Sessile drop methods

FIGURE 1.12 Schematic illustration of capillary rise method. 

FIGURE 1.13 Schematic illustration of sessile drop method. 

---

The following values for the surface tension of a 10 Af solution of sodium oleate at 25°C are reported by various authors (a) by the capillary rise method, y - 43 mN/m (b) by the drop weight method, 7 = 50 mN/m and (c) by the sessile drop method, 7 = 40 mN/m. Explain how these discrepancies might arise. Which value should be the most reliable and why ... 

The most commonly used techniques for contact angle measurements are the sessile drop method and the Wil-helmy plate method. Results obtained from these two methods are in good agreement. 

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. 

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]. 

For the DMFC, Zhang et al. [127] used the sessile drop method to study the wettabilities of liquid methanol solutions on the surface of the anode DLs and MPLs. They were able to observe that the contact angles of the materials were the smallest with low PTFE content. In addition, the effect of Nafion ionomer content on the MPL (to increase hydrophilicity see Section 4.3.2) was also shown through the contact angle measurements (i.e., smaller contact angles compared to MPLs with PTFE). 

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... 

The antistatic properties of the step 2 product were evaluated by preparing tapes of blends with polystyrene by extruding in a twin-screw extruder using a flat die at 200°C. The contact angle of the tapes was measured using the sessile drop method and water as the measuring liquid. Testing data not supplied by author. 

Important techniques to measure the surface tension of liquids are the sessile drop method, the pendant or sessile bubble method, the Du-Notiy ring tensiometer, and the Wilhelmy-plate method. 

Some of the commonly used techniques for measuring contact angle [215, 216, 217] are the sessile drop method, captive bubble method and Wilhelmy plate method. These techniques have been extensively used and well documented for characterisation of modified PE surfaces [218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230] for various applications. Whitesides et al. [231 ] studied the wetting of flame-treated polyethylene film having ionisable organic acids and bases at the polymer-water interface. The effect of the size of substituted alkyl groups in amide and ester moieties on the surface hydrophilicity was also studied [232]. The biocompatibility of the polyethylene film surface modified with various water-soluble polymers was evaluated using the same technique [233]. The surface properties of hy-perbranched polymers have been very recently reported [234]. 

The Young-Laplace equation forms the basis for some important methods for measuring surface and interfacial tensions, such as the pendant and sessile drop methods, the spinning drop method, and the maximum bubble pressure method (see Section 3.2.3). Liquid flow in response to the pressure difference expressed by Eqs. (3.6) or (3.7) is known as Laplace flow, or capillary flow. 

For emulsions, the interfacial tension is usually of most interest. Here, the du Noiiy ring, Wilhelmy plate, drop volume, pendant, or sessile drop methods are the most commonly used. The spinning drop or captive drop techniques are applicable to the very low interfacial tensions encountered in the enhanced oil recovery and microemulsion fields. The maximum droplet pressure technique can be used when there is little or no density contrast between the phases, such as in bitumen-water systems at elevated temperature. 

The pendant drop method and the sessile drop methods for surface tension appear quite different, but are similar in principle. In each case a drop is formed which either sits on a plane surface (sessile, see Figure 3.10) or hangs as a pendant drop... 

The sessile drop method can also be used for the determination of contact angle (Section 3.4). 

The captive drop and captive bubble methods, variations of the sessile drop method, have been developed for the determination of very low values of surface or interfacial tension [140,141], including at elevated temperature and pressure [141]. 

The sessile drop method, described in Section 3.2.3, can also be used for the determination of contact angle. A sessile drop is created as for surface tension measurement, but instead of measuring drop shape, the contact angle (0) is measured through the liquid phase (Figure 3.16). This can be done using a goniometer. 

RH and at 25°C. using the sessile drop method. Surface tensions were measured at 25°C. using the duNouy ring method. Unless otherwise noted in the text, the contact angles were reproducible to 2° and the critical surface tensions to 0.5 dynes/cm. 

The wettability of diamond with Ni-Mn alloys having Ga, Ge, Sn, Mg, A1 and Si additives was determined by means of the sessile drop method at high pressure (5.5 GPa) and temperature (1400° C) following a special procedure [3,4]. The metal pellet melted and attained its equilibrium form owing to surface forces on the diamond plane surrounded by molten NaCl that gives a quasi-hydrostatic enviroment and suppress oxidation of the metal. 

Naidich and Grigorenko 1992, Passerone and Ricci 1998) is making the sessile drop method more and more reliable and accurate. However, it should be noted that substantial effort has been required historically to derive a surface energy by curve fitting, and many authors have suggested simplifications which require far fewer measurements. Thus, Bashforth and Adams, (1883), rewrote equation (3.6) in the form ... 

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. 

---