Water - Teflon - octane system

In the water-Teflon-octane system, the contact angle, 9, is related to the different surface tensions as follows ... 

In these studies the rate of the mass and contact diameter of water and -octane drops placed on glass and Teflon surfaces were investigated. It was found that the evaporation occurred with a constant spherical cap geometry of the liquid drop. The experimental data supporting this were obtained by direct measurement of the variation of the mass of droplets with time, as well as by the observation of contact angles. A model based an the diffusion of vapor across the boundary of a spherical drop has been considered to explain the data. Further studies were reported, where the contact angle of the system was 9 < 99°. In these systems, the evaporation rates were found to be linear and the contact radius constant. In the latter case, with 9 > 99°, the evaporation rate was nonlinear, the contact radius decreased and the contact angle remained constant. 

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). 

The evaporation behaviour was found to vary from system to system. The data In Fig. 2 shows the evaporation process of water-or n-octane (C8) drops when placed on glass or teflon surface, respectively. In contrast to the other systems, the evaporation of water drops placed on a glass surface, and n-octane drops placed on a teflon surface was found to be stationary process, (l.e., the rate of evaporation of the liquid drop Is constant during evaporation process). This occurs when the size of evaporating drop decreases, and the contact angle decreases while the radius of the liquid-solid... 

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