Sessile drop experiments

The contact angle is by far the most often quoted characteristic to be derived from sessile drop experiments, but reference will be made later in this Section to other liquid parameters and particularly to the liquid surface energies, (tlv. While values for both 0 and rLv can be determined from measurements made during a single sessile drop experiment it is often convenient to use differently sized drops. Small drops will assume the profiles of nearly spherical caps and this regularity assists the estimation of 0 values from their dimensions, while the gravitational 

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The five main requirements for conduct of a sessile drop experiment relevant to high temperature capillary phenomena are characterisation of the materials, a flat horizontal substrate, a test chamber to provide a controlled and generally inert gaseous environment, a facility that heats the sample to a predetermined temperature and a means of measuring the geometry and size of the sessile drop. Satisfying these requirements demands careful and precise experimental procedures. 

Thus conduct of even a simple sessile drop experiment can be difficult and require a complex array of sophisticated equipment, such as that shown in Figure... 

A sessile drop experiment can take one of several forms, as shown schematically in Figure 3.7. The classic form shown in Figure 3.7.a calls for a small piece of solid sessile drop material, typically 0.01 ml in volume, to be placed on a substrate and then heated above its melting temperature. The sessile drop can be composed of a pure material or it can be an alloy or a chemical compound. It is not always convenient to prepare alloys or compounds prior to conducting the sessile drop experiment, and a variant of the classic technique, shown in Figure 3.7.b, is to form the alloy or compound in situ from a mechanical mixture of the components, (Mortimer and Nicholas 1973). In some work done in the former Soviet Union, metal alloys were formed in situ by plunging a piece of solid solute into a molten sessile drop of solvent (Naidich and Zhuravlev 1971). 

Figure 3.7. Methods of conducting sessile drop experiments (a) classic technique, (b) in situ formation of an alloy, (c) dispensed drop, (d) transferred drop, (e) double substrate, (0 tilted plate.

Sessile drop experiments are also used to measure the effects of temperature on liquid surface energies. Because the temperature coefficient dliquid metals and oxides is usually a very small, negative, value (—0.05 to —0.5 mJ.m-2.K-1), a temperature rise of several hundred degrees is necessary to produce decreases in the surface energy that can be reliably detected by measurements of drop profiles. Even in this case, the error on the temperature coefficient lies between 30% and 100% (see Section 4.1.1). 

Sessile drop experiments have been used extensively to derive quantities characterising spreading and penetration phenomena, such as the work of adhesion and work of immersion, given by equations (1.45) and (1.54), using a single sessile drop experiment to measure both the contact angle and the liquid surface energy. 

In the example of Table 4.4, the surface energy of solid W in equilibrium with a saturated vapour of Cu is lower than °sv due to adsorption of Cu atoms on the W surface. This is generally characteristic of metallic A-B pairs having a low mutual miscibility (Eustathopoulos and Joud 1980). For this reason, results of sessile drop experiments for such systems cannot be interpreted by taking for the surface energy of the solid metal the value of equilibrium with its own vapour (see Sections 1.4.2 and 5.2). 

Selected results of Bailey and Watkins (1951-52) are given in Table 5.5. These demonstrate that configuration 2.a of Figure 5.7 is achieved with a wide range of couples that form intermetallics compounds or in which the liquid member dissolves in the solid. For Ag/Fe, the drop-forming behaviour accords with the results of sessile drop experiments which identified contact angles of 36°-57° as... 

In principle, the Young contact angle can be measured only for a metal presaturated with C. In practice, two major difficulties exist first, the high roughness and porosity of polycrystalline graphite, second, the difficulty of performing standard sessile drop experiments with metals fully presaturated in C (Hara et al. 1995). This second difficulty arises because substantial dissolution of... 

This special class of brazes reacts chemically with the surfaces of ceramic components to produce wettable products with metallic characteristics, such as TiO, TiC x or TiN x as described in Sections 6.3 and 7.2. Thus the wetting is due to an in situ metallization . By definition, the brazes must contain chemically reactive elements such as Ti that are often added to eutectic brazes similar to those developed for joining metal components. Many sessile drop experiments have shown that active metal brazes can wet a wide range of ceramics when a suitable inert environment is used. Particularly high standards of environmental inertness... 

FIGURE 4.20 Scheme of the experimental setup for sessile drop experiments (left) and for captive air bubble measurements (right) using ADSA. Source Uhlmann et al. [41]. Reproduced with permission of Amaican Chemical Society. 

If the spreading coefficient is positive, the spreading of the Hquid over the solid surface is spontaneous. If Young s equation is substituted into the Dupre equation the work of adhesion can be determined from sessile drop experiments, since only and 0 remain as variables and both are readily determined by this method ... 

Figure 9. Schematic drawing for (a) and (b) for bubble containing inclusion for wetting and non-wetting condition, respectively, (c) and (d) sessile drop experiments for wetting and non-wetting conditions, respectively.

To characterize spreading/penetration characteristics of binder/powder systems, sessile drop experiments are performed using commercial goniometers. Binders... 

Sessile drop experiments, using a formic acid-nitrogen gas mixture as a flux, performed by Goslin et al. [173] on Au-coated Ni substrates (Fig. 56), indicate that the addition of Ag in the alloy reduced spreading. Fig. 57 shows the spreading results for the same alloys except that the... 

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