Apparent work of adhesion

The process of viscoelastic braking just described has certain parallels with the dynamic adhesion of elastomers. When, for example, a rubber strip is peeled from a rigid substrate, the effective, or apparent, work of adhesion, W, is usually much greater than the intrinsic, or reversible, energy of adhesion, Wq, given by the Dupre equation [15] ... 

These improvements have been achieved without sacrificing the antistiction characteristics of the film, in that water and hexadecane contact angles, apparent work of adhesion, and coefficient of static friction data are found to be similar to those of OTS, (see Table... 

Schapery goes on to show that for an opening crack (72) the apparent work of adhesion is given approximately by... 

For a closing crack a different approach is required. Schapery 13) introduces a strain intensity factor to replace K in equation (14). The length d of the Dugdale zone is still given approximately by equation (15), but the apparent work of adhesion is now given approximately by... 

The problem of measuring adhesion, in general, is that the curves for peeling have a similar shape, with an apparent work of adhesion plus a large kinetic adhesion drag, but we are not sure exactly where the equilibrium is. So it is important to devise experiments to study both making and breaking the joint in order to define the precise equilibrium point. Three typical experiments are shown in Fig. 33 [13]. 

Upon comparison of Eqs. 29 and 36, it is readily apparent that both theories predict the same power law dependence of the contact radius on particle radius and elastic moduli. However, the actual value of the contact radius predicted by the JKR theory is that predicted by the DMT model. This implies that, for a given contact radius, the work of adhesion would have to be six times as great in the DMT theory than in the JKR model. It should be apparent that it is both necessary and important to establish which theory correctly describes a system. 

It is apparent from Fig. 1 that the water evolution profile is qualitatively similar for water-sized and silane-treated glass fibers. Table 4 shows, however, that the desorption volume of physically adsorbed water (peak 1) is significantly larger for water-sized glass than for silane-treated specimens. This result is in qualitative accord with evidence from wetting experiments demonstrating that silane deposition diminishes the non-dispersive component of the work of adhesion with water [2-5], When bare and silane-treated fibers were equilibrated with water for 6 months, as opposed to several hours in this study, the desorption volumes of... 

Amides are preferentially adsorbed from aqueous solutions. Maximum adsorption occurs close to the pzc. The controlling factor appears to be the squeezing-out effect of the water rather than specific interaction of the amide molecule with the electrode even though this apparently occurs according to the high work of adhesion of form-amide and DMF (Table 7.1.1). 

These results gave similar behavior to that of silicone on acrylic, with an apparent equilibrium work of adhesion at low speeds, plus a velocity-dependent... 

A list of values for various aerogel/water pairs is given in Table 4.4. As the contact angle (9) increases (approaches to 180°), the respective works of adhesion diminish (VFa = 0.54 N/m, for 173°) because the solid-liquid interactions (contacted areas) become smaller and smaller which becomes apparent upon closer inspection of (4.3). For complete non wetting (9 = 180°), the Wa equals to zero. This means that the drop is levitating on the surface. 

It is immediately apparent that reduction in the thickness of the viscoelastic layer attenuates the work of adhesion and, at high peeling speeds, causes a decrease of adhesion with speed. This effect may be understood when it is appreciated that increasing speed brings an increasing volume of material into the high dissipation... 

Ta is the work of adhesion, which is equivalent to the Gibbs free energy decrease (per unit area) when an interface is formed from two pure component surfaces. The work of adhesion increases as the interfacial attraction increases, leading to a decrease in interfacial tension. It is apparent from (18) that, if the two compmients are identical, an expression can be obtained that relates the surface tensimi (7,- to the work of cohesion (Wed for component i ... 

In this example, at low speeds, the adhesion levelled off at a low value, corresponding to an apparent reversible work of adhesion of 0.3 J m at very... 

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