Wetting force

In a small-diameter capillary tube, wetting forces produce a distortion of the free liquid surface, which takes a curvature. Across a curved liquid surface, a difference of pressure exists and the variation of pressure across the surface is the Laplace or capillary pressure, given by ... 

The wetting energies give repulsive forces exceeding that of the van der Waals attractive force under certain conditions. The combined van der Waals and wetting force is given by equation 7 in which h is the distance perpendicular to the interface that the particle has moved from its equilibrium position. 

Figure 9.2 Measurement of contact angle by the Wilhelmy technique. The wetting force is measured by a sensitive electrobalance and the contact angle is calculated from changes in the wetting force.

The measured force, F, in Eq. (26.2) is the difference in the interfacial force between the water and sample plate (wetting force) and the buoyancy force from the immersed portion of the plate. The total force, F, is divided by the sample plate perimeter, L, to give force per unit length, FjL (expressed in mN/m). 

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 ... 

Application. This is the basic action of turning a more or less dry substrate or web into a wet one, and it can be carried out before or after the metering function, and even as part of the distribution function. The heart of application is the dynamic wetting zone, which looks like a line and is often called one, although it is in fact a microscopic or submicroscopic region in which wetting forces are active and the coating liquid replaces gas at the solid surface. 

For measurement of the work of adhesion to the surface of the fibres the Wilhelmy method is known to be appropiate /15/. Although here the contact angle cannot be measured directly. But the method allows us to measure the wetting force F and the tear-off force F q with an electrical balance. The contact angle can be obtained according to one of the following two equations. 

Equation (8) assumes that the cross section of the fiber is circular. If the cross section is not circular, the ird term is replaced by the actual circumference of the fiber. The circumference may be determined by measuring the wetting force of the fiber in a liquid that completely wets the fiber [cos( = 0] as described by Herb et al. [57]. 

Figure 6 Advancing wetting force eurves in sueeessive immersions of untreated hair fiber and the same hair fiber treated with 1% JR-400.

TRI has developed a different method for low-load friction (43), which uses a modification of the interfiber fiictional apparatus mentioned above. In this instrument, the forcemeasuring device is a Kahn recording microbalance on which the vertical fiber is mounted. The horizontal fiber can be moved by a micrometer to establish the exact angle of deflection between the fibers. The system is shown schematically in Figure 21. The apparams is an attachment to the TRI/SCAN surface force analyzer, which was described earlier in the discussion of wetting force measurements with the appropriate software. 

In the following section the focus is on the methods that allow the characterization of the wetting behavior indirectly the contact angle is derived by measuring the wetting force exerted from a meniscus of a liquid on a vertical fiber or by determining the penetration rates of liquids in fiber assemblies. 

The rate of rise to maximum wetting force This will show the level of oxides present on the surface in the case of a gold-plated part, and how thick the deposit is. This again will provide information on process control and/or age and storage of the parts being tested. 

The maximum wetting force produced The higher the force the better—provides information on the basis metal solderabiUty, the surface preparation, or lack thereof. Low forces recorded on a test, even if the wetting time is fast, is of major concern. 

Once the part wets and produces positive wetting forces, do they remain consistent (ideal), continue to increase (indicative of oxide reduction), or begin to de-wet (the worst... 

The impact of stressing (steam exposure or dry bakes or temperature/humidity) on the robustness of the plating can easily be determined as a function of reduced wetting forces or increased wetting times. 

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