Blowing Bubbles: How Does Gravity Affect the Shape of Soap Bubbles?

Problem

How does gravity affect the shape of soap bubbles?

Materials

• cup (63 ml) dishwashing liquid
• small bowl
• cup tap water
• spoon
• 1 teaspoon (5 ml) sugar
• large empty thread spool

Procedure

NOTE: This is an outdoor experiment.

1. Pour the dishwashing liquid into the bowl.
2. Add the water to the bowl.
3. Stir the sugar into the soapy mixture to give strength to the bubbles.
4. Dip one end of the thread spool into the mixture.
5. Place your mouth against the dry end of the spool, and gently blow through the hole.



6. Blow a large bubble, and then place your finger over the hole you blew through to prevent the air from leaking out of the soap bubble.
7. Study the bubble's shape until it breaks.
8. Observe and record any movement on the surface of the bubble.

Results

A bubble that is slightly pointed on the bottom hangs from the spool. Tiny, threadlike streams of liquid quickly swirl down the sides of the bubble and collect at the bottom, where they form drops and fall.

Why?

The molecules of dishwashing liquid and water link together to form a thin skin of stretchy liquid around the air blown into it. Gravity pulls the rounded bubble downward, forming a slight point at the bottom. Excess liquid on the edge of the spool is pulled down to the lowest point, collects in drops, and drips from the bottom of the bubble. The molecules that make up the thin film of the bubble are also pulled downward, causing the bubble's skin to become thinner at the top until it finally breaks.

Let's Explore

1. Does the size of the bubble affect its shape? Blow a large bubble and leave the hole in the top of the spool open to allow the air to leak out. Observe and record any changes in shape as the bubble decreases in size.



2. G is the symbol used to rate the force of gravity. The earth's gravity is used as a standard and given the value of 1 G. Pulling the spool upward quickly increases the G-force on the bubble. Determine the shape of a bubble blown where the G-force is greater than that on the earth by blowing a bubble and observing its shape as you move the spool quickly upward. Repeat the procedure, using different upward speeds. Science Fair Hint: Record the results and make drawings of the shapes of the bubbles. Use these as part of a project display.
3. Does the shape of the opening in the spool affect the bubble's shape? Would a square or triangular opening make flat-sided bubbles and thus change the effect that gravity might have on the bubble? Use wire to make openings of different shapes; then use them to blow bubbles.

Show Time!

1. Take photographs while bubbles are being blown from the different-shaped openings and display them along with the written results of the shape produced.
2. What shape would a soap bubble be in space? In space, the pull of gravity is so weak that scientists call it microgravity. Everything in space is virtually weightless. Find out the shape of drops of liquids released in a spacecraft, and make drawings of the shape of bubbles in and out of a strong gravitational field. Use these drawings as part of a project display.

Check it Out!

Some objects made on the earth have flaws created by the pull of gravity. How does gravity negatively affect the shaping of things like marbles or ball bearings? Make a survey of materials that are difficult to make perfectly round because of gravity. Would these things be easier to make in a spacecraft?

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