Molecular adhesion energy

The interesting thing about this molecular model of adhesion is that energy and force of the molecular bonds do not decide the adhesion. Instead it is evident that molecular adhesion depends on the product of molecular adhesion energy and range. 

Fig. 17. Adhesion energy G measured as a function of the surface density of the interfacial chains. It may noted that the strength measured in a peel test (a) is about 5 times larger than that measured using the JKR method (b). Further, a maximum exists in the value of G as function of the surface chain density. This is because of swelling effects at larger values of surface chain density. The open symbols represent the data for elastomer molecular weight Mo = 24,000 and the closed symbols represent the data for Mo = 10,000.

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. 

As we see in Chapter 6, surface tension and contact angle measurements provide information on liquid-liquid and solid-liquid adhesion energies (Fig. 1.26c). Contact angles measured under different atmospheric environments or as a function of time provide valuable insights into the states of surfaces and adsorbed films and of molecular reorientation times at interfaces. 

Initially, with the completion of development wells, such fields, for a short time, give high yields of oil accompanied by a sharp drop in reservoir pressure. During that stage, the elastic forces of viscous oil and the energy of dissolved gas constitute the principal reservoir drives. Bound formation water also assists in displacing the oil from the reservoir by weakening the forces of molecular adhesion betwen the oil and the rocks. 

Clearly, our results for adhesion of lipid bilayers in fibrinogen and albumin solutions are consistent with the (non-adsorption) depletion type of assembly process. This deduction is based on (i) the null observation that no fiuorescently labelled material was detected in the gap between bilayers, (ii) the continuous increase of the free energy potential with concentration even for fairly large volume fractions, and (iii) the transfer of adherent vesicle pairs with subsequent separation which showed that adhesion energy depended only on the composition of the medium exterior to the gap but not the gap composition. Similar results have been obtained for adhesion of lipid oilayers in solutions of high molecular weight dextran polymers (Figure 4, J ). Hence, we have chosen to carefully examine (non-adsorption) depletion-based theories in conjunction with these experiments. 

Although the fibril extension stress can be predicted from the nonlinear elastic properties of the adhesive, in practice the important property that one wishes to predict is the adhesion energy rather than simply the plateau stress of the fibrillar zone. This prediction would require a better understanding of which molecular features control the detachment of the fibril from the surface, once it is... 

However, molecular adhesion is very different. This falls off in a very short distance of separation. As a consequence, these molecular adhesion forces cannot be measured with a meter ruler, but need a nanometer scale. The adhesion force may be high when the molecules are touching, but even a separation of one nanometer causes the force to drop almost to nothing. Thus the surfaces snap apart in a brittle fashion, totally different from the other types of adhesion force. The area under the curve is very small, hi other words, the energy of molecular adhesion may be negligible. Taking this energy for one square metre of joint, we define the work of adhesion ITin Joules per square metre. 

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

When we first think of adhesion experiments, we imagine that we can carry out the sort of tests shown schematically in physical chemistry textbooks, as shown in Fig. 7.8(a), in order to measure the surface forces as the bodies approach and begin to adhere. This was the experiment of Tabor and Winterton, and of Israelachvili. hi fact, they could only obtain gaps down to around 20 nm by this means. Remembering that 99% of adhesion energy is below 1 nm, we know that full molecular adhesion was not being measured in those tests. 

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. 

After the aack has extended such that the crack length c is comparable to h, then Equation (14.3) applies and the scaling is with h/c to the power 1. This obviously requires a lower force. When the crack extends still further, the elastic energy term becomes constant with anck length and then the force is independent of elasticity, giving a direct linkage between force and molecular adhesion, corresponding to the Rivlin peel equation F = Wb, which leads to a still smaller detachment force (see Section 7.7). 

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