Contact angle evaporation effect

Most of the older theories of adsorption hysteresis made explicit use of the Kelvin equation. Zsigmondy (1911) was the first to suggest that the phenomenon was due to a difference in the contact angles of the condensing and evaporating liquid. This explanation may account for some of the anomalous effects produced by the presence of surface impurities, but in its original form it cannot explain the permanence and reproducibility of the majority of recorded loops. However, as will become evident, die notion of delayed condensation is still of great importance. 

Liquid Evaporation Effects During Contact Angle Measurement... 

So far, we have not considered the presence of the flat substrate, which restricts the space into which vapor may diffuse and so reduces the evaporation rate. It has been predicted theoretically that a completely spherical small droplet possessing a 180° contact angle and sitting on a flat solid surface will have a rate of mass loss reduced by (ln2), compared with an identical, completely spherical droplet far removed from any solid surface. When the effect of the presence of horizontal substrate is taken into account, we need to modify Equation (659) as... 

Besides these applications, droplet evaporation reveals several intriguing phenomena and is a prime example of a microfluidic multiphysics system, which is noticeably complex due to the interplay of mass and heat transfer, hydrodynamics of multiphase flow, interface energy effects as contact angle hysteresis, voliunetric forces as gravity, and Marangoni flows. 

One important lesson is that measurement of the apparent contact angle of a sessile drop will not give a true measure of the wettability - or even surface free energy - of the fiber surface. The apparent contact angle will always be affected by microscopic surface property (i.e. fiber surface), macroscopic surface geometry (fabric), and capillary effects. In case of hydrophilic substrates the sessile droplet will penetrate the porous textile, typically in seconds, and thus effectively prohibit the measurement. On a hydrophobic surface, the apparent contact angle will always differ from the true contact angle on the fiber surface. Capillary effects occur even on hydrophobic substrates and compete with evaporation of the liquid. 

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