Liquid-vapor surface tension, adhesion

It has generally been observed that urethane systems, which are cured on the surface of some low energy materials, are also free from adhering to them, i.e. they are self-releasing. Materials which show this characteristic relative to urethanes in a very effective way include at least three different types of plastics polytetrafluoroethylene, polyethylene, and polypropylene. These materials all have a defined critical surface tension sc less than about 30 dynes/cm. Assuming that this value is near, the liquid vapor surface tension lv value of an effective IMR urethane systems, then the work of adhesion as given by Equation 4 is as follows ... 

Initial premises for the thermod3mamic description of adhesion are the characteristics of two surfaces their surface tension and interfacial tension at the interface between the two bodies in contact. In the simplest case of two liquids with surface tension y and 7,2 > their surface tension at the interface (interfacial tension) is always lower than the highest sruface tension at the interface with saturated vapor ... 

Theoretically, we should expect, as indicated by Eqs. (IV.39) and (IV.40), that the magnitude of the capillary forces will depend on the particle size, the surface tension of the liquid making up the bridge formed by vapor condensation (see Fig. IV.6), and the wettability of the contiguous bodies. Since capillary forces are proportional to particle size, the forces of adhesion should be identical for particles of a given size in any case in which the capillary component of the adhesive forces is dominant. The difference in adhesive force for particles of a... 

Thus we see that the capillary forces that are responsible for particle adhesion will be greater for higher surface tensions of the liquid form of the vapor surrounding the dust-covered surface these capillary forces will also be greater for larger particle sizes and for better wetting of the contact surfaces. A liquid interlayer between the particles and the surface will eliminate or greatly reduce the effect of electrical forces. Simultaneous action of capillary and electric forces is almost impossible in practice. 

Our prior discussion on surface tension again offers some insight. Since the surface tension and adhesive forces scale down with the lengths of the liquid, solid, and vapor boundaries, they drop much slower in comparison to the gravity force on a liquid droplet that is proportional to the volume of the droplet. Hence we could envision that at some smaller scale, the surface tension would be a much more dominant force compared to gravity, and if so, the effect of gravity onto the shape of the droplet would be negligible. 

The study of surface tension is really a branch of surface chemistry, and its development has been exceedingly rapid in the last decade. Thus, adhesion can be considered to be partially an exercise in wetting and spreading of liquids on solid surfaces. The flotation of ores is accomplished by gravity differences as well as by the adhesion of the solid particle to an air bubble, and it involves solid-liquid-gas interfaces. It is possible to reduce the vaporization of water from bodies of water with large surfaces such as reservoirs and lakes, by adding a monolayer of a substance such as hexadecanol or other surface-active agents. The action of soaps produces a decrease in surface tension on water. Many other applications in our modem environment can be readily identified. 

Theoretically we should expect [see Eqs. (III.36) and (III.38)] that the value of the capillary forces would depend on the particle size, the surface tension of the liquid formed as a meniscus by vapor condensation (see Fig. III.11), and also the capacity of the contiguous bodies to become wetted. Since the capillary forces are proportional to the particle dimensions, in cases in which the capillary component of the adhesive forces is dominant, the adhesion should be the same for all particles of the same size, while the difference between the adhesive forces of particles belonging to a particular fraction with a spread of particle sizes should not exceed the ratio of the dimensions of the extreme members of this fraction. For example, the adhesive forces calculated from Eq. (ni.36) should be 4.52 dyn for particles 100 /x in diameter and 5.43 dyn for those 120 p. in diameter. Experimental results disagree with the calculated values. Actually the adhesive forces for particles 100-120 M in diameter (for = 97-25%) fluctuate between 0.4 and 4.7 dyn, i.e., they vary by a factor of 12 over the fraction in question. Thus the scatter of the experimental data is much greater than would be expected, and hence the capillary effect fails to eUminate the indeterminacy of adhesive properties. 

Thus the capillary forces producing the adhesion of particles are the larger, the greater the surface tension of the liquid, the vapor of which surrounds the dust-laden surface, the greater the particle dimensions, and the better the wettability of the surface in contact. A liquid interlayer between the particles and the surface eliminates or greatly reduces the effect of electrical forces. The simultaneous action of capillary and electrical forces is practically excluded. 

The difference between the equilibrium surface energy of solid—vapor and solid—liquid is sometimes called adhesion [11]. Note that the work of adhesion and adhesion tension involves the solid—vapor equilibrium rather than that of the solid—liquid ... 

Adhesion — (a) When two compact materials, be they solid or liquid, are in intimate contact, attractive forces may act between their surface atoms or molecules. These forces are typically - van der Waals forces and electrostatic forces. The work of adhesion W (b)b(a) between the two phases (denoted A and B) is WAB = yA+yB -yAB> where yA and yg are the - interfacial tensions of A and B when each is interfaced only with the vapor phase, and yAB is the interfacial tension of the interface between A and B. In a more rigorous treatment (at thermodynamic equilibrium) each phase is regarded as saturated with the other phase [i]. In the case of liquid phases the equation for the work of adhesion is referred to as the -> Dupre equation. Adhesion forces between particles, or between particles and surfaces, dominate gravity for small particle sizes (pm and sub-pm range). In electrochemistry, increasing attention is being given to various phenomena related to the adhesion of vesicles [ii], particles [iii], droplets [iv], cells [v], etc. to electrode surfaces. 

In the early 1990s, Chaudhury and Whitesides proposed using a standard adhesion test developed in 1971 by Johnson, Kendall, and Roberts (known as the JKR test) to determine interfacial tensions between a solid and a liquid or a solid and a vapor. The procedure consists of bringing a hemispherically shaped solid, of radius typically 1 mm, in contact with a planar surface made of the same material (Figure 2.25). 

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