Non-ideal Rough Solid Surfaces
Unlike ideal surfaces, real surfaces do not have perfect smoothness, rigidity, or chemical homogeneity. Such deviations from ideality result in phenomena called contact-angle hysteresis. Contact-angle hysteresis is defined as the difference between the advancing (θa) and receding (θr) contact angles
In simpler terms, contact angle hysteresis is essentially the displacement of a contact line such as the one in figure 3, by either expansion or retraction of the droplet. Figure 6 depicts the advancing and receding contact angles. The advancing contact angle is the maximum stable angle, whereas the receding contact angle is the minimum stable angle. Contact-angle hysteresis occurs because there are many different thermodynamically stable contact angles on a non-ideal solid. These varying thermodynamically stable contact angles are known as metastable states.
Such motion of a phase boundary, involving advancing and receding contact angles, is known as dynamic wetting. When a contact line advances, covering more of the surface with liquid, the contact angle is increased and generally is related to the velocity of the contact line. If the velocity of a contact line is increased without bound, the contact angle increases, and as it approaches 180° the gas phase will become entrained in a thin layer between the liquid and solid. This is a kinetic non-equilibrium effect which results from the contact line moving at such a high speed that complete wetting cannot occur.
A well-known departure from ideality is when the surface of interest has a rough texture. The rough texture of a surface can fall into one of two categories: homogeneous or heterogeneous. A homogeneous wetting regime is where the liquid fills in the roughness grooves of a surface. On the other hand, a heterogeneous wetting regime is where the surface is a composite of two types of patches. An important example of such a composite surface is one composed of patches of both air and solid. Such surfaces have varied effects on the contact angles of wetting liquids. Cassie–Baxter and Wenzel are the two main models that attempt describe the wetting of textured surfaces. However, these equations only apply when the drop size is sufficiently large compared with the surface roughness scale.
Read more about this topic: Wetting
Famous quotes containing the words rough, solid and/or surfaces:
“Women of fashion and character—I do not mean absolutely unblemished—are a necessary ingredient in the composition of good company; the attention which they require, and which is always paid them by well-bred men, keeps up politeness, and gives a habit of good-breeding; whereas men, when they live together without the lenitive of women in company, are apt to grow careless, negligent, and rough among one another.”
—Philip Dormer Stanhope, 4th Earl Chesterfield (1694–1773)
“Miss Western: Tell me, child, what objections can you have to the young gentleman?
Sophie: A very solid objection, in my opinion. I hate him.
Miss Western: Well, I have known many couples who have entirely disliked each other, lead very comfortable, genteel lives.”
—John Osborne (1929–1994)
“Footnotes are the finer-suckered surfaces that allow tentacular paragraphs to hold fast to the wider reality of the library.”
—Nicholson Baker (b. 1957)