Adhesion at the Molecular Level

It is widely recognized that specific interactions between polymers and mucins play an important role in mucosal adhesion at the molecular level. Khutoryanskiy et al. [126] have isolated and estimated the contribution of different physical interactions through manipulating chitosan structure via partial acetylation and adding NaCl, ethanol or urea. Although it was not possible to completely switch off selected interactions, Khutoryanskiy et al. have demonstrated that mucoadhesive interactions between chitosan and mucin are complex with contributions from electrostatic attraction. 

Although the diffusion mechanism can be seen as mechanical but occurring at molecular dimensions, van der Waals intermolecular interactions and conformational entropic energy provide an additional mechanism that increases adhesion [62]. It is interesting to note the analogy that exists between this mechanism at the molecular level with the adherence, adhesion and viscoelastic deformations concept applied for a macroscopic adhesive. 

Tribological behavior has its origin at the molecular level. Current theories suggest that adhesive forces have an important effect on die magnitude of frictional forces measured between two surfaces. At the molecular level, the two... 

Capillary action When water is placed in a narrow container such as a graduated cylinder, you can see that the surface of the water is not straight. The surface forms a concave meniscus that is, the surface dips in the center. Figure 13-15 models what is happening to the water at the molecular level. There are two types of forces at work cohesion and adhesion. Cohesion describes the force of attraction between identical molecules. Adhesion describes the force of attraction between molecules that are different. Because the adhesive forces between water molecules and the sihcon dioxide in glass are greater than the cohesive forces between water molecules, the water rises along the inner walls of the cylinder. 

Thus the paradox of adhesion being unpredictably large or small can be explained by these two complexities of contact at the molecular level contamination and toughness. 

Consider a definition of molecular adhesion which allows it to be distinguished from aU the other known forms of attractions between bodies molecular adhesion is the force experienced when bodies make contact at the molecular level, with gaps near molecular dimensions. 

Just as Perrin concluded that a fluid s apparent repose is merely an illusion because the fluid molecules are in a state of eternal and spontaneous motion, so must we believe that all molecules adhere strongly, even though macroscopic objects appear nonsticky. The apparent lack of adhesion we see in engineering situations is really an illusion because adhesion is universal at the molecular level, according to the first law of adhesion above. However, there is a serious conundrum here because it seems impossible that particles can be in constant Brownian movement, where it is necessary for particles to collide and bounce off each other, yet also sticking together, which would cause agglomeration and... 

We normally observe adhesion at the macroscopic level, say by peeling a film from a surface as in Fig. 3.9. In order to understand the force which is necessary to pull the fihn off, we have to connect this mechanical picture with the molecular adhesion forces which we know to be universal at the nanometer level. 

The skeptical engineer is wary of the idea that molecules leap into contact with their surrounding snrfaces. After all, car parts on an assembly line do not suddenly jump together and adhere strongly to fashion the finished vehicle. However, at the molecular level it is quite obvious that such events occur naturally, and a nanoscale car could be self-assembled by these normal adhesive forces. The problem is that the nano-engine would not work because all its parts would seize togeflier. 

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