Question

4) When a glass capillary is dipped into water, the liquid will rise inside of the tube. This phenomenon is called capillary action. The picture on the right shows a glass capillary dipping into a beaker filled with water. What will happen to the water in the capillary if you pull the tube slowly out of the water? Please explain. Would the capillary action change if the capillary was made out of polypropylene, a hydrocarbon-like polymer? Please explain.

Answer 1

Solution:

On pulling out the capillary tube slowly out of the beaker, the capillary action will be disrupted because of the absence of surface tension. Though some water will remain in the tube.

The capillary action would change if the glass capillary is replaced with a polypropylene. This is because the contact angle for water-polypropylene interface would be more than 90 ° and as a result very little attraction exists between the polar water molecules and the polypropylene which is a non-polar material.

Note:

The force of attraction between molecules of the same kind is known as cohesion.The force of attraction between unlike molecules is called adhesion. Capillary action comes about as a consequence of both the adhesive forces between the liquid and the tube and the surface tension of the liquid. The adhesive forces cause water molecules to try to "stick," or "adhere," to solid surfaces. The water molecules respond to an attractive "adhesive" force pulling them towards the glass walls. Near the walls of the cylinder, the adhesive force pulling the water towards the glass walls is stronger than the cohesive force pulling the water molecules together. This attractive force pulls the water up the sides of the glass tube against the downwards pull of gravity

If a capillary tube is placed vertically into a liquid, capillary action will raise or suppress the liquid inside the tube depending on the combination of substances. The actual effect depends on the relative strength of the cohesive and adhesive forces and, thus, the contact angle θ. If θ is less that 90°, then the fluid will be raised; if θ is greater than 90°, it will be suppressed.