Gravity-Driven Fluid Flow

OBJECTIVE:

To observe the gravity-driven fluid flow (buoyancy-driven convection) that is caused by differences in solution density.

BACKGROUND:

Many crystals grow in solutions of different compounds. For example, crystals of salt grow in concentrated solutions of salt dissolved in water. Crystals of proteins and other molecules grown in experiments on the Space Shuttle are also grown in similar types of solutions.

Gravity has been shown to cause the fluid around a growing crystal to flow upward. "Up" is defined here as being opposite the direction of gravity. This flow of fluid around the growing crystal is suspected to be detrimental to some types of crystal growth. Such flow may disrupt the arrangement of atoms or molecules on the surface of the growing crystal, making further growth non-uniform.

Understanding and controlling solution flows is vital to studies of crystal growth. The flow appears to be caused by differences in the density of solutions which, in the presence of gravity, create fluid motion around the growing crystal. The solution nearest the crystal surface deposits its chemical material onto the crystal surface, thereby reducing the molecular weight of the solution. The lighter solution tends to float upward, thus creating fluid motion. This experiment recreates the phenomenon of gravity-driven fluid motion and makes it visible.

PROCEDURE: Step 1. Fill the large glass container with very salty water. Step 2. Fill the small vial with unsalted water and add two or three drops of food coloring to make it a dark color.

Step 3. Attach a thread to the upper end of the vial, and lower it carefully but quickly into the salt water in the large container. Let the vial sit on the bottom undisturbed. Step 4. Observe the results. Step 5. Repeat the experiment using colored salt water in the small vial and unsalted water in the large container. Step 6. Observe the results. Step 7. Gently remove the two vials and examine the water in them. Are any layers present?

QUESTIONS:

1. Based on your observations, which solution is denser (salt water or un-salted, dyed water)?
2. What do you think would happen if salt water were in both the small vial and the large container? What would happen if unsalted water were in both the small vial and the large container?
3. What results would you expect if the experiment had been performed in a microgravity environment?
4. How does this experiment simulate what happens to a crystal growing in solution?

FOR FURTHER RESEARCH:

1. Repeat the experiment, but replace the water in the small vial with hot, unsalted water. Replace the salt water in the large container with cold, unsalted water.
2. Repeat the experiment with different amounts of salt.
3. Try replacing the salt in the experiment with sugar and/or baking soda.
4. Attempt to control the observed flows by combining the effects of temperature and salinity in each container.
5. Try to observe the fluid flows without using food coloring. You will have to observe carefully to see the effects.

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March 25, 1997

Adapted from NASA's "A Teacher's Guide With Activities", produced by the Microgravity Science and Applications Division, Office of Space Science and Applications, and NASA's Education Division, Office of Human Resources and Education.

author/curator: Bryan Walls
NASA Official: Dr. Gregory S. Wilson