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Interfacial Surface Energy and Contact Angle
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Now consider a drop of fluid resting on a solid surface. The underside of the liquid is in contact with the solid surface. The remainder of the solid’s surface is in contact with some other gas above – typically the liquid’s vapor and/or air. Similarly, the remainder of the liquid’s surface area is in contact with the same above gas – again, typically, the liquid’s own vapor and/or air. Accordingly, there are three interfaces described by a drop of fluid resting on a solid surface: 1) a liquid-solid interface, 2) a solid-gas interface, and 3) a liquid-gas interface.
Because surface areas exist at the three interfaces, three interfacial surface energies also exist. Each of the interfacial surface areas are interested in lowering their surface energy. As a result, the interfacial surface energies compete with one another to lower their own energy level – without regard to the surface energy levels of the other interfaces. The “battle of interfacial surface energies” causes changes to the liquids shape resting on the solid surface until a state of equilibrium is reached.
At equilibrium, the droplet takes on a customary dome shape. The dome shape of the droplet can be described by a contact angle – an angle that exists between a line drawn along the liquid-solid interface and a line drawn tangential to the liquid at the liquid-solid and liquid-gas interface. Even though the contact angle is a resultant of a system in equilibrium, we can use the contact angle to determine the relative strengths of the three interfacial surface energies.
If the interfacial surface energies are varied, the contact angle and droplet shape changes until equilibrium is reestablished. Note the changes that occur in the liquids contact angle when the liquid’s surface energy is first increased and then decreased. Then, note the changes that occur to the contact angle when the solids surface energy is first increased and then decreased. In both cases, the shape of the droplet - hence, the shape of the droplet’s contact angle - conform to balance interfacial surface energies.
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