Wilhelmy balance

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. 

When the surface of the liquid is drawn up on the solid we say that the liquid wets the solid. This results from a substantial attraction between the liquid and the surface of the solid which is indicated by a small value ySL, the solid-liquid surface tension, and a large downward net surface tension force. A device that measures the force on a plate, such as shown in Fig. 6, is called a Wilhelmy balance. 

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. 

Surface tension measurements were performed on the aqueous solutions by means of a Wilhelmy balance, using a sand-blasted platinum plate, and are equilibrium values. All measurements were carried out at room temperature (23° 2°C.). 

The spectra clearly show that the adsorption of charged surfactants at the CCLj/H20 interface at nanomolar aqueous phase surfactant concentrations results in a significant modification of the interfacial water behaviour. Wilhelmy balance surface pressure measurements [89] show that at these concentrations (headgroup areas of >4000 molecule ), the water molecules responsible for the observed spectral... 

The Wilhelmy balance apparatus is composed of an electronic balance interfaced with a PC computer. The tensiometer measures the force exerted by a partially immersed thin-sample plate in water. A beaker containing water is moved up (increasing the immersion) and down (emerging the sample) by a constant speed, and the corresponding force change is recorded. 

The Wilhelmy apparatus is suitable for both static and dynamic contact angle measurements. However, it is particularly suited for the measurement of dynamic wetting properties of polymer surfaces, and most Wilhelmy balance is used in the dynamic mode. 

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. 

EFFECT OF WILHELMY BALANCE PARAMETERS ON FLUID HOLDING TIME... 

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. 

The software driven apparatus allows different types of area changes step and ramp type, square pulse and trapezoidal as well as sinusoidal area deformations. The construction ensures that area changes are almost isotropic. Area changes used in transient and harmonic relaxation experiments are of the order of 1 to 5%. The surface tension response measured via the Wilhelmy balance has an accuracy of better than 0.1 mN/m. 

Serrien et al. (1992) measured the damping of planar longitudinal waves, generated with a barrier on a Langmuir trough and detected with a Wilhelmy balance. The longitudinal waves correspond to slow periodic compressions/dilations of the protein adsorption layer (BSA, casein). The results for BSA are shown in Fig. 6.18. Further experiments using a stress... 

All surface-active molecules, such as soaps and lipids, can be prepared as monomolecular layers at the air-water interface. Driven by the reduction of the surface free energy of water, these molecules spread when applied to the surface, for example, from a volatile solvent (Fig. 10). The physical properties of these monolayers were first investigated in the 1940s by Langmuir after his first work on molecules in insoluble monolayers [41]. A so-called Langmuir trough filled with water defines an exactly known area for the spread molecules (Fig. 10). At low lateral density, these molecules behave like a quasi-two-dimensional gas. If the area for these molecules is reduced by a movable barrier, this lateral compression will eventually lead to a measurable lateral pressure n (force F per unit length of barrier). Which can be measured by a so-called Wilhelmy balance (see Kuhn et al. [42]), It is the difference between the surface tension of the free, y, and the layer-covered, y, water surface ... 

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. 

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. 

More work, including surface-tension measurements (e.g., via Wilhelmy balance investigations), will be needed to better explain the details of the behavior of mixed organophosphate SAMs on rough (e.g., SLA type) titanium (oxide) surfaces. The motivation for further investigation is twofold we believe that the interfacial energetics is important in the context of protein adsorption and... 

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.,... 

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. 

The Wilhelmy balance is an excellent technique for indirectly measuring contact angles on a flat plate of known perimeter of the plate cross-section. The technique is based on the following relationship ... 

Table 14.3. Summary of contact angle, 6, and surface tension, ypv, data obtained by the Wilhelmy balance technique and the solidification front technique for carbon and Kevlar fibres, respectively (from ref. (75))...

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. 

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