Wilhelmy balance method

The sessile droplet contact angle measurement is a simple and accurate method to obtain information pertinent to the surface energy of a sample. The Wilhelmy balance method, on the other hand, is a very useful method to investigate the surface dynamic aspect of a sample, which will be described in the following sections. The instability of some of plasma-treated polymer surface observed by the Wilhelmy balance method is also described in Chapter 30. 

The FHT measured by the Wilhelmy balance method can be effectively used to compare the liquid holding capabilities of different surfaces. The value of FHT depends on the experimental parameters and cannot be used in an absolute sense. In general, the aqueous film stability is obtained when spontaneous wetting occurs on imperturbable surfaces. However, moderately hydrophilic and possibly even some hydrophobic surfaces that are perturbable by water were found to be capable of holding continuous films of water. Multicomponent fluid, such as a dilute solution of protein used as a simulated tear fluid, may yield misleading liquid holding characteristics of surfaces due to preferential adsorption of components on a surface. 

K Grundke, P Uhhnann, T Gietzelt, B Redlich, HJ Jacobasch. Studies on the wetting behaviour of polymer melts on solid surfaces using the Wilhelmy balance method. Colloid Surface A 116 93-104, 1996. 

Most studies of the surface wettability of fibers by the Wilhelmy balance method use relatively short specimens because of the difficulty of immersing long fibers vertically in the liquid. In an effort to overcome this disadvantage of the bulk immersion method, TRI has developed the liquid membrane method for the scanning of long filaments (23). The experimental arrangement for this method is shown schematically in Figure 8. 

Surface tension of liquids can be measured by either of the two methods static and dynamic. The static methods are based on the assumption that the liquid has attained surface equilibrium. For pure liquids and solutions of crystalloids the process of attainment of equilibrium is very fast and the static methods are best suitable. But for colloidal solutions a considerable time is required to reach the equilibrium state and therefore the dynamic methods of measuring surfacf tension are preferred. The dynanJc methods measure the tension of a liquid before the surface film has had time to form. TTiere are other methods too which fall between the static and the dynamic methods. Among the static methods, the most commonly used ones are (0 the capillary rise method, (ip the du Nouy ring method, (Up the Wilhelmy balance method, and (iv) the drop-weight method.,... 

The surface tension of BNNTs was measured using the Wilhelmy balance method with nonpolar and polar fluids, obtaining a value of 27 mN m , comparable to values obtained for CNTs (see section 10.3.1). Contact angle measurements using different liquids showed an increase in value as a function of the liquid s polarity, with a value of 85 5° water. 

In the Wilhelmy balance method, the polymer film is suspended vertically from the balance and then lowered slowly until it is in contact with the liquid. If the surface tension at the liquid-air interface is known, the contact angle can be calculated from the difference in sample weight when in and out of contact with the liquid. 

The film pressure is defined as the difference between the surface tension of the pure fluid and that of the film-covered surface. While any method of surface tension measurement can be used, most of the methods of capillarity are, for one reason or another, ill-suited for work with film-covered surfaces with the principal exceptions of the Wilhelmy slide method (Section II-6) and the pendant drop experiment (Section II-7). Both approaches work very well with fluid films and are capable of measuring low values of pressure with similar precision of 0.01 dyn/cm. In addition, the film balance, considerably updated since Langmuir s design (see Section III-7) is a popular approach to measurement of V. 

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. 

For a plate of rectangular cross section having length f and thickness t, P = 2(f + t) these dimensions can be accurately measured. By suspending the plate from a sensitive balance, we can also measure w with considerable accuracy. The apparatus is called a Wilhelmy balance, and the technique the Wilhelmy plate method. Thus, if the contact angle is known from an independent determination by, say, the tilted-plate method, then 7 can be evaluated by Equation (2). 

We saw in Section 6.2 that the Wilhelmy plate offers an accurate method for measuring 7 cos 6. We thus have one experiment with two unknowns. The Wilhelmy balance measures the weight of the meniscus in this section we examine the height to which the meniscus climbs on the same surface. We shall see that this distance — which may be accurately measured with a traveling microscope or cathetometer — also depends on 7 and 6. The functional relationship between these parameters and the experimental variables is different from the case of the meniscus weight. Therefore we have two experiments with two unknowns that can be solved for 7 and 6. 

An extensive discussion of the Langmuir balance technique and a comparison with the Wilhelmy plate method are given by MacRitchie (1990). This book also discusses modifications that are possible to the techniques and other experimental details. 

Devise a method to accurately use two Wilhelmy plates (see Fig. 6) to measure the surface tension of a Langmuir film. Do not forget about the gravitational and buoyancy forces. Such a device is called a Wilhelmy balance. 

The Wilhelmy plate method, as shown in Figure 4.10, is similar to du Nouy s ring method, but it uses a thin mica plate or microscope slide. The plate is suspended from a balance and dips into the liquid. The force, F, required to detach the liquid meniscus surrounding the plate depends on the surface tension or interfacial tension by ... 

In Wilhelmy s method, the downward pull exerted by a liquid on a thin vertical glass or metal plate hanging partly immersed in the liquid and wetted by it, is measured by suspending the plate from one arm of a balance and... 

There are two modifications to the Wilhelmy plate method. In the first modification, the cup carrying the liquid is mobile and is lowered until the previously immersed plate becomes detached from the liquid surface, and the maximum vertical pull, / max on the balance is noted, similarly to the ring method. Then the capillary force, for the zero contact angle, can be given as... 

The Wilhelmy slide method is somewhat similar to the ring-pull method. A very thin plate, such as a microscope cover glass or a sheet of mica, is hung from one arm of a balance and allowed to dip in the solution (Fig. 18.3). If p is the perimeter of the slide, the downward pull on the slide due to surface tension is yp. If F and F are the forces acting downward when the slide is touching the surface and when it is suspended freely in air respectively, then... 

The essential difference between the two studies is that Elliott and Riddiford, Schwartz et al. (and many others) used optical methods to determine 0 whereas Johnson et al. used the Wilhelmy plate method. The Wilhelmy balance derives the contact angle from a force measurement. 

The measurement of the surface tension of a liquid by the Wilhelmy - the force balance method - is widely used and well documented. The same apparatus may be used to... 

This difference is, of course, independent of the buoyancy correcUon and, accordingly, when used in this way the buoyancy correction is unnecessary. One absolute requirement of the Wilhelmy method is that the substrate solution must wet the slide, i.e., the angle of contact between the slide and solution must be zero. The Wilhelmy method has been employed by Dervichian (25), by Abribat and Dognon (26), by Harkins and Anderson (27) and by others. The reviewer has found tUn microscope cover glasses which are 6X6 serve as excellent Wilhelmy plates. A battery of these plates fixed in a parallel position and properly spaced from each other can be suspended from an arm of a chmn-o-matio balance and rather extreme sensitivity can be achieved. With the Wilhelmy balance thoe is no question of leakage arotmd barriers as the q>read film is compressed. 

A stand-alone static method is the popular Wilhelmy plate method (Figure 1.18). In this method, a completely wetted platinum plate is brought into contact with a liquid surface, and a pull force is applied to the plate. Equilibrium is achieved when that force, corrected by the buoyancy force acting on the immersed part of the plate, is balanced by the surface tension, that is, F + dbHpg = 2(d + b)a. The force F is measured with a sensitive dynamometer, which typically forms the core of modem surface tension meters. 

A commonly used alternative to the Langmuir film balance method of determining surface pressure is to measure surface tension using a Wilhelmy plate (Section 4.3.1), dipped into the monolayer at different stages of compression. 

The Wilhelmy plate method (Ludwig Ferdinand Wilhelmy [1812-1864]) can be used to measure surface or interfacial tension at an air-liquid or liquid-liquid interface. In this method, a thin plate is oriented perpendicular to the interface, and the force exerted on it is measured. The plate is often made from glass or platinum which may be roughened to ensure complete wetting. The plate is cleaned thoroughly and attached to a scale or balance via a thin metal wire. The force on the plate due to wetting is measured via a tensiometer or microbalance and used to calculate the surface tension (y) using the Wilhelmy equation ... 

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