The Bubble Blower

Location:

Concepts Shown:

Grains, grain boundaries, and grain growth

Equipment:

Necessary for this demonstration is the bubble blower itself, the little tube to blow bubbles with, the solution used to make the bubbles, and a transparency of a bubble demonstration done by C.S. Smith (Metal Interfaces, pp. 80-81). The cost for making a bubble blower is fairly cheap. It should not cost more than twenty dollars. The tube and the Plexiglas required to make it can be found in nearly any department store. Its dimensions are shown on the diagrams that are attached. As shown on the side view, the bubbles are given a space of 6 mm. This is important because a larger thickness may allow the bubbles to break up too fast, and a smaller thickness could slow down the bubble breaking process too much. The transparency shows for each picture the growth of bubbles and the time in minutes from the beginning of the experiment. The solution I have used that worked best for me is a couple of drops of Palmolive to a cup of water. This weak solution is best for the breaking up of bubbles to show grain growth.

Procedure:

This is a good demonstration to perform during the lecture concerning grain structures, boundaries, and especially grain growth. Simply put two or three tube fulls of bubble solution (a couple of millimeters) into the bubble blower. Leaving the end of the tube a couple of millimeters from the edge of the blower, blow into the tube at a slow, steady rate. This way most of the bubbles will be about a centimeter wide, which is the size I found to show grain growth the best. If the ends of the tube is placed against the edge of the blower, too many small bubbles may result, and if the bubbles already formed to beak up before the blower can be filled. Once the blower if full of bubbles, simply place it on the overhead projector for the class to view. IF room permits, the transparency showing the results of C.S. Smith's bubble experiment may be put on the overhead next to the bubble blower. Right away the breaking of bubbles and the growth of big ones will begin. The instructor can then proceed with other topics while the bubbles continue to break up and grow.

Science:

The demonstration is perfect for showing grain growth. As you watch it, you will notice the little bubbles breaking and being overtaken by the big bubbles, just as large grains grow and small grains dissipate. It also shows convex and concave grain boundaries between the bubbles. It even displays how the boundaries meet at 120 degree angles just like they ideally do to fill space correctly. Remarks: The 3-D grain model could also be shown to compliment this demonstration. It could be explained that the grains represented by the bubble blower are actually surfaces or cross section of a three dimensional grain structure. A piece of sheet metal with visible grains may also be shows with this demonstration. This would show that the bubbles you see represent the grains that you see on the metal. The particular piece that we have is a galvanized sheet of steel. This just means that it is coated with zinc, which makes the steel more resistive to corrosion. Another aspect about the metal is that it not only shows visible grains, it also shows dendritic growth. References: Smith, Cyril S., "Grain Shapes and Other Metallurgical Applications of Topology", in Metal Interfaces, American Society for Metals, Cleveland, OH, 1952, pp. 80-81. [eq].

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