Contact angle static liquid droplet

The line that delimits the wetted portion of a surface by a liquid is called the contact line. In two-dimensional flows the contact line reduces to a point (see Fig. 3). For a static droplet, the angle of the interface between the liquid and the solid surface is caUed the static contact angle 0. 

When the air pressure difference applied to the droplet equaled the internal capillary pressure caused by the difference in the curvature between the two liquid-air interfaces of the droplet, the liquid-air interface at the junction could protrude from the microchannel and be steadily piimed along the edges. The shape of the other liquid-air interface, on the other hand, depended on the static contact angle of the liquid on the channel material under homogeneous pneumatic pressure. A liquid microlens was thus formed from the droplet. 

For a static system, solid surface interfacial tensions can be calculated from the measured contact angles by using a mechanical equilibrium relation derived by Young in 1800 [38]. The liquid droplet contact angle on any solid surface can be defined by the mechanical equilibrium of the droplet under the action of three interfacial tensions. The interfacial tension a ij is defined as the amount of work that must be performed in order to separate a unit area of fluid j from k. The term aj is the surface tension between substance j and its own vapor phase (Figure 5.28). The work to separate two phases is expressed as... 

Figure 6.1 schematically depicts the three interactions between a liquid droplet and a surface. These three interactions are actually governed by the movement of the contact line. When the liquid first wets the surface, the contact line advances outward, and the first information one seeks is wettability. The adjectives to describe the surface are wettable and non-wettable. As for the liquid, it will either wet or partially wet the surface or repel from it. As discussed in Chap. 5, wettability is measured by the advancing angle Oa- Once the liquid partially wets the surface, a static sessile drop is formed. There exist two interactions between the sessile drop and the surface. In the vertical direction, it is the adhesion and it is measured by the receding angle 0r. The only motion for the contact line is receding, and an interface (liquid-solid) is eliminated when the liquid droplet is detached from the surface. 

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