| OBJECTIVE:
To study surface tension and the fluid flows caused by differences
in surface tension.
BACKGROUND:
The spherical shape of liquid drops is a result of surface tension.
Molecules on the surface of a liquid are attracted to their neighbors
in such a way as to cause the surface to behave like an elastic
membrane. This can be seen in drops of rain, drops of oil, dewdrops,
and water beading on a well-waxed car.
Beneath the surface of a liquid, molecules are attracted to each
other from all directions. Because of this attraction, molecules
have non tendency to be pulled in any preferred direction. However,
a molecule on the surface of a liquid is pulled to each side and
inward by neighboring molecules. This causes the surface to adjust
to the smallest area possible, a sphere. Surface tension is what
allows objects such as needles, razor blades, water bugs, and pepper
to float on the surface of liquids.
The addition of a surfactant, such as liquid soap, to a liquid
weakens, or reduces, the surface tension. Water molecules do not
bond as strongly with soap molecules as they do with themselves.
Therefore, the bonding force that enables the molecules to behave
like an elastic membrane is weaker.
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In a microgravity environment, buoyancy-driven fluid flows and
sedimentation are greatly reduced. When this happens, surface tension
can become a dominant force. Furthermore, microgravity makes it
easier to study surface tension-driven flows then to study them
in a normal gravity environment. An analogy to this process would
be like trying to listen to a flutist (the surface tension-driven
fluid flows) during a thunder-storm (the buoyancy-driven convection).
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MATERIALS NEEDED:*
Beaker, clear jar, or drinking glass
Shallow dish or petri dish
Stirring rod
Water
Black pepper
Clear liquid soap
Toothpick
*per group of students
PROCEDURE:
Step 1. Fill the beaker, jar, or glass with water.
Step 2. Sprinkle some pepper on the water surface. Observe
what happens to the pepper.
Step 3. Stir the water vigorously. Observe what happens to
the pepper.
Step 4. Add new water to the container and mix in a few drops
of liquid soap. Carefully stir the water to dissolve the detergent
but try not to create any bubbles.
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Step 5. Sprinkle pepper on the
water surface. Observe what happens to the pepper.
Step 6. Fill the shallow dish or petri dish with water.
Step 7. Sprinkle some pepper on the surface. Observe any movement
of the pepper on the surface.
Step 8. Touch one end of the toothpick into a drop of liquid
soap to pick up a small amount of the soap. Carefully touch the end
of the toothpick to the surface of the water in the center of the
dish. Be careful not to disturb the water. observe any movement.
Step 9 (optional). Steps 6-8 can be demonstrated to the entire
class by placing the dish on the stage of an overhead projector.
QUESTIONS:
1. Why did the pepper float on the water?
2. Why did the pepper sink when the water was stirred?
3. Does the amount of liquid soap affect the results of the
experiment? Is more or less detergent better?
4. How does liquid soap enable us to wash dishes?
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