Subject surface forces

Friction can now be probed at the atomic scale by means of atomic force microscopy (AFM) (see Section VIII-2) and the surface forces apparatus (see Section VI-4) these approaches are leading to new interpretations of friction [1,1 a,lb]. The subject of friction and its related aspects are known as tribology, the study of surfaces in relative motion, from the Greek root tribos meaning mbbing. 

Erom the previous sections it is clear that there are a number of different possible models that can be applied to the contact of an elastic sphere and a flat surface. Depending on the scale of the objects, their elasticity and the load to which they are subjected, one particular model can be more suitably applied than the others. The evaluation of the combination of relevant parameters can be made via two nondimensional coordinates X and P [16]. The former can be interpreted as the ratio of elastic deformation resulting from adhesion to the effective range of the surface forces. The second parameter, P, is the load parameter and corresponds to the ratio of the applied load to the adhesive puU-off force. An adhesion map of model zones can be seen in Figure 2. 

In this chapter, the basics of surface forces will be described, and examples will be given where the system is dependent on these forces. The principles of surface forces are the building blocks that lead to the understanding of the subject. These forces interact at both the liquid-liquids and liquid-solid interfaces. 

In the past decades, it has become more and more obvious that students and scientists of chemistry and engineering should have some understanding of surface and colloid chemistry. The textbooks on physical chemistry tend to introduce this subject insufficiently. Modern nanotechnology is another area where the role of surface and chemistry is found of much importance. Medical diagnostics applications are also extensive, where both microscale and surface reactions are determined by different aspects of surface and colloid chemical principles. Drug delivery is much based on lipid vesicles (self-assembly structure) that are stabilized by various surface forces. 

The surface force apparatus is now being used routinely to study the equation of state of solutions confined between opposed, molecularly thin solid films. The apparatus is also used in one laboratory to study electrochemistry of thin films at electrodes a few nanometers thick and in a few other laboratories to study the behavior of molecularly thin films subjected to shear and flow [7]. 

The precise direct measurement of surface forces is a subject of current interest, since it provides sufficiently reliable distinction of the forces, along with the elucidation of their mutual influence, their dependence on the distance between the interacting surfaces in systems of different composition, temperature, etc. All this enables a more critical application of the theories (old and new) of the known surface forces. On the other hand, the direct measurement of surface forces stimulate theoretical analyses. 

Microscopic foam films are most successfully employed in the study of surface forces. Since such films are small it is possible to follow their formation at very low concentrations of the amphiphile molecules in the bulk solution. On the other hand, the small size permits studying the fluctuation phenomena in thin liquid films which play an important role in the binding energy of amphiphile molecules in the bilayer. In a bilayer film connected with the bulk phase, there appear fluctuation holes formed from vacancies (missing molecules) which depend on the difference in the chemical potential of the molecules in the film and the bulk phase. The bilayer black foam film subjected to different temperatures can be either in liquid-crystalline or gel state, each one being characterised by a respective binding energy. 

In contrast to the high affinity biotin-(strept)avidin bonds, carbohydrate-L-selectin bonds with modest affinity stop white cells at vessel walls in the circulation. Numerous bonds to other surface (integrin) receptors then form between the white cell and vessel wall to sustain adhesion and enable subsequent movement into the surrounding tissue. On its initial arrest from the blood flow, the white cell can be subjected to forces of 100 pN in a time frame of milliseconds, which implies loading rates of lO -lO pN s . With this functional requirement in mind, we now examine recent tests of carbohydrate-L-selectin bonds under dynamic loading in probe tests. 

Atoms, groups of atoms, ions, molecules, macromolecules, and particles always are subject to forces between them. These interaction forces may cause chemical reactions to occur, i.e., cause the formation of other molecular species, but they are also responsible for the existence of condensed phases (solids and liquids), for adherence of a liquid to a solid surface, or for aggregation of particles in a liquid. In short, all structures form because of interaction forces. Generally, formation of a structure causes a decrease in entropy, and this may counteract the tendency of formation, depending on its magnitude compared to that of the energy involved. 

Colloidal interaction forces act primarily in a direction perpendicular to the particle surface forces primarily acting in a lateral direction were discussed in Chapter 10. We merely consider internal forces, which find their origin in the properties of the materials present. This excludes forces due to an external field, such as gravitational, hydrodynamic, and external electric forces these are involved in some subjects of Chapter 13. 

In the absence of external hydrodynamic forces, the stability of a colloid depends on partides interaction caused by surface forces electrostatic repulsion and molecular attraction [52]. In order for the partides to interact with each other under influence of these forces, they need to be sufficiently close to one another. The partides approach in a liquid occurs under the action of Brownian motion, due to the influence of external forces, for example, gravity, or due to hydrodynamic forces. Studies of stability of the colloid systems should be carried out with due consideration of all the factors listed. Generally, this problem is very difficult, and therefore we consider first the interaction of particles under the action only of electrostatic and molecular forces. The theory of stability of a colloid system subject to such interactions is called DLFO theory as an acronym of its founders - Derjaguin, Landau, Ferwey, and Overbeck [53]. 

The discussion of the particles capture in the previous section was based on the assumption, that the particles are not subject to the forces from the system liquid - collector. Actually a particle near the surface of collector, is subject to the surface forces of (molecular and electrostatic) interactions, as well as the hydrodynamic force of viscous resistance from the liquid film between the particle and collector. The account of these forces considerably complicates the problem of determining the particles capture efficiency for the given obstacle. 

In general, two classes of methods exist for subjecting molecules to F in calculations. In the first class of methods, selected atoms in the simulated system are subjected to forces that are directed toward artificial points that are external to the molecule. These points correspond to the locations at which the external groups used to apply F in experiments would be located. Consider the extension of a surface-bound polymer in an AFM experiment as outlined above. In that case, the polymer, or a small portion thereof, would be simulated explicitly, whereas the surface and AFM tip would be replace by artificial points at appropriate locations around that molecule. Mechanochemical conditions could then be simulated by subjecting the atoms at either end of the polymer to forces directed toward the nearest artificial external point. Methods that employ artificial external points to apply F are described in Sect. 2.1.1. 

The continuum material that assumes is continuously distributed throughout its volume have an average density and can be subjected to body forces such as gravity and surface forces. Observed macroscopic behavior is usually illustrated by ignoring the atomic and molecular structure. The basic laws for continuum model are ... 

If a liquid is subjected to an electric field, a charge is induced on the liquid surface and mutual charge repulsion results in an outwardly directed force. Under suitable conditions, for example extrusion of the liquid through a needle, the electrostatic pressure at the surface forces the liquid drop to form a cone shape (Figure 31.1). 

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