Gravitational Field Demonstrated in a Soap Bubble

To the scientist, soap bubbles are more than a visual delight. They demonstrate and model many principles of chemistry and physics: adhesion, cohesion, surface tension, and elasticity.

Cohesion is the attraction between molecules of the same substance, such as soap solution to itself. Adhesion is the attraction between molecules of different substances, such as soap to a countertop. The soap bubble exhibits surface tension (contraction) as the membrane of soap is stretched around a bubble of slightly pressurized air. The bubble is elastic: it stretches and contracts.

You can also use soap bubbles to model physics phenomena, in this case, gravitational fields. (Gravity is a weak mutual force of attraction between all matter.)

Materials

• mixing bowl
• metric measuring cup and spoons
• water
• dishwashing liquid
• glycerin (available at drugstores)
• stirring rod or spoon
• wineglass
• wire (Coat hanger wire is fine.)
• wire cutters
• metric ruler
• quick-set waterproof epoxy
• baking pan
• ring stand and clamps
• rubber tubing to hold dropper
• dropper pipette or eyedropper
• food coloring
• toothbrush

Procedure

To do a quick demonstration of the bubble ring, simply invert a wineglass, dip the rim in your soap solution, and lift the glass up.

1. In a mixing bowl, mix up a batch of soap bubbles. The best solutions are not pure soap, but rather a specific solution of different ingredients. Start with a ratio of 500 mL of cold tap water to 50 mL of dishwashing liquid and 3 mL of glycerin. Different brands of detergent give different results, so switch brands if you are not satisfied with your bubbles. Mix the ingredients well by gently stirring.
2. Devise your bubble-generating apparatus. Instead of being blown, the bubbles will hang from a ring-shaped frame.
   1. Fashion the bubble ring from a 35-cm piece of wire. Join the ends of the ring together with quick-set waterproof epoxy and allow the epoxy to harden.
   2. Make a U-shaped wire bar and attach it to opposite sides of the ring, with two wire struts, each 8 cm long, rising up as shown. The struts will allow you to handle the ring. Join the pieces of wire with epoxy.
   3. Fill a baking pan with bubble solution.
   4. Arrange a ring stand to clamp the bubble ring above the pan so that your hands are free.
   5. Remove the bulb of a dropper, and attach 1 m of rubber tubing to the exposed base of the dropper. The tube should fit snugly over the base.
3. Generate bubbles and simulate planetary orbits, gravitation, and formation.

   You may notice colorful interference patterns in the soap film during the first few minutes of its life. (See chapter 34 "Wave Interference.")

   

   A line that is tangential to a circle touches it at one point, as shown.

   When two drops collide, the more massive one usually absorbs the smaller one, like a black hole colliding with a star. By coloring one drop with food coloring and waiting until both drops merge, you can see that they will spin violently about an axis.

    

   1. Gently dip the ring into the pan, allowing it to get completely covered in bubble solution.
   2. Slowly lift the ring out so that a circular bubble film forms in the ring. Fasten the struts to the stand. Notice that the film hangs from its own weight.
   3. Dip the dropper in the soap solution to get a drop in the tip. It may help to put your finger over the open end of the tube after submersing the dropper tip.
   4. Release the drop in the bubble film. Aim it, as best you can, tangential to the circumference of the circle. The drop should spiral inward toward the center.
   5. Watch the drops move in an orbit similar to that of planets about the Sun. You may vary the initial speed of the drop by giving a gentle blow from the open end of the tube. Try to release two or more drops in the same run. They will travel and interact like multiple planets in a solar system; or like moons about a planet.
   6. To simulate the formation of a star or planet, use a toothbrush to spray fine droplets of solution onto a fresh bubble film and let the droplets coalesce into one drop. The spray represents an interstellar gas cloud, and the drop a star.

References

Black Holes and Neutron Stars: antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html

Family Fun magazine on soap bubbles: www.family.go.com/features/family_1997_07/famf/famf77bubbles/

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