Wilhelmy slide technique

In addition to the sessile drop method which measures the contact angle directly, Neumann and Renzow (1969) have developed the Wilhelmy slide technique to measure it to 0.1° precision. As shown in Fig. 2.20, the meniscus at a partially immersed plate rises to a finite length, h, if the contact angle, 0, is finite. 6 is calculated from... 

Fig. 2.20. Wilhelmy slide technique for contact angle measurement. After Adamson (1982).

This method suffers from two disadvantages. Since it measures 7 or changes in 7 rather than t directly, temperature drifts or adventitious impurities can alter 7 and be mistakenly attributed to changes in film pressure. Second, while ensuring that zero contact angle is seldom a problem in the case of pure liquids, it may be with film-covered surfaces as film material may adsorb on the slide. This problem can be a serious one roughening the plate may help, and some of the literature on techniques is summarized by Gaines [69]. On the other hand, the equipment for the Wilhelmy slide method is simple and inexpensive and can be just as accurate as the film balance described below. 

Several methods for measurement of contact angle are summarized by Adamson (28). Neumann and Renzow (31) describe the Wilhelmy slide method which offers significantly higher precision than other techniques. ... 

As soon as the coated slides were removed from the oven and cooled to room temperature, dynamic contact angles were measured by the Wilhelmy plate technique [21], We obtained the wetting curve with an electrobalance (Cahn Model... 

The Wilhelmy slide method requires no correction, in contrast with the previous techniques, and is simple to use. It is based on the principle that a thin plate immersed vertically in the liquid supports a meniscus whose weight, measured statically or by detachment, is given very accurately by the equation... 

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... 

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