Water, tangential layer

Ideally, models of vicinal water should eventually "explain all established experimental facts. There is a long way to go However, some general observations have been made. One is that, against a variety of hydrophobic phases (silver iodide, mercury, air) water molecules appear to be oriented with the negative ends of the molecules pointing outward (sec. 3.9). In other words, the polarization of water adjacent to silver iodide and mercury is similar to the spontaneous polarization of water surfaces. The implication is that near such surfaces water-water interactions play at least an important role as water-surface interactions. Another observation, relevant for the interpretation of electroklnetic phenomena, is that tangentially immobile surface layers do occur near both hydrophilic and hydrophobic surfaces. 

On a molecular scale there is no sharp boundary between hydrodynamically stagnant and movable solvent molecules. As discussed In sec. 2.2, the, say tangential, diffusion coefficient of water near many surfaces may be somewhat lower than in bulk, but it is not zero. The very existence of ionic conduction In the layer(s) adjacent to surfaces also points to non-zero mobility. Yet, phenomenologically such layers behave as immobilized. This looks like a paradox, but the phenomenon is encountered in other places as well. For Instance, a few percent of gelatin added to water may hydrodynamically immobilize the liquid completely, without markedly impairing ionic conduction or self-diffusion of dissolved ions. Macroscopic immobilization of a fluid is not in conflict with mobility on a molecular sceile. 

Consider a piece of the ocean covered with a monolayer with film pressure 11 = crM - a, where crw and a are the surface tensions of pure water and film, respectively. We further assume that the film is stagnant. Beneath the film, however, there is a horizontal motion (taken to be in the x-direction) with a corresponding boundary layer. Consequently, there is a tangential viscous stress, rvisc, on the film which must be balanced by a gradient in n. 

Let us consider the problem, similar to Stokes first problem, in which a stagnant viscous incompressible fluid occupying the half-space Y > 0 is set in motion for t > 0 by a constant tangential stress to acting on the fluid surface (the simplest model of flow in the near-surface layer of water under the action of wind) [140]. In this case, the initial and boundary conditions for Eq. (1.9.1) are written as follows ... 

Diffusion across an interface. Consider a pond containing pure water. If the air above it is dry, water will evaporate from the surface, especially if the wind is blowing. The air flow will readily be turbulent, so that water vapor can be transported from the pond surface by convection. Now a surfactant is added, enough to produce a monomolecular layer on the pond, and the evaporation rate is markedly reduced. It is often assumed that the surfactant layer provides resistance to evaporation because water cannot readily diffuse through it. However, the layer is very thin (a few nanometers) and can only cause a small resistance to diffusion (see Section 5.3.3). The main explanation of the reduced evaporation must be that the wind over the surface causes a y-gradient, so that the surface can now withstand a tangential stress hence a laminar boundary layer of air will be formed near the surface, and the diffusion of water vapor through the boundary layer (which may be about a millimeter thick) will cause a considerable decrease in transport rate. 

Within the layers limited by hydrophobic walls, the water molecule dipoles are oriented parallel to the surface. The effect of ordered orientation spreads to a considerable distance that is, it is of a long-range nature. Such an orientation of water molecules causes a decrease in density near the walls and an increase in the mobility of the molecules in the tangential direction. This situation is interpreted as a decrease in the viscosity of the boundary layers. From a macroscopic point of view, this effect can manifest itself as the slipping of water on the hydrophobic substrate. 

In layers bound by the hydrophilic surfaces, the situation changes. Near such a surface, the water dipoles are oriented normal to the surface. This geometry results in an increase in the density of water and a decrease in the tangential mobility of water molecules within layers that are several nanometers thick. From a macroscopic point of view, there should be an increase in the viscosity of the boundary layers of water. With a decrease in the radius of the quartz hydrophilic capillary, the average viscosity of water increases. 

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