Wilhelmy apparatus

Strictly speaking, Equation (2) allows the vertical component of surface tension to be measured. Since this equals 7 cos 0, we are actually making a single measurement that involves two parameters. If 7 were independently known, the Wilhelmy plate method could also be used to determine 0. Whether we seek to evaluate 7, 0, or both, two experiments are needed, and these may not both involve the factor cos 0. In Section 6.8a we discuss a second type of measurement that can be made with the Wilhelmy apparatus that supplies a complementary observation so both 7 and 0 can be determined on a single instrument. 

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. 

Fig. 11-13. Apparatus for measuring the time dependence of interfacial tension (from Ref. 34). The air and aspirator connections allow for establishing the desired level of ftesh surface. IV denotes the Wilhelmy slide, suspended from a Cahn electrobalance with a recorder output.

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. 

Figure 1. Schematic diagram of the dynamic Wilhelmy Plate Apparatus.

The Wilhelmy plate is placed at the surface of the water, and the oil phase is added until the whole plate is covered by the latter. The apparatus must be calibrated with known IFT data such as water-hexadecane (52 mN/m 25°C) (Table 2.4). 

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). 

Next, an experiment such as that shown in Figure 7.2b could be conducted. The apparatus consists of a pair of Wilhelmy plates attached to two arms of a balance. One plate contacts the clean surface and the other the surface with the monolayer. Note that the barrier separates the two portions of surface. The surface tension will be different in the two regions, and the weight (and volume) of the meniscus entrained by the plate will be larger for the clean surface... 

Interfaciai Tension Procedure. IFT measurements were made by the Wilhelmy plate method. The apparatus was the same as that described previously (2). A standard protocol was followed for all IFT determinations. The desired interface was formed at a specified temperature by partially filling a thermostatted sample holder with the desired aqueous phase. This phase, distilled water (mono triple) or a supernatant aqueous phase isolated from a complex coacervate system, completely covered the Wilhelmy plate (roughened platinum). The desired citrus oil was carefully layered onto the aqueous phase. It had been preheated (or cooled) to the same temperature as the aqueous phase. Once the citrus oil/aqueous phase interface was formed, the Wilhelmy plate was drawn completely through the interface and into the oil phase where it was zeroed. 

Apparatus and Procedure. Surface Isotherms. The technique for determining the n-A and AV-A curves of the lipid films has been described (6). Briefly, the Wilhelmy plate method was used to measure surface tension, from which the surface pressure was calculated (n = 7h2o—yfiim) The surface potential was measured by means of a radioactive (226Ra) air electrode and a saturated calomel electrode connected to a high impedance model 610 B Keithley electrometer (Keithley Instruments, Cleveland, Ohio). 

FIG. 2 Apparatus for Wilhelmy technique (1) measuring plate, (2) glass fiber or rod, (3) electrobalance, (4) recording device, (5) measuring cell, (6) liquid, (7) movable platform, (8) screw or gear mechanism to raise or lower the platform, (9) motor, (10) clamp and support, (11) lid. 

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 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. 

The adhesion tension, r, was measured with Guastalla s wetting balance [13]. This apparatus was designed according to Wilhelmy s principle [30] for measuring the surface tension of liquids. 

The dynamic advancing and receding contact angles of modified PU sheets were evaluated with a Wilhelmy plate contact angle apparatus (WET-TEK FlOO, Biomaterials Int.) in double-distilled water at constant temperature (20 °C) and humidity (30X RH) The velocity of the translation stage was 20 mm/min. 

Figure 2.8. Surface tension data obtained using the modified Wilhelmy plate apparatus compared with the results of other techniques (o, PP Wilhelmy A, PTFE Wilhelmy -----, PP literature —, PTFE literature). After Sauer and Dipaolo (1991).

The measurement of the surface tension of a liquid by the Wilhelmy - the force balance method - is widely used and well documented. The same apparatus may be used to... 

Iranian Heavy crude oil/distilled water interfaces were aged for set intervals on the dilatational modulus apparatus. The interfacial length was initially set as 13.5 cm and amplitudes of area oscillation of 0.5 cm were used. The Wilhelmy plate (platinum, hydrophobic) was always aligned parallel to the oscillating barrier. The X-Y recorder was used to produce Lissajou figures of interfacial tension-interfacial Wilhelmy... 

There have been various methods for measuring the surface tension [2]. The Wilhelmy plate is commonly employed in the usual LB apparatuses to monitor the change in the surface pressure. In the Wilhelmy method [8], the force balanced with the surface tension is determined by a plate vertically suspended and partially immersed in the water phase. Figure 3 illustrates the experimental configuration. 

Fig. 1 Schematic illustration of the apparatus view from side A and above B. Key a = trough walls, b = lower (aqueous) phase, c = barrier, d = upper (oil) phase, e = hydrophobic Wilhelmy plate, f = siphon, g = direction of stepper motor drive for compression...

FIG. 21 Schematic of an apparatus allowing the determination of surface tensions of pol5mer melts by immersing libers based on the modified Wilhelmy principle [98],... 

The interfacial tension was estimated on the basis of surface tension measurements by the Wilhelmy Plate apparatus, the contact angle... 

---