Spreading spontaneous

Complete wetting, i.e. spontaneous spreading should always be sought to maximize adhesion. This condition occurs when, with reference to Fig. 4, it is not possible to satisfy the horizontal force balance, i.e. ys > Vl + Ysl- The thermodynamic driving force for the spreading process is the spreading coefficient. 

When spontaneous spreading occurs, the bulk of the advancing liquid is preceded by a precursor film, usually a few millimeters in width and a few hundred nanometers or less in thickness [58], as pictured in Fig. 12. The observed dynamic contact angle is that which is made by the bulk liquid against the precursor film, and it itself depends on the rate of the advance of the nominal interline. The relationship between the rate of spontaneous spreading, i.e. the rate of movement of the nominal interline normal to itself, (/, and the dynamic contact... 

Table 12 shows the equilibrium spreading pressures of each diacid. It is immediately apparent that for three of the diastereomeric pairs there are statistically significant differences. These distinctions relate stereochemical preferences in the spontaneous spreading of (+)- versus meso-monolayers in equilibrium with their respective crystalline phases. However, there appears to be no discernible trend in either the ( )- or meso-ESPs as a function of carbonyl position despite clear trends seen in their monolayer properties in the absence of any bulk crystalline phase. 

Complete wetting of a solid is only possible if a drop of the liquid spreads spontaneously at the surface, i.e. for 9 = 0 or cos 9=1. The limiting value cos 6 = 1 is a constant for a solid and is named critical surface tension of a solid y... Therefore, only liquids with yl < Vc have the ability to spontaneously spread on surfaces and wet them completely. Tab. 4.2 gives an overview of critical surface tension values of different polymer surfaces [40]. From these data it can be concluded that polytetrafluoroethylene surfaces can only be wetted by specific surfactants with a very low surface tension, e.g. fluoro surfactants. 

De Gennes and Cazabat proposed an alternative continuum model [290]. They describe the spontaneous spreading of a liquid by a competition between the driving force, which is the disjoining pressure in the precursor film and the core region, and the friction between layers of liquid with the solid [287],... 

A complete wetting of a solid is only possible for spontaneous spreading of a drop of liquid at the surface, i.e. for 6 = 0 or cos = 1. For a specific solid surface of low surface energy, a linear correlation is observed between cos 6 and the surface tension. This is demonstrated for polytetrafluoro ethylene in Figure 3.10. 

A 45° contact angle is given for a liquid on a smooth solid surface. What roughness, i, must the solid surface have to induce spontaneous spreading ... 

Most conventional polymers are moderately hydrophobic (i.e., they possess low surface energy) thus, spontaneous wetting by water does not occur on such polymers. The improvement of the wettability of polymeric solids usually requires some type of surface modification. Spontaneous spreading of a liquid on a solid surface occurs when the surface tension of the liquid is less than the surface tension of the solid, i.e., A(yg)s given by Eq. (25.6), is negative, in which case the contact angle is zero. Thus, contact angle = 0 is the postulated necessary condition for the formation of stable and continuous water films, but some moderately to poorly... 

Figure 3.1. Spontaneous spreading of a monocomponent liquid of amphlpolar molecules.

Returning to our present issue, the dynamics of spontaneous spreading, we can at least make the statement that flow in the film is two-dimensional (onedimensional if the length of the film is small as compared to the radius of the droplet), whereas it is three-dimensional for the bulk of the droplet. Intuitively, one might expect a(dyn) to increase with Ca. Experimentally, this trend is corroborated with, at the extremes, a(dyn) -> 0 for very low Ca (down to 10 ) and a(dyn)- n for very high Ca (10 and higher). Cubic relationships of the nature... 

The kinetic model of matter and the concept of entropy both explain the process of diffusion, which is the spontaneous spreading of liquid or gas particles throughout a volume. Explain, in terms of both kinetics and disorder, why a drop of a dye placed into a beaker of water will eventually color the entire volume of water. 

Figure 2-7. A sketch of the contours of constant hydrostatic pressure within a drop that is initially placed on a flat surface in air. As discussed in the text, the fact that the horizontal component of the pressure gradient is nonzero implies that this fluid configuration is unstable and will undergo a spontaneous spreading motion to decrease the hydrostatic pressure gradients in any horizontal plane. In this case, this process will continue indefinitely but at a decreasing rate as the contours of hydrostatic pressure become increasingly horizontal and flat If the edge of the drop encounters container walls these will stop further spread and the drop will continue to move only until it has filled the container to a constant depth.

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