Contact Angle of Liquid Drops on Solids

Turning now to the solid/water/oil measurements, we compare the predicted Ooic values according to Equations 14 and 15 with experimental values in Tables IV-VII for the substrates investigated. In the case of Teflon, where it is possible to test Equation 11, values are given for heptane and n-hexanol. The dispersion and polar components of the surface tension of water-n-hexanol, i.e. water saturated with n-hexanol, and hexanol-water were obtained by measuring the contact angle of liquid drops on paraflSn wax (ys = 25.5 dynes/cm), which served as a... 

Young s equation at liquid —solid l iq uid2 has been investigated in various systems where it has been found that the liquid-solid-liquidi surface tensions meet at a given contact angle. For example, the contact angle of water drop on Teflon is 50° in octane (Chattoraj and Birdi, 1984 see Figure 5.3). 

FIGURE 1.14 Schematic presentation of a liquid droplet on a horizontal solid substrate, which is slowly pumped through the liquid source in the drop center. R is the radius of the drop base 0 is the contact angle (1) liquid drop, (2) solid substrate with a small orifice in the center, (3) liquid source (syringe). 

The first method, wettability, can be evaluated from the contact angle of a drop of liquid deposited on the flat surface of the solid. This method hardly applies to powders like silicas because special care must be taken to control the surface porosity of a silica disk made from compressed silica particles. For a chromatographic silica, Kessaissia et al. (1) determined a Ys value close to 100 mJ/m2 whereas the polar component of the surface energy was found to be 46 mJ/m. Hence, the silica exhibits a large surface energy. 

Next, a method was developed to determine the initial peripheral contact angle, 9, of sessile drops on solid surfaces from the diffusion controlled rate of drop evaporation, for the constant drop contact radius mode. Application of this method requires use of the product of the vapor diffusion coefficient of the evaporating liquid, with its vapor pressure at the drop surface temperature (l)APv), which can be found directly from independent experiments following the evaporation of fully spherical liquid drops in the same chamber. It is then possible to calculate 9,p, from... 

If the starting material is available as a solid with smooth and homogeneous (at the macroscopic level) surface, a method widely used for surface free energy dctennination is based on the measurement of the contact angle of a drop of selected liquid, deposited on the surface. The discussion of the results is made in terms of Young s equation ... 

The surface properties of polymers are important in technology of plastics, coatings, textiles, films, and adhesives through their role in processes of wetting, adsorption, and adhesion. We will discuss only surface tensions of polymer melts that can be measured directly by reversible deformation or can be inferred from drop shapes. Those inferred from contact angles of liquids on solid polymers ( critical surface tension of wetting ) are excluded, as their relations to surface tensions are uncertain. 

From Young s Equation 11.2, then, we obtain for the contact angle of a drop of liquid, L, on a solid surface, S, the following working equation ... 

Several authors [119] have tried to relate the resistance to movement of liquid drops on a tilted surface to the surface tension and the contact angles (advancing and receding) of liquid droplets with the solid surface. The detailed analysis given by Furmidge [119] is summarized below. 

In 1805 Young established a relationship between the contact angle of a drop of liquid and the interfacial tensions at the three-phase contact line between a solid, a liquid, and its vapor at equilibrium (Eq. 10.1). J. Willard Gibbs in 1928 then derived the contact angle 9, from thermodynamic quantities, the surface free energies for the three interfaces. Substantial refinements in terms of the nature of the forces involved in the wetting process have been made since then, and interested readers can consult fundamental texts on surface forces. ... 

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