Adhesion energy, range

SFA has been traditionally used to measure the forces between modified mica surfaces. Before the JKR theory was developed, Israelachvili and Tabor [57] measured the force versus distance (F vs. d) profile and pull-off force (Pf) between steric acid monolayers assembled on mica surfaces. The authors calculated the surface energy of these monolayers from the Hamaker constant determined from the F versus d data. In a later paper on the measurement of forces between surfaces immersed in a variety of electrolytic solutions, Israelachvili [93] reported that the interfacial energies in aqueous electrolytes varies over a wide range (0.01-10 mJ/m-). In this work Israelachvili found that the adhesion energies depended on pH, type of cation, and the crystallographic orientation of mica. 

Among all the low energy interactions, London dispersion forces are considered as the main contributors to the physical adsorption mechanism. They are ubiquitous and their range of interaction is in the order 2 molecular diameters. For this reason, this mechanism is always operative and effective only in the topmost surface layers of a material. It is this low level of adhesion energy combined with the viscoelastic properties of the silicone matrix that has been exploited in silicone release coatings and in silicone molds used to release 3-dimensional objects. However, most adhesive applications require much higher energies of adhesion and other mechanisms need to be involved. 

It should be noticed that the angle 0 cannot be set to zero in this case because of the singularity due to the short-range nature of the van der Waals interaction. Therefore the sum of Eqs. (4), (5), (10), and (12) gives the total interaction energy per coating particle, namely, the adhesive energy. 

The mechanism of the encapsulation is still unknown. Nonetheless, the encapsulation implies high adhesion energy between Pt and iron oxide, which could, in principle, be derived from the structural information, obtained by STM on the particle size and shape, using the modified Wulff construction [81].The analysis yielded an energy in the range of 3.8-4.2 J/m2, which is, indeed, considerably larger than those obtained for Pd particles on FesO lll) and alumina films (i.e. 3.1-3.3 J/m2), for which the encapsulation has not been observed [77]. Note also, that CO adsorption experiments indicated Fe-Pt surface intermixing with the onset at ca. 600 K [82], probably as the first step in the encapsulation. 

PROBLEM OF THE WIDE RANGE OF ADHESION ENERGY VALUES... 

Figure 7.1. Range of measurc>d adhesion energies compared to theoretical clean molecular...

In conclusion, there are many mechanisms which can alter the level of molecular adhesion between solid bodies. Some of these may be elastic and reversible while others are time dependent, leading to drag and hysteresis. Thus, adhesion measurements cannot generally be explained in terms of a simple adhesion energy or range. The mechanisms of adhesion can magnify or reduce the adhesion force by several orders of magnitude. 

Adhesion of cells is one of the most fascinating topics. It is directly relevant to our human, cellular condition. It is vital to disease, hygiene, cancer, growth, memory, and so forth. More technically, cell adhesion falls at the boundary of molecular and engineering adhesion, where Brownian motion is still important and the adhesion energy is around kT, such that the bonds form and break easily under ordinary conditions. It is also the most complex form of adhesion in this book there are difficult geometries, complicated viscoelastic and structural behavior, a variety of chemical reactions, colloidal forces, and enormous ranges of polymer molecules present. 

---