Contact angle capillary rise technique

The capillary rise technique is considered to be very useful for DL materials, especially if the angle is less than 90° and/or for measurements that are taken under different temperature conditions [192]. In this method, a sample material is immersed in a container filled with water and the meniscus height is measured with a microscope (see Figure 4.23). Contact angles between the water and DL are calculated using Equation 4.1 and by measuring the... 

Table 14.2 shows a comparison of measured contact angles using ADSA and capillary rise. It can be seen that the capillary rise technique is also able to produce contact angle data with high accuracy, and there is excellent agreement between the two techniques. 

The contact angle measurements with the capillary rise technique can be time-consuming, and hence automation of the measurements is desirable. Figure 14.18 shows a schematic of an automated capillary rise apparatus. In the automated capillary rise measurements, the cathetometer, which is used to measure the capillary rise, is replaced by a computer and a microscope on motorized positioning stages. Three computer-controlled motorized translational stages (Micro-control) are used to move a horizontally mounted microscope fitted with a video scanner (Cohu CCD) in three directions, i.e. X, y and z. The x-, y- and z-axis stages... 

Table 14.2. Comparison of measured contact angles, 0 (degree), using the capillary rise technique and ADSA-P for two solid surfaces, FC-721 and FEP. The error limits are 95% confidence limits...

The automated capillary rise technique has been employed to carry out various dynamic advancing and receding contact angle measurements (54) at different immersion speeds ranging from 0.008 to 0.9 mm/min. This technique has also been utilized to study both dynamic advancing and receding contact angles on dry, pre-wetted, and soaked fluorocarbon FC-722 (3M Inc.) surfaces (55). Such measurements reveal that the... 

Budziak, C. and Neumann, A. W., Automation of the capillary rise technique for measuring contact angles. Colloid Surf., 43, 279-293 (1990). 

The capillary rise on a Wilhelmy plate (Section II-6C) is a nice means to obtain contact angles by measurement of the height, h, of the meniscus on a partially immersed plate (see Fig. 11-14) [111, 112]. Neumann has automated this technique to replace manual measurement of h with digital image analysis to obtain an accuracy of 0.06° (and a repeatability to 95%, in practice, of 0.01°) [108]. The contact angle is obtained directly from the height through... 

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]. 

The situation shown in Figure 6.2b is one in which surface tension and contact angle considerations pull a liquid upward in opposition to gravity. A mass of liquid is drawn up as if it were suspended by the surface from the supporting walls. At equilibrium the upward pull of the surface and the downward pull of gravity on the elevated mass must balance. This elementary statement of force balance applies to two techniques by which 7 can be measured if 6 is known the Wilhelmy plate and capillary rise. 

Many different techniques have been developed for the measurement of contact angles 17.8). Of these, the three most useful methods are the Wilhelnty technique, the technique of capillary rise at a vertical plate, and the drop shape methods. These techniques require the solid surface to be flat and smooth. Direct measurement of contact angles on fibers (of uniform thickness) can also be performed using the Wilhelmy technique. For nonflat surfaces or particles, indirect methods such as capillary penetration into columns of powders, sedi-... 

In the following subsections, the three most commonly used contact angle techniques, the Wilhelmy technique, capillary rise at a vertical plate, and drop shape methods, are described in detail. 

TABLE 1 Comparison of Measured Contact Angles H (deg) Using the Capillary Rise at a Vertical Plate Technique and ADSA-P for the Two Solid Surfaces. FC-72I and Teflon (FEP)... 

In principle, the capillary depression caused by a fiber floating on a liquid surface can be used to calculate the contact angle 7], In practice, however, the relatively small depth of immersion can make it difficult to obtain sufficient accuracy. Recently, a technique utilizing image analysis has been developed to analyze the capillary rise profile around fibers or cylinders 79],... 

D. Y. Kwok. D. Li. and A. W. Neumann. "Capillary Rise at a Vertical Plate as a Contact Angle Technique." in Applied Surface Thermodynamics (A. W. Neumann and J. K. Spelt, eds ). Surfactant Science Series. Vol 63. Marcel Dekker. New York. 1996. pp. 413-440. 

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. 

Because ceramic powders usually have macropores, mercury porosimetry is more suitable than gas adsorption. The principle of the technique is the phenomenon of capillary rise, as shown schematically in Fig. 4.4 [19]. When a liquid wets the walls of a narrow capillary, with contact angle, 9 < 90°, it will climb up the walls of the capillary. If the liquid does not wet the walls of a capillary, with contact angle, 9 > 90°, it will be depressed. When a nonwetting liquid is used, it is necessary to force the liquid to flow up the capillary to the level of the reservoir by applying a pressure. For a capillary with principal radii of curvature rj and r2 in two orthogonal directions, the pressure can be obtained by using the Young and Laplace equation ... 

Gu, Y., Li, D. and Cheng, P., A novel contact angle measurement technique by analysis of capillary rise profile around a cylinder (ACRPAC), Colloid Surf., A, 122, 135-149 (1997). 

These and other techniques for measuring contact angle, such as capillary rise at a vertical plane plate, are included in the review by Neumann and Good. ... 

---