Bubble Rafts

Creating a plane of bubbles on top of water to demonstrate atomic packing in crystals.

Safety

• Needles can puncture skin. Use caution and proper handling techniques.
• Use caution to not break the glass Pasteur pipet when adusting it.

Procedure

1. Mix together enough of the soap solution so that the liquid is at least 1 cm deep in the tray. The ratio of water to soap to glycerin is roughly 20:1:1. Distilled water is best but not entirely necessary.
2. Set up a ring stand with a clamp to hold the tip of the pipet or syringe needle in the water. You may want to use a ring to hold a camera above the tray if video or pictures are needed.
3. Attach a length of tubing from the air pump to the needle valve/flow meter and then another from whichever you use to the syringe and needle. You will need to cut the flange of the syringe to fit the tubing.
4. If there is another outlet on the pump or if you have a second pump, attach tubing with a 1 mm syringe at the end. This second pump will be used to blow air over the bubbles as they come out of the needle to direct where they go.
5. Turn on the pump(s) and adjust the air flow so that uniformly sized bubbles are produced. The uniformity of the bubble size is paramount, they should be around 1 mm in diameter. Try a thinner needle if this cannot be achieved by adjusting air flow.
6. Continue directing the bubbles by blowing on them either with your breath, the other tube, or both. When a large enough raft is formed for the demonstration, turn off the pump(s).

Questions 

1. Can you see areas where the bubbles are in a hexagonal arrangement? If not, try to remake the raft with smaller, more uniform bubbles.
2. Can you see different orientations of these hexagonal arrangements in the raft? If the bubbles were particles of a material, what would we call these areas? What would we call the areas in between these areas? The areas where the hexagonal orientation is uniform are models of individual crystals or grains of a material. In between these grains are what material scientists call grain boundaries. 
3. Try popping some bubbles in the middle of the raft. then take two popsicle sicks or tongue depressors and use them to squeeze the raft together. This is like applying a compressive load to a material. What do you observe? You should see dislocations moving along a straight line. These dislocations model what material scientists call slip. 
4. Repeat the last step and then immediately after compressing the raft, try moving the popsicle sticks away from each other to model a tensile load. What do you observe? How does it compare to what you observed in the last step? You should see the direction of each dislocation reverse when the load is reversed. 
5. Now, try popping many bubbles by running a glass rod (or your fingers) across the bubble raft. You should see bubbles in a highly irregular arrangement. What you just did models what material scientists call quenching,or rapidly cooling a hot metal. Watch what happens for a few seconds of gently blowing on the raft. What do you see happening? How is the arrangement changing as you wait? 

What should happen is the raft should become more and more arranged the longer you blow on it. This models atoms when a metal is annealed, or slowly cooled after being heated up.