Wilhelmy-plate technique

In the Wilhelmy plate experiment, the solid surface has to be fabricated in plate form with weU-defined dimension both sides. ExperimentaUy, it is immersed into the testing liquid to form a liquid-solid-air interface. The forces exerted on the sample are gravity g), surface tension (Fwening) and buoyance (FB oyance) forces, and the force by the tensiometer (i.e., action-reaction forces, it is also the force measured by the tensiometer). The free body diagram of the plate is shown in Fig. 2.19 [51]. The equation for the force balance is described as [52]  

Similarly the receding force (Fg) wiflidrawing at a speed of V is given by  

2 Contact Angle Measurements and Surface Characterization Techniques 

On smooth flat surfaces, the dynamic 0a/0r increases/decreases very slowly in the low-speed regime, and the rate of change increases at higher speed. The values extrapolate to F=0 would be comparable to those measured from the drop expansion/contraction method. Typical results can be found in the work of Hayes and Ralston [56], and an illustrative plot is reproduced in Fig. 2.21. In most practical situations, the surface or sometimes the liquid are not static, e.g., liquid movement 

Finally, WiUielmy plate has also been used extensively to measure liquid surface tensions and liquid-liquid interfacial surface tensions by using a roughened platinum/iridium plate rendering excellent wetting by test liquids with a contact angle of 0°. 

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Formation Wettability Studies that Incorporate the Dynamic Wilhelmy Plate Technique... 

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 computer interface system lends itself well to the determination of interfacial tension and contact angles using Equation 3 and the technique described by Pike and Thakkar for Wilhelmy plate type experiments (20). Contact angles for crude oil/brine systems using the dynamic Wilhelmy plate technique have been determined by this technique and all three of the wetting cycles described above have been observed in various crude oil/brine systems (21) (Teeters, D. Wilson, J. F. Andersen, M. A. Thomas, D. C. J. Colloid Interface Sci., 1988, 126, in press). The dynamic Wilhelmy plate device also addresses other aspects of wetting behavior pertinent to petroleum reservoirs. 

This same technique should be helpful in understanding wetting properties important in the oil industry since wetting is very dependent on mineral surface energies. The use of contact angle hysteresis information may allow a better understanding of the effects of surface heterogeneities of natural mineral samples. The dynamic Wilhelmy plate technique is ideally suited for such experiments ... 

Each term on the right hand-side of Eq. (2.9) is measured independently providing a direct determination of n, also under low pressures [30]. It is necessary as well to eliminate the fluctuations in the atmospheric pressure and to measure more precisely the differential pressure pg - p, (Fig. 2.12). This is done with a sensitive draft-range differential pressure transducer from Omega (Model PX750-06DI) with an accuracy of 0.5 Pa. The capillary radius r is determined by a Kondon micrometer calliper while the surface tension of the surfactant solution - by the Wilhelmy-plate technique, and the height hc in the capillary tube - with a Wild cathetometer (Model KM-274). 

These equations may be compared with those for cylinders, see for instance [1.3.21. For flat plates one does not have to worry about complications of the details of the profile, but this advantage is offset by the much lower rise. Typically, h is of order i.e. h = O (mm) and y is proportional to whereas it scales with ah in capillaries. Over the last few decades laser-optical techniques for scanning the meniscus and establishing h down to about 10" mm have become available In a modem variant of the Wilhelmy plate technique, to be described in sec. 1.8a, the force needed to pull the plate out of the liquid is measured as a function of the height above the zero level. In this way the surface tension and contact angle can be determined simultaneously. Alternatively, the method can be used to obtain contact angles, i.e. from [1.3.161 after y has been measured by some other technique. 

The Wilhelmy plate technique has also been modified to measure surface or interfacial tensions under special conditions. By way of illustration we mention the application to the measurement of electrocapillanj curves for the mercuiy-aqueous electrolyte solution interface by Montgomery and Anson ), extending earlier work by Smith ). Electrocapillary curves are plots of y as a function of the potential applied across the interface, which should be polarizable. In fig. II.3.48 we already gave a few such curves. Montgomery and Anson found that their curve in 10 2 M NaF agreed within 0.005 mN m- with data obtained using the maximum bubble pressure. 

The force at the barrier may be measured directly by a calibrated torsion wire that is mechanically attached to the barrier. However, nowadays the surface tensions at both sides of the barrier are measured Independently using one of the appropriate methods in sec. 1.8, mostly by the static Wilhelmy plate technique. The latter method has the advantage that einy leakage of monolayer material across the barrier can be easUy detected. 

Palmitic and myristic acids (Applied Science Laboratories, State College, Pa.) and oleic acid (Hormel Institute, Austin, Minn.) were applied to the Langmuir trough in a hexane solution. Constant pressure-variable area measurements were obtained at 25 °C with a floating barrier and piston oils as previously described (23). Castor oil, tri-m-tolylphos-phate, and linoleyl alcohol (Hormel Institute, Austin, Minn.) were the piston oils they exerted surface pressures of 17 0.7, 9.5, and 33.5 dynes/cm. Variable pressure-variable area measurements were obtained at 24°-26°C with a movable barrier propelled by a high-torque motor (27). tr was measured by the Wilhelmy plate technique (27). 

Steady State Expansion Measurements. The dynamic surface tension in steady state expansion was determined using a modified Langmuir trough equipped with six barriers fixed to an endless belt which were moved caterpillar-wise one after another over the liquid surface.10,24 Surface tension was determined using the Wilhelmy plate technique. Measurements were performed going from the highest to the lowest expansion rate. 

In 2008, Qian et al. synthesized two series of four-arm and eleven-arm star polystyrene with six different molar masses for each case. They determined surface tension of single component polymer film using a Wilhelmy plate technique... 

Contact Angle Measurement. Dynamic contact angle measurement was performed on the dried hydrogels at 25°C by the Wilhelmy plate technique using equipment manufactured by Shimadzu Inc. (Automated System for Dynamic Contact Angle Measurement, ST-IS type) Water used for the measurement was purified by de-ionization after double distillation. Five measurements on different parts of the film were averaged. The movement rate of the water vessel into which the gel specimen was immersed was kept at 0,3 mm sec , ... 

As soon as the coated slides were removed from the oven and cooled to room temperature, dynamic contact angles were measured by the Wilhelmy plate technique [21], We obtained the wetting curve with an electrobalance (Cahn Model... 

FIG. 2 Dipping velocity effect at various speeds using cleaned glass plates in the Wilhelmy plate technique (a) 10 mni/min (b) 30mm/min (c) 40mm/min (d) 60mm/... 

Dynamic contact angle Wilhelmy plate technique Banana stem and bunch 26... 

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