Measurement of contact angles

Li and Neumann sought an equation of state of interfacial tensions of the form 7 l = /(Tlv. TSv). Based on a series of measurements of contact angles on polymeric surfaces, they revised an older empirical law (see Refs. 216, 217) to produce a numerically robust expression [129, 218]... 

An astronaut team has, as one of its assigned experiments, the measurement of contact angles for several systems (to test the possibility that these may be different in gravity-free space). Discuss some methods that would be appropriate and some that would not be appropriate to use. 

Tables of single substance surface tensions y can be built up tlirough tire measurement of contact angles [134] a few examples are collected in table C2.14.1. These can be combined pairwise according to ...

To investigate the influence of swelhng of the substrate by the contacting liquid, the contact angle 6 of sessile drops of tricresylphosphate, TCP (drop volume 2 p,L, viscosity t = 70 cP, surface tension = 40.9 mN m ), has been measured as a function of time after deposition, t, on flat, smooth, horizontal surfaces of soft and rigid solids at 20°C. The method of measurement of contact angle is the same as in Section Ill.A. 

The measured contact angle of mercury on various samples can range from 112 to 170° [39], but for most applications the average value of 140° is used. It should be noted, however, that the accuracy of the pore radii measurement is limited by the accuracy of the contact-angle measurement [40]. Contact angles can readily be measured on flat surfaces or compacts of powders [6], and the measurement of contact angles with powder systems has also been reported [41]. 

The measurement of contact angles for a sessile drop or bubble resting on or against a plane solid surface can be measured by direct microscopic examination. 

Lim, C. Wang, C. Y. Measurement of contact angles of liquid water in PEM fuel cell gas diffusion layer (GDL) by sessile drop and capillary rise methods. Penn State University Electrochemical Engine Center (ECEC) Technical Report no. 2001 03, Perm State University State College, PA, 2001. 

As shown in Fig. 2.31, a contact angle can be experimentally measured by looking at two adhesive droplets. However, a direct determination from side views is rather difficult unless the contact angle is large enough, as in Fig. 2.31. A more convenient way to achieve measurements of contact angles consists of measuring the radius of two adhesive droplets, a and d2, and the radius of the adhesive film between the droplets, r (Fig. 2.32) [104-107]. One has ... 

Carroll, B.J, (1976). The accurate measurement of contact angle, phase contact areas, drop volume, and Laplace excess pressure in drop-on-fiber system. J. Colloid. Interface Sci. 57, 488-495. 

Optical devices are sometimes employed for the measurement of contact angle, wherein the operator must attempt to establish the tangent to the contact angle of a drop of mercury resting on a plane surface. This method has never proven sufficiently accurate because of its inherent subjectivity. Different experimenters will inevitably measure substantially different contact angles and even the same person will observe different angles on the same material on different occasions. 

FIG. 6.2 Illustrations of liquid film formation, contact angle, and measurement of contact angle (a) a wire loop with a slide wire on which a liquid film might be formed and stretched by an applied force F. (b) profile of a three-phase (solid, liquid, gas) boundary that defines the contact angle 0. (c) the tilted plate method for measuring contact angles. 

We have now established that both 7 and 6 have thermodynamic significance and have seen that their values as well as their temperature coefficients are of interest. In addition, we have seen that the measurement of contact angles presents some complications of its own. All this adds up to a need for more reliable and more accurate methods for the measurement of these parameters than those presented in Section 6.2. One of the most powerful strategies for this involves a second measurement made with the Wilhelmy plate. 

Eadie, in Ref 69, reports on a considerable amount of work done on the ability of beeswax and paraffin wax to remain coated on HMX surfaces when immersed in liq TNT. Thru measurements of contact angles, a technique used earlier on RDX/wax systems reported on by Rubin in Ref 23, it was determined that the TNT preferentially wets the HMX and the wax is stripped away. He concludes that the most important property of a desensitizing wax is that it should be readily dispersed uniformly thruout the TNT phase. He also suggests that a better desensitizer for investigation for use would be a wax or substituted hydrocarbon having a low interfacial tension with TNT. The smaller the wax droplet size the more efficiently it will be distributed and the more effectively it should desensitize. Williamson (Ref 64) in his examination of the microstructures of PETN/TNT/wax fusion-casts detected that wax is dispersed thru the cast as isolated descrete globules which he refers to as blebs or irregular or streak-like areas, surrounded by TNT (see also Ref 54)... 

Fig. 2.39 Measurement of contact angle between drop of liquid and solid substrate...

The measurement of contact angles is a wet chemical technique and since TTe is determined with a liquid film adsorbed from vapor in equilibrium with the liquid, it could also qualify as a wet chemical technique in contrast with vacuum techniques. 

Given a flat solid surface, the actual measurement of contact angles to 1° is relatively straightforward. The complications associated with contact angle measurement relate to the system itself and include the following ... 

Figure 6.3 Tilting plate method for the measurement of contact angles...

The wettability of a polymer film normally is determined by static contact angle measurements. The surface free energy (SE) of a polymer can be determined by wettability measurements with two different liquids. The dispersion force and polar contributions to SE, 7 d and 7 p respectively, are also calculated normally by using the Owens and Wendt, and Kaelble methods [146,147], The measurements of contact angles (CA) on a given solid surface is one of the most practical ways to obtain surface free energies. 

R. Aveyard, B. P. Binks, P. D. I. Fletcher, and C. E. Rutherford, Measurement of contact angles of spherical monodisperse particles with surfactant solutions, Colloids Surf. A 83, 89-98 (1994). 

Measurement of contact angles hysteresis. The experimental determination of contact angles is complicated by two factors first, that the angle depends on the surface tension of the liquid surface second, that the angle is rarely definite, but may have any value between two extremes, according as the liquid is tending to advance over a dry surface, or to recede from a previously wetted one. 

Measurement of contact angles hysteresis. Transference of monolayers from water to solid surfaces. Multilayers... 

For the measurement of contact angles, by the reflection of light from the edge of a drop of liquid, Langmuir2 describes a simply constructed instrument, said to give an accuracy of about one degree, which is amply sufficient for nearly all purposes. 

The following three main routes are available to assess the wettability. The first method is dependent on the measurement of contact angles, while the second and third make use of energy of immersion and adsorption isotherm data. 

Other discrepancies between the black film behaviour and DLVO-theory are related to the difference in the critical electrolyte concentration, corresponding to the transition between the two black films types (see Section 3.4.2) the existence of a second minimum in the 11(A) isotherm the sharp rise in the disjoining pressure (after the second minimum). All this is evidenced by the measurements of contact angles between the film and bulk phase. 

In many areas of applied surface thermodynamics, measurement of contact angles plays an important role. The range of applications of contact angle measurement is remarkable. It can be used as a simple tool to assess, for example, the cleanliness of the surfaces, or it can be a highly sensitive scientific measurement aimed at obtaining information on the solid surface tension and the... 

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