Liquid-solid interactions receding

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. 

Experimentally, the measurement of contact angles is somewhat tricky, hence absolute values should not be considered too seriously. In particular, there is generally a difference between the an e found when the liquid advmices over the solid, and the angle found when the liquid recedes this is because the previous condition of the surface affects its wettability. But a series of measurements made under the same conditions certainly indicates the right ordering of fiber materials with regard to interaction with water. 

Adam has listed two additional causes of contact angle hysteresis. One is physical interaction between the fluid and the solid. In the case of water, this would take the form of hydration, dissolution, or possible deposition of soluble contaminants. The other cause is chemical interaction between the liquid and solid. Thus, with water it could be hydrolysis or water-induced decomposition. In both cases, the solid surface in contact with water changes so that the receding contact angle is now defined by a different solid-vapor interface. 

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