Contact angle capillary meniscus height method

FIGURE 11.12 Experimental setup of the capillary meniscus height method for measuring the external contact angle [93]. (For color version of this figure, the reader is referred to the online version of this book.)... 

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... 

Application to the capillary height method. Sugden1 has pointed out that, when the contact angle is zero, the ratio r/6 of the radius of the tube to the radius of the lowest point of the meniscus is the x/b of Bashforth and Adams s tables. By (6)... 

Ferguson4 has also devised two ingenious methods for determining the surface tension of very small amounts (I or 2 cu. mm.) of liquid. The pressure required to force the meniscus, located at the end of a capillary tube, to a plane form, is measured it is equal to 2yjr for tubes so narrow that the meniscus would naturally be spherical. In one method the tube is vertical in the other it is horizontal, and in this modification the density of the liquid need not be determined. The contact angle must be zero, as in all variants of the capillary height method. 

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. 

Later, Neumann developed the static Wilhelmy plate method which depends on capillary rise on a vertical wall, to measure 6 precisely. A Wilhelmy plate whose surface is coated with the solid substrate is partially immersed in the testing liquid, and the height of the meniscus due to the capillary rise at the wall of the vertical plate is measured precisely by means of a traveling microscope or cathetometer. If the surface tension or the capillary constant of the testing liquid is known, then the contact angle is calculated from the equation, which is derived from the Young-Laplace equation... 

Surface tension can be measured in many ways. One of the most accurate and conceptually simple methods is to measure the rise of a liquid in a capillary (Fig. 4.7a). The surface tension is related to the height of liquid supported by gravity, the tube radius, the contact angle of the liquid meniscus and the density difference between liquid and vapour. The determination of surface tension using this capillary rise method is easiest when the liquid completely wets the capillary wall, i.e. when the contact angle (Section 4.5.1) is near zero. 

---