Wetting or contact angles

A drop of liquid at rest on a solid surface is under the influence of three forces or tensions. As shown in Fig. 10.2, the circumference of the area of contact of a circular drop is drawn toward the center of the drop by the solid-liquid interfacial tension, 7sl- The equilibrium vapor pressure of the liquid produces an adsorbed layer on the solid surface that causes the circumference to move away from the drop center and is equivalent to a solid-vapor interfacial tension, ygy- The interfacial tension between the liquid and vapor, y y, essentially equivalent to the surface tension y of the 

At mechanical equilibrium the tensions or forces cancel to yield 

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Figure 5.1. Schematic diagram of a crystallite (or droplet) on a support with wetting or contact angle 0.

The experimental method of mercury porosimetry for the determination of the porous properties of solids is dependent on several variables. One of these is the wetting or contact angle between mercury and the surface of the solid. 

The wetting or contact angle 0 between mercury and solid is usually 130°, and the surface tension of the mercury, y, is 0.48 N/m. Pressure is expressed in atmospheres and d in nanometers (10 A). This technique is satisfactory for pores down to 50 A diameter however, this is a function of the instrument capability. Maximum diameters measured are usually 106 A. 

In the pendular state, shown in Figure la, particles ate held together by discrete lens-shaped rings at the points of contact or near-contact. For two uniformly sized spherical particles, the adhesive force in the pendular state for a wetting Hquid (contact angle zero degree) can be calculated (19,23) and substituted for H. in equation 1 to yield the foUowing, where y is the Hquid surface tension in N/m. 

Why does a drop of pentane spread into a thin film when placed on a water surface, whereas a larger hydrocarbon such as dodecane breaks up into smaller droplets This is not an academic question, as should be evident from the importance of wetting and contact angle phenomena that we discussed in Chapter 6. Why is it that we can produce relatively stable bubbles with a soap solution but not with pure water Water droplets on an oily surface, dewdrops on a blade of grass, and soap bubbles or foams are so common in our daily life that they rarely engage our attention, but to a scientist they are a constant reminder of the ubiquitous van der Waals forces ... 

Platinum plates with 1 cm area per side were metallographically polished to a mirror finish, rinsed with distilled water, and flamed just before exposure to sea water for either ellipsometric or contact angle measurements. The platinum plates were wetted with photo-oxidized sea water before immersing into experimental sea water, so that passage of the surface through the air—sea water interface would not cause any film present there to be transferred to the plate. For the same reason plates removed from the experimental sea water were immediately immersed while still visibly wet in photo-oxidized sea water and then rinsed. 

The wettability of hair surfaces can be determined using the Wilhelmy technique, in which the force exerted by the wetting liquid (usually water) on an individual fiber is scanned along the fiber length [194,195], The wetting force (Fw) or contact angle (9) is then given by the Wihelmy equation ... 

When adsorption of the surfactant onto the substrate is strong, however, Fowkes found that the rate of wetting was determined not by the bulk phase concentration of the surfactant, but by the rate of diffusion of the surfactant to the wetting front. In this case the concentration of surfactant present at the advancing liquid front was so depleted by adsorption that the surface tension (or contact angle) there, and... 

Wetting hysteresis (or contact angle hysteresis) is defined as the ability of a liquid to form on a solid surface several stable (or metastable) contact... 

When a three-phase contact line is formed by a solid phase and two liquid phases, a selective wetting of the solid phase by one of the liquids takes place. Usually, there is competition between the polar phase (e.g., water) and the nonpolar phase (e.g., hydrocarbon or oil ) in the wetting of the polar and nonpolar solid surfaces. By convention, in selective wetting, the contact angle, 0, is measured into the more polar phase. The solid surface is referred to as hydrophilic ( oleophobic ) when it is predominantly wet by water (0 < 90°), and hydrophobic ( oleophilic ) when it is predominantly wet by a nonpolar liquid (0 > 90°), as illustrated in Figure 1.8. 

Here a - surface tension pa - atmospheric pressure 9 - contact angle of crack s wall wetting by penetrant n - coefficient, characterizing residual filling of defect s hollow by a penetrant before developer s application IT and h - porosity and thickness of developer s layer respectively W - minimum width of crack s indication, which can be registered visually or with the use of special optical system. The peculiarity of the case Re < H is that the whole penetrant volume is extracted by a developer. As a result the whole penetrant s volume, which was trapped during the stage of penetrant application, imbibes developer s layer and forms an indication of a defect. 

A zero or near-zero contact angle is necessary otherwise results will be low. This was found to be the case with surfactant solutions where adsorption on the ring changed its wetting characteristics, and where liquid-liquid interfacial tensions were measured. In such cases a Teflon or polyethylene ring may be used [47]. When used to study monolayers, it may be necessary to know the increase in area at detachment, and some calculations of this are available [48]. Finally, an alternative method obtains y from the slope of the plot of W versus z, the elevation of the ring above the liquid surface [49]. 

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