Capilarity

Capillarity is the event rise or fall on the liquid surface of the capillary tube, as shown in Figure 1.1 In the picture, the inside diameter of the capillary tube from left to right are getting smaller. If the diameter of the capillary tube degrease , the water level rise in the capillary tube will be increase .

Figure 1.1 capillary tube

The surface liquid that wetting  the wall , such as water, will rise. And while it does not wet the walls, such as mercury, will go down. In everyday life, examples of capillary phenomenon are as follows.  kerosene up through the wick in the kerosene lamps or kerosene stove,  wet walls during the rainy season, water in the soil up through the timber vessels.

kerosene lamps

kerosene stove

wetting wall by water events, or mercury does not wet the wall can be explained by considering the attractive force among particles . The force of attraction is called cohesion if among particles are similar, while attractive forces are not similar in the among particles called adhesion. Water soaks glass wall due to the cohesive forces of water antarpartikel smaller than the adhesion force between water particles and particles of glass walls. Meanwhile, mercury has a cohesive forces greater than the adhesion force with a glass wall so it does not wet the glass wall. Adhesion force of water greater than the cohesion of the water causes the meniscus-shaped concave, while the force is greater than the cohesion of mercury causing surface adhesion force mercury meniscus convex shape. If the liquid is inserted into a capillary tube, a liquid surface will be curved. Curved surface of liquid in the pipe is called the meniscus.

Figure 1.2 y force of surface tension on the fluid in the capillary tube. Fluid rises if θ <90 ° and down if θ> 90 °.

Figure 1.2 shows the surface tension force of  fluid in the capillary tube. Shape of the surface liquid in a capillary tube depends on the contact angle (θ) fluid. The surface of the liquid will rise if θ < 90 ° and down if θ> 90 °. Increase or decrease the liquid surface can be determined by the following equation.

mg = F cosθ

Vg = ρ cosθ γ l

r2hg ρ π = γ 2π r cosθ

by:
h = increase or decrease in liquid (m),

γ = surface tension (N / m),

g = acceleration due to gravity (m/s2), and

r = radius of base tube / pipe (m).

If a liquid wetting the pipe wall, the contact angle of less than 90 ° and the liquid rises until it reaches the equilibrium height. Contaminants are added to the liquid will change the angle of contact, such as detergents alter the magnitude of the contact angle greater than 90 ° is smaller than 90 °. In contrast, substances that make waterproof fabric (waterproof) causes the contact angle of water with the cloth becomes larger than 90 °.