Solids sessile drop

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 C2.11.8. An illustration of the equilibrium contact (i.e. wetting) angle, ( ), fonned by the balance of interfacial energies for or a liquid (sessile) drop on a flat solid surface.

Fig. 3. Sessile drop with equilibrium contact angle 6 displacement of edge a distance dx (s solid 1 = liquid v = vapour).

As was shown above (Section 2.2, Eq. 5), Young s equation (Eq. 4) may be derived by considering the small displacement from equilibrium of a sessile drop on a plane surface. If the same derivation is applied to the situation where the solid surface has a roughness factor (Eq. 19) of r, it is readily seen that Eq. 5 becomes [28]... 

Fig. 5. Sessile drop on a rough surface true contact angle BTA and apparent contact angle BTH. Thick curve = surface of solid (s) thin curve = surface of liquid (1) v = vapour. T is the triple point HTR a horizontal AT a tangent to the solid surface BT a tangent to the liquid surface.

We can consider the spreading of a sessile drop on a soft, lossy substrate rather like the advance of a negative crack and thus use fracture mechanics concepts, as was the case in the derivation of Eq. (15) for the separation of an elastomer from a rigid solid. The term negative is used since the spreading of a drop leads to the creation of solid/liquid interface rather than separation. 

To investigate the influence of swelhng of the substrate by the contacting liquid, the contact angle 6 of sessile drops of tricresylphosphate, TCP (drop volume 2 p,L, viscosity t = 70 cP, surface tension = 40.9 mN m ), has been measured as a function of time after deposition, t, on flat, smooth, horizontal surfaces of soft and rigid solids at 20°C. The method of measurement of contact angle is the same as in Section Ill.A. 

The sessile drop method has several drawbacks. Several days elapse between each displacement, and total test times exceeding one month are not uncommon. It can be difficult to determine that the interface has actually advanced across the face of the crystal. Displacement frequency and distance are variable and dependent upon the operator. Tests are conducted on pure mineral surfaces, usually quartz, which does not adequately model the heterogeneous rock surfaces in reservoirs. There is a need for a simple technique that gives reproducible data and can be used to characterize various mineral surfaces. The dynamic Wilhelmy plate technique has such a potential. This paper discusses the dynamic Wilhelmy plate apparatus used to study wetting properties of liquid/liquid/solid systems important to the oil industry. 

The measurement of contact angles for a sessile drop or bubble resting on or against a plane solid surface can be measured by direct microscopic examination. 

One of the most common ways to characterize the hydrophobicity (or hydrophilicity) of a material is through measurement of the contact angle, which is the angle between the liquid-gas interface and the solid surface measured at the triple point at which all three phases interconnect. The two most popular techniques to measure contact angles for diffusion layers are the sessile drop method and the capillary rise method (or Wihelmy method) [9,192]. 

Provides measuring techniques of contact angle, surface tension, interfacial tension, and bubble pressure. Suitable methods for both static and dynamic inteifacial tension of liquids include du Nous ring, Wilhelmy plate, spinning drop, pendant drop, bubble pressure, and drop volume techniques. Methods for solids include sessile drop, dynamic Wilhelmy, single fiber, and powder contact angle techniques. 

If a moderately large area of flat solid surface is available, contact angles are usually measured directly from a projection of a sessile drop of the liquid. Alternatively, the tilting-plate method illustrated... 

Static contact angle measurement of the sessile drop. The contact angle, 6C, is the angle formed by a liquid drop at the three-phase boundary where a liquid, a gas, and a solid intersect. It depends on the interfacial surface tensions between gas and liquid nGL, liquid and solid nLS, and gas and solid IIGS, as given by Young s6 equation of 1805 ... 

Figure 2.18. Dissolut i ve wetting the approach of local (c) and total (d) equilibrium in the sessile drop configuration. In liquid metal/solid metal systems, the characteristic times of the different stages are ti 10-2s,t 10Jsandt3 10s s. According to the calculations of Warren et al. (1998).

Figure 3.1. Schematic illustration of the profiles assumed as a solid cylinder melts to form a sessile drop, with an initial contact angle of 120°, which then spreads over the substrate, displaying transient contact angles to achieve a final equilibrium contact angle 0F of 30°.

The wetting balance technique is a variant of the maximum pull (or detachment) method used to measure liquid-vapour surface tensions (Keene 1993). It is nowadays widely employed in the electronics industry to quantify wetting of solders, but has also been used for wetting studies in metal/ceramic systems (Naidich and Chuvashov 1983b, Nakae et al. 1989, Rivollet et al. 1990). As compared to the sessile drop method which needs planar substrates, solids of various geometry can be studied by this technique. 

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... 

Figure 10.1. Schematic illustration of the influence of contact angles on the profiles of (a) sessile drops, (b) fillets between T configuration components, (c) the entry of liquid into capillary gaps and (d) the microscopic contact between the liquid and the solid components.

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