Wilhelmy technique

Many different techniques have been developed for the measurement of contact angles 17.8). Of these, the three most useful methods are the Wilhelnty technique, the technique of capillary rise at a vertical plate, and the drop shape methods. These techniques require the solid surface to be flat and smooth. Direct measurement of contact angles on fibers (of uniform thickness) can also be performed using the Wilhelmy technique. For nonflat surfaces or particles, indirect methods such as capillary penetration into columns of powders, sedi-... 

In the following subsections, the three most commonly used contact angle techniques, the Wilhelmy technique, capillary rise at a vertical plate, and drop shape methods, are described in detail. 

FIG. 2 Apparatus for Wilhelmy technique (1) measuring plate, (2) glass fiber or rod, (3) electrobalance, (4) recording device, (5) measuring cell, (6) liquid, (7) movable platform, (8) screw or gear mechanism to raise or lower the platform, (9) motor, (10) clamp and support, (11) lid. 

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 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 ... 

At present time, the most sensitive technique for measuring surface osmotic pressures is the Wilhelmy technique (1968) 4 it consists in measuring a lowering of surface tension. Let r0 be the surface tension of a pure liquid, and r0+, the surface tension of a liquid whose surface has been spread with a defined quantity of solute. It will be shown that the surface osmotic pressure is simply... 

In the case of small areas A, the film surface pressure n [mNm"l] was determined with an accuracy of 0.2 mNm l from the difference Yq - Y between the surface tensions of the pure and of the film covered substrate respectively. The tension was measured by the Wilhelmy technique using a roughened Pt slide and a Sanborn force transducer made by Hewlett-Packard. The accuracy of the surface pressure manometer Lauda is about 0.2 mNm l too. The variation of the surface pressure n with average area per molecule a (not shown) was established for the mixed films PVPC6 + PI for which xpjt is 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 and where xpj is the mol fraction... 

The Wilhelmy technique was employed to study adhesion on the coated carbon fibres. 

Pike FD, Thakkar CR Interfacial tension measurement by an improved Wilhelmy technique. In Kerker M (ed) Colloid and interface science, p 375 Wilhelmy LF (1863) PoggAnn 119 177 Matsunaga T, Ikada Y (1981)J CoUoid Interface Sci 84 8 Li D, Neumann AW (1992) J CoUoid Interface Sci 148 190... 

Using the Wilhelmy technique the wetting tension (yiv-cos0) exerted by a liquid meniscus on a vertical fiber can be measured directly, using the balance ... 

Until the present day, only very few methods are available for the study of the wetting behavior of libers directly by polymer melts. The modified Wilhelmy technique [89,93] can be used to measure directly the wetting tension (7iy cos 0). When the surface tension of the polymer melt is known, then the contact angle can be calculated [94]. In addition, direct measurement methods can be used. Such methods enable the calculation of the contact angle of drop-on-fiber systems from the drop shape [18 20]. 

It should be noted that the Wilhelmy technique can be readily used to measure liquid surface tensions, e.g. with a straight piece of platinum wire of a diameter of 1 -2 mm, which can be cleaned readily by heating it in a methanol flame. The contact angle is then zero for many liquids, as required. 

Interfacial tension between two fluid phases is a definite and accurately measurable property depending on the properties of both phases. Also, the contact angle, depending now on the properties of the three phases, is an accurately measurable property. Experimental approaehes are described, e.g., in Refs. 8,60, and 63 and in Ref. 62, where especially detailed discussion of the Wilhelmy technique is presented. Theories sueh as harmonie mean theory, geometric mean theory, and acid base theory (reviewed, e.g., in Refs. 8, 20, and 64) allow caleulation of the solid surfaee energy (beeause it is difficult to directly measure) from the contaet angle measurements with selected test liquids with known surface tension values. These theories require introduction of polar and dispersive components of the surface free energy. 

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