OBJECTIVE: To illustrate the effects of gravity and surface tension on fiber pulling. BACKGROUND: Fiber pulling is an important process in the manufacture of synthetic fabrics such as nylon and polyester and more recently, in the manufacture of optical fibers for communication networks. Chances are, when you use the telephone for long distance calls, your voice is carried by light waves over optical fibers.

Fibers can be drawn successfully only when the fluid is sufficiently viscous or "sticky." Two effects limit the process: gravity tends to cause the fiber to stretch and break under its own weight, and surface tension causes the fluid to have as little surface area as possible for a given volume. A long slender column of liquid responds to this latter effect by breaking up into a series of small droplets. A sphere has less surface area than a cylinder of the same volume. This effect is known as the "Rayleigh instability" after the work of Lord Rayleigh who explained this behavior mathematically in the late 1800's. A high viscosity slows the fluid motion and allows the fiber to stiffen as it cools before these effects cause the strand to break apart.

Some of the new exotic glass systems under consideration for improved optical fibers are much less viscous in the melt than the quartz used to make the fibers presently in use: this low viscosity makes them difficult to draw into fibers. The destructive effects of gravity could be reduced by forming fibers in space. However, the Rayleigh instability is still a factor in microgravity. Can a reduction in gravity's effects extend the range of viscosities over which fibers can be successfully drawn? This question must be answered before we invest heavily in developing expensive experiment apparatus to test high temperature melts in microgravity. Fortunately, there are a number of liquids that, at room temperature, have fluid properties similar to those of molten glass. This allows us to use common fluids to model the behavior of molten materials in microgravity.

PROCEDURE: (for several demonstrations) Step 1. While wearing eye and hand protection, use the propane torch or Bunsen burner to melt a blob of glass at one end of a stirring rod. Touch a second rod to the melted blob and pull a thin strand downward. Measure how long the fiber gets before it breaks. Caution: When broken, the fiber fragments are sharp. Dispose of safely. Step 2. Squirt a small stream of water from the syringe. Observe how the stream breaks up into small droplets after a short distance. This breakup is caused by the Rayleigh instability of the liquid stream. Measure the length of the stream to the point where the break-up occurs. Do the same for other liquids and compare the results. Step 3. Touch the end of a cold stirring rod to the surface of a small quantity of water. Try to draw a fiber. Step 4. Repeat #3 with more viscous fluids, such as honey. Step 5. Compare the ability to pull strands of the various fluids with the molten glass and with the measurements made in step 2. Step 6. Pour about 5 centimeters of water into a small test tube. Drop the ball bearing into the tube. Record the time it takes for the ball bearing to reach the bottom. (This is a measure of the viscosity of the fluid.) Step 7. Repeat #6 for each of the fluids. Record the fall times through each fluid.

MATERIALS NEEDED: Propane torch or Bunsen burner Small-diameter glass stirring rods (soft glass) Disposable syringes (10 ml) without needles Various fluids (water, honey, corn syrup, mineral oil, and light cooking oil) Small ball bearings or BBs Small graduated cylinders or test tubes (at least 5 times the diameter of the ball bearing) Stopwatch or clock with second-hand Eye protection Protective gloves Metric ruler

QUESTIONS:

1. Which of the fluids has the closest behavior to molten glass? Which fluid has the least similar behavior to molten glass? (Rank the fluids.)
2. How do the different fluids compare in viscosity (ball bearing fall times)? What property of the fluid is the most important for modeling the behavior of the glass melt?
3. What is the relationship between fiber length and viscosity of the fluid?

FOR FURTHER RESEARCH:

1. With a syringe, squirt a thin continuous stream of each of the test fluids downward into a pan or bucket. Carefully observe the behavior of the stream as it falls. Does it break up? How does it break up? Can you distinguish whether the breakup is due to gravity effects or to the Rayleigh instability? How does the strand break when the syringe runs out of fluid? (For more viscous fluids, it may be necessary to do this experiment in the stairwell with students stationed at different levels to observe the breakup.)

2. Have the students calculate the curved surface area (ignore the area of the end caps) of cylinders with length to diameter ratios of 1, 2, 3, and 4 of equal volume. Now, calculate the surface area of a sphere with the same volume. Since nature wants to minimize the surface area of a given volume of free liquid, what can you conclude by comparing these various ratios of surface area to volume ratios? (Note: This calculation is only an approxi-mation of what actually happens. The cylinder (without the end caps) will have less surface area than a sphere of the same volume until its length exceeds 2.25 times its diameter from the above calculation. Rayleigh's theory calculates the increase in surface area resulting from a disturbance in the form of a periodic surface wave. He showed that for a fixed volume, the surface area would increase if the wavelength was less than p times the diameter, but would decrease for longer waves. Therefore, a long slender column of liquid will become unstable and will break into droplets separated by p times the diameter of the column.)