Bubble Tray - Create giant bubbles!

  

Bubble Tray

 

Create giant bubbles

Bubbles are fascinating. What gives them their shape? What makes them break or last? What causes the colors and patterns in the soap film, and why do they change? 

 

 

• Measuring cups and spoons.
• Dawn™ or other dishwashing liquid.
• Glycerine (available at drugstores).
• Tap water.
• A wire coat hanger.
• A shallow tub or tray about 18 inches (45 cm) in diameter such as a potted-plant drain dish, a pizza pan, or a catering tray).
• Optional: Yarn.

 

(30 minutes or less)

Mix up a bubble solution of 2/3 cup (160 ml) Dawn™ dishwashing liquid and 1 tablespoon (15 ml) glycerine in one gallon (3.8 l) of water. We have found that more durable bubbles form if you let this solution age for at least a day, preferably for a week.

Bend the coat hanger into a flat hoop with the hook sticking up at an angle to serve as a handle. Bubbles will form more consistently when the hoop is as circular as possible. If you wrap yarn tightly around the wire of the hoop, the yarn will absorb the bubble solution, which will make the hoop easier to use.

If you prefer a more elegant apparatus, a bubble tray complete with a bubble hoop is available at the Exploratorium Store for about $20. 
 

(15 minutes or more)

Fill the shallow tray with bubble solution and submerge the hoop in the solution. Then tilt the hoop toward you until it is almost vertical, and lift it from the tray. You should have a bubble film extending across the hoop. Swing the hoop through the air to make a giant bubble. When you have a big bubble, twist the hoop to seal it off at the end.

What shapes do the bubbles take once they are free of the hoop? What roles do convection and air currents play in the bubble's movement? Look for patterns and colors in the bubbles. Dip the hoop in the solution and hold it up to the light without forming a bubble. What patterns (and changes in patterns) do you observe? 
 

The strong mutual attraction of water molecules for each other is known as surface tension. Normally, surface tension makes it impossible to stretch the water out to make a thin film. Soap reduces the surface tension and allows a film to form.

Because of surface tension, a soap film always pulls in as tightly as it can, just like a stretched balloon. A soap film makes the smallest possible surface area for the volume it contains. If the bubble is floating in the air and makes no contact with other objects, it will form a sphere, because a sphere is the shape that has the smallest surface area compared to its volume. (Wind or vibration may distort the sphere.)

The patterns of different colors in a so
ap bubble are caused by interference. Light waves reflected from the inner and outer surfaces of the soap film interfere with each other constructively or destructively, depending on the thickness of the bubble and the wavelength (that is, the color) of the light. For example, if the soap film is thick enough to cause waves of red light to interfere destructively with each other, the red light is eliminated, leaving only blue and green to reach your eyes.

You can make other devices to create large bubbles. One of the easiest is a length of string (or, still better, fuzzy yarn) threaded through two drinking straws, with the ends tied to make a loop any size you want. Not only will this device make large bubbles, but you can twist the straws to make film surfaces with different shapes.

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Bubble Suspension - Soap bubbles float on a cushion of carbon dioxide gas

   

Bubble Suspension

 

Soap bubbles float on a cushion of carbon dioxide gas

This beautiful experiment illustrates the principles of buoyancy, semipermeability, and interference 

 

• A small aquarium
• Dry ice
• Bubble solution You can use a commercial solution like Wonder Bubbles™, or use the Exploratorium's recipe: 2/3 Cup (160 ml) Dawn™ dishwashing liquid and 1 tablespoon (15 ml) glycerine (available at most drugstores) in 1 gallon (3.8 l) of water. Aging the solution for at least a day before use significantly increases the lifetime of the bubbles.
• Gloves
• Adult help

 

(5 minutes or less)

Place a slab of dry ice flat in the bottom of the aquarium. (CAUTION: Use gloves when handling the dry ice; do not touch it with bare skin.) Allow a few minutes for a layer of carbon dioxide gas to accumulate. 
 

(15 minutes or more)

Blow bubbles so they float down into the aquarium. The bubbles will descend and then hover on the denser layer of carbon dioxide gas. After a few minutes, notice that the bubbles begin to expand and sink. Notice the color bands on the bubbles. Notice how some of the bubbles freeze on the dry ice. 
 

As dry ice turns from a solid to a vapor, or sublimes, it produces carbon dioxide gas. Carbon dioxide is denser than air. (Carbon dioxide molecules have an atomic mass of 44 amu [atomic mass units]. Air is made up of nitrogen, 28 amu, and oxygen, 32 amu.) The denser carbon dioxide gas forms a layer on the bottom of the aquarium.

A bubble is full of air. It floats on the carbon dioxide layer just like a helium balloon floating in the air. You might expect that the air in the bubble would cool and contract near the dry ice, but the bubble actually expands slightly. The soapy wall of the bubble allows carbon dioxide to pass through but does not allow air molecules to pass through. Initially, the concentration of carbon dioxide gas is low inside the bubble and high outside the bubble.

The gas gradually diffuses into the bubble, a process called osmosis. The bubble film is a semipermeable membrane--a surface that allows some substances to pass through while preventing others from passing through at all. The cells in your body have the same property. Water, oxygen, and carbon dioxide easily enter some cells, whereas other molecules do not. The added carbon dioxide makes the bubble denser, causing it to gradually sink. The carbon dioxide at the bottom of the tank is cold enough to freeze the bubble.

You can do many experiments with these bubbles.

What happens when bubbles of different sizes collide? Sometimes they make a single larger bubble, other times they join as two bubbles with a flat or bulging wall between them. If the two bubbles are the same size, the wall is flat between them, since the pressure is equal on both sides. If the two bubbles are of different sizes, the wall will bulge away from the smaller of two bubbles, since the smaller bubble will have a higher pressure inside.

How does a bubble respond to a comb that has been charged by rubbing it with a wool cloth? The neutral bubble is electrically polarized by, and attracted to, the charged comb.

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Bronx Cheer Bulb

Bronx Cheer Bulb

 

Some light sources may appear to wiggle and flash when you give them the raspberry, but the only thing wiggling is you

Some light sources flash on and off many times a second. When you give them the "Bronx cheer," you can see the their hidden flickering. 

 

• A digital radio or clock radio that uses light-emitting diodes (LEDs) These have red numbers. Or
• A circuit tester with an LED on it Or
• A neon glow lamp available from your local hardware store, such as a GE Guide lamp, and an extension cord. (Any nightlight labeled "1/4 watt" has a neon glow lamp in it)

 

(5 minutes or less)

No assembly is required for the digital radio, circuit tester, or neon glow lamp; just plug them in and observe them from a few feet away.

A simple source for a neon glow lamp is a button-type nightlight. These are small orange night-lights advertised as 1/4 watt bulbs. They do not have a regular replaceable small lighibulb. Plug the nightlight into the wall or into an extension cord that is plugged in. 
 

(5 minutes or more)

Observe the light source from 3 to 10 feet (90 to 300 cm) away and give it the "Bronx cheer." (A Bronx cheer, also known as a "raspberry," is a rude noise made by blowing air through your lips in a way that makes them vibrate.) Notice that the light seems to wiggle back and forth and flicker. Try shaking your head rapidly and notice whether the light still flickers. See if you can find other body motions that make the light flicker. Try the Bronx cheer on other light sources, such as incandescent lightbulbs. Notice whether the light flickers. 
 

No part of the LEDs or the neon glow tube move when you give the Bronx cheer. Instead, your whole body is vibrating, including your eyes. you can feel this vibration by putting your hand on your head as you blow. The LEDs flash on and off sixty times a second (a neon glow tube glows on and off 120 times a second). This flashing is so fast that your eyes normally can't separate the "blinks." But when your body is vibrating, your eyes are in a different position each time the bulb flashes. As the image of the bulb traces a path across your eyes, it looks like the bulb is moving and flickering. An incandescent bulb won't flicker when you give the Bronx cheer, because the bulb doesn't flash on and off. Incandescent bulbs give a steady glow.

Plug a commercial neon night-light into an extension cord. Tape it firmly in place. Twirl the light around in a circle. (Be careful not to let it hit anything.) Notice that you can see the light flashing. Since the light is moving, it's in a new position each time it flashes. The light traces a path across your eye, and its flashes become spread out and visible. Find an oscilloscope and set it up so that the beam goes straight across the middle of the screen in about 1/1ooth of a second. Ask a couple of friends to stand back a few yards from the scope. Tell them that the oscilloscope is an eating detector. Have your friends watch the scope at the same time. Have one of them eat a peanut and the other one not eat. The person eating the peanut will see the beam jump up and down. Eating causes vibrations of your skull, including vibrations of your eyes. If your eyes are moving, the dot of light scanning across the oscilloscope shines on different parts of your eyes and appears to jump around.

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