Why Soap Bubbles Liquid drops take spherical shape ? Surface Tension Experiments Soap film taking minimum area

11P09D Surface Tension / Mechanical Properties of Fluids

Why Soap Bubbles, Liquid Drops take Spherical Shape ? Soap Film taking minimum Area.

Surface Tension is one topic that is slightly difficult for the students. Simple home experiments can help to grasp the idea.

Students Shreya and Jagan Surface Tension Experiments

When the soap film inside the thread is broken, the soap film outside the thread tries to attain minimum area, so it tries to pull the thread from all the directions causing it to take a circular shape.

Soap bubbles take spherical shape. It is the same surface tension that causes the water to take spherical shapes (minimum surface area) in the absence of gravity as seen in space stations.

 Surface tension = Force / Length = Surface Energy / Surface Area

 Potential Energy of a ball is lowest in the ground level. So the ball always likes to fall down to reach the lowest potential energy. Similarly the Surface energy of a liquid such as water wants to keep the value as minimum as possible. If the surface tension of a given liquid is a constant then minimum area will give minimum surface energy.

 Also Surface Tension can be imagined as a stretched membrane. This is why it is even able to support the needle without sinking in.

What is Surface Tension?

Why spherical shape for droplets?

Why does a water droplet or soap bubble tends to take spherical shape, why not it take a cubical or ellipsoidal or cuboidal or conical shapes?

Minimum surface potential energy can be achieved with minimum surface area. For a given volume the minimum surface area is achieved with spherical shape. (A free falling ball falls to the ground because that is where the potential energy is minimum)

Soap bubble Experiment 1:

Make a thread loop to float on the surface of soap bubble. Then prick the inner part with a sharp piece. You will see that the thread expands to form a perfect circle. Why?

When the soap bubble part inside the thread is pricked, we will see that the thread takes up a circular shape. This is because the soap bubble on the rim should be minimum to minimize the surface energy. To achieve this the thread should take up a shape that will be of maximum inner area. For a given perimeter circle will give the maximum area.

Soap bubble Experiment 2:

Make a thread loop to float on the surface of water. Then touch the inner part with a soap. You will see that the thread expands to form a perfect circle. Why?

When the inner part of the thread is touched with a soap, the soap solution will have a surface tension lower than that of water in the outer part of ring. So the outer part of the surface tension in water pulls the thread outward to keep the surface tension minimum by minimizing the area.