Work of wetting

Wo-tting Single. Pa.rticles. When a particle is submerged in a Hquid, the work of wetting a surface, n>, is the change in interfacial energy density times area, a, as the soHd—gas and Hquid—gas interfaces are replaced by a soHd—Hquid interface. 

Another important wetting parameter is the work of wetting [70] or adhesion tension W , defined as ... 

The authors have also applied the method of molecular dynamics (see Chapter III,4) for the investigation of peculiarities in the formation and rupture of coagulation contacts at the atomic-molecular level [29], It was established that at the nano-level under the conditions of a complete lyophilicity (macroscopically, at extremely low interfacial tension at solid-liquid interface) the particle did not always readily separate from the substarate, because its separation required that a particular gap was to be formed before the dispersion medium could penetrate underneath the particle and fill this gap. This means that molecular attractive forces had to be mostly overcome before the work of wetting could be performed, which required either the work of external forces or waiting for a long period of time for a suitable fluctuation. Periodic oscillations of force, experimentally observed by J. Israelachvili [30], were also present at several molecular distances. 

Frisch and Frisch speculated that the minimum in surface tension might be due to a large entropic contribution to the reversible work of wetting. This, in turn, may have been caused by an elastic straining of the immediate surface layers near a critical point of inversion. One of the network components may have been leaving the interface, and the other migrating there at the minimum. 

The above force is opposed by the surface force resulting from wetting and dewetting of the leaf surface as the droplet slides downwards. In moving down, an area w dl of the leaf is wetted by the droplet and a similar area is dewetted by the trailing edge. The work of wetting per unit area of the surface is equal to + 1)j... 

At the same time, the similarity between the solid and liquid phases, as evidenced by good wettability and high values of the work of wetting and the work of adhesion, suggests that the interfacial energy 012= Osl is much lower than the OsGof the same glass specimen in air. In Chapter 2, we will further address contact interactions in the case of surfactant adsorption. 

At the same time, the same figure shows independent experimental results for the work of wetting of methylated surfaces, = Ol cos 0, by aqueous solutions of ethanol. These data are presented in the form of the difference /2Aof(SG) - W ((l)). The fact that these two independent data sets coincide confirms the conclusion that polar fluids are expelled from the gap between hydrophobic surfaces and hence confirms that it is valid for one to regard this difference as the interfacial free energy, Oi2=asL. 

The rupture of a contact in the case when the particle moved with a constant velocity under the applied external force is illustrated in Figure 4.29. Here, we encounter gaps between the particle and the substrate that are on the order of molecular dimensions. In order to realize the work of wetting, the molecules of the medium must penetrate into the gap. This was possible only after the gap became comparable in size to the molecules. However, by this point in time, the attractive forces had... 

In addition. Figures 7.4(b) and 7.5 schematically illustrate further thermodynamic expressions for solid-liquid interactions, i.e. the work of wetting or work of immersion and the work of cohesion and the work of adhesion , respectively. As can be easily seen, each of these parameters represents the work associated with a de-wetting process. 

Figure 7.4. Definitions of thermodynamic parameters for (a) work of spreading, and (b) work of wetting...

As can be seen from Figure 7.4(b), there is a third wetting parameter, called the work of wetting , (or more commonly, the wetting tension or adhesion tension ). This parameter is defined as the work expended in eliminating a unit area of the solid-liquid interface while exposing a unit area of the solid-vapour interface, as follows ... 

This reversible work of wetting possesses a maximum at... 

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