rigidity of the resin arises from intermolecular friction binding the chains together in a rigid network. On heating, these firictional forces are weakened to allow the plasticizer molecules to lubricate the chains. Once incorporated into the polymer, the plasticizer molecules shield the chains from each other, thus preventing the reformation of the rigid network (1). 

The molecular level characteristics which create friction forces are the intermolecular attraction forces of adhesion. If the two materials that make up the sliding surfaces in contact have a high degree of attraction for each other, the coefficient of friction is generally high. This effect is modified by surface conditions and the mechanical... 

The inhibitor contributes to increasing mobility and to the macromolecules alignment on the direction of the field of forces. In this way, intermolecular frictions are diminished, which is confirmed by the values recorded for the extrusion forces. Table 3.94. 

The physical foundation of any treatment dealing with adhesion and friction forces is the knowledge of interatomic (intermolecular) potentials. If the pairwise (two-body) potential between two specii is described by a potential V(r) as a function of the distance, r, between the particles, the force F(r) and the potential are related through simple differential (integral) equations ... 

If we assume that the development of a standard state, i.e. ideal metal-vacuum interface, was in fact the key to the advancements in metallic contact analysis we should also expect that this will also be the case in the polymer, or solid organic, friction or adhesion analysis. The consequence is that polymer surface characterization under various environments becomes the most important issue at hand. The achievement of the standard surface state in polymer or solid organic system will be most difficult due to the relatively weak intermolecular bonding forces and the normal existance of a wide range of impurities within the material itself. Ambient vacuum conditions are required for most of the physics oriented surface characterization techniques, e.g. LEED, FIM, Auger electron spectroscopy and under these conditions the surface can be modified by... 

Israelaohvili J N 1991 Intermolecular and Surface Forces (London Aoademio) The most often cited reference about surface forces and SFA. Hutohings I M 1992 Friction and Wear of Engineering Materials (London Arnold) A good introduction to tribology. 

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