After this activity, students should be able to:

• Describe some aspects of buoyancy-driven flows.
• Understand that gravity, shear force and surface tension can affect fluid flow.
• See that visualizing a fluid flow can both show the physics that are occurring, and be beautiful.

Figure 3. Red-colored water dropped into vegetable oil forms spherical drops due to oil's hydrophobic force. click for copyright

(Optional: During the activity introduction, show students the images provided in the attached Flow Visualization Images PowerPoint presentation.)

What is the difference between a solid and a liquid? One answer is that when you push sideways on top of a solid, it can resist you without changing its shape. (Demonstrate by sliding your hands along the surface of a desk). This is known as resisting a shear force. By contrast, when you push sideways on the top of a liquid, it must move. This is why liquid films are slippery. Inside a car engine, a liquid film of oil is used as a cushion between moving parts. Ice is slippery when there is a liquid film of water on it. When you ice skate the pressure of your skate melts the ice right underneath it, so it slides easily.

When you drop a dense solid object into water, it just falls to the bottom. The water cannot resist being pushed out of the way. But when you drop a dense (heavier) liquid into a less dense liquid, different things can happen. If the liquids do not mix, like water into oil, then the denser fluid forms a sphere, and sinks mostly in that shape (as shown in Figure 3). This is called negative buoyancy, when the denser fluid sinks. (Optional: Demonstrate negative buoyancy as a demo by dropping colored water into a glass of oil, or add it to the negative buoyancy portion of the activity. The reason that oil and water do not mix is actually kind of complicated, and we will not cover it here, but the result is that oil is hydrophobic; i.e., tends not to combine with water.)

Figure 4. Droplet deformation. click for copyright

But if the liquids can mix, like food coloring or alcohol in water, then the shear forces (or forces acting sideways) between the two fluids can make beautiful shapes as the denser fluid sinks, or a lighter fluid rises (positive buoyancy). In these cases, the shear forces are created by viscosity(that is, friction) as gravity pulls the droplet downward. A droplet shape that starts out as a sphere gets deformed into a pill shape as the fluid in front of the droplet is pushed out of the way, and drags some of the droplet fluid with it. Next, the droplet is deformed into a shape like a blood cell (thinner in the middle), as shown in Figure 4.

Figure 5. A droplet of cream falls through hot tea. click for copyright

Sometimes the fluid in the middle is pulled to the outer edge, leaving a vortex ring (as shown in Figure 5) such as a smoke ring. Eventually the ring of falling fluid may break up into smaller droplets, leaving a trail of partly mixed fluid behind it (as seen on the left side of Figure 1).

Similar things happen when a less dense fluid rises through a denser fluid. In Figure 6, dyed alcohol was injected into the bottom of the wine glass. It rises, first through corn syrup, and then through water. Notice the little vortex rings here and there. Another example is the right side of Figure 1, where a droplet of food coloring was dropped from above into sugar syrup. Its momentum carried it to the bottom, and then buoyancy made it rise.

Figure 6. Buoyant plume of dyed alcohol rises through an interface between corn syrup and water. click for copyright

In all of these images, the moving fluid was colored with a food coloring dye, and the surrounding fluid was left clear. This is a type of flow visualization, which is used to see what is going on in a fluid. Pure food coloring is slightly denser than water, but we will mix it with rubbing alcohol to make a mixture that is less dense. Flow visualization is important for understanding fluid flows for scientific and engineering purposes, such as the design of airplanes, water treatment processes, even soda pop bottling plants, but it is also beautiful to watch.

Fluids move any time a net (unbalanced) force acts on them. After a fluid is in motion, it has momentum, so more force is required to stop it, or change its direction. What are the forces here? Gravity is pulling downward, more strongly on the denser fluids (higher weight per volume), so lighter, less dense fluids are squeezed upward. If the fluids do not mix, surface tension pulls the droplets into spheres, and surface tension may hold some of the dye at the surface of the water. Friction from molecules bouncing off each other in the fluid results in viscosity, which then makes the shear force that deforms the droplets. As you go through these activities today, think about what forces are acting on the fluid and causing the motions you observe.

• Divide the class into teams of three students each.
• Have half the teams start with the negative buoyancy experiment, and half with the positive, and then swap materials.
• This activity can also be performed as a class demonstration.
• Gather materials and make copies of the Positive Buoyancy Worksheet and Negative Buoyancy Worksheet, one each per person, and the Student Instructions Handout, one per group.
• Set up one buoyancy station per group, half with the negative buoyancy experiment and half with the positive, as described below.

Before the Activity

Assemble one buoyancy station per student group.

1. It works best to set up the stations at locations with a white surface and a white background. Or, use a horizontal and vertical sheet of white paper, as shown in Figure 7.
2. At each station, place 1 glass container, 1 paper cup, paper towels, 1 syringe, 2 small squeeze bottles of food coloring (blue, green), and 1 length of plastic tubing.
3. Place the container on top of the white paper on the tabletop with its long side aligned with the long side of the paper on the tabletop.
4. For the positive buoyancy stations, add 1 bottle of alcohol and 1 bottle of corn syrup.
5. Distribute worksheets and instructions.
6. Place a digital camera on each desk (optional).

Figure 7. Initial buoyancy station and background setup. click for copyright

Positive Buoyancy Station

1. Pour into the glass container about 1.5 inches (3.8 cm) of corn syrup.
2. Pour 2 inches (5 cm) of cool water on top of the corn syrup.
3. Mix 3 drops of food coloring with about ¼ cup (59 ml) of isopropyl alcohol (blue food coloring recommended) in the cup.
4. From the cup, pull the colored fluid up into syringe by submerging the tube tip and pulling back on the plunger.
5. Push slightly to fill the tube until almost to the end.
6. Insert the tube carefully into the lowest layer of fluid within the container, below the surface of the corn syrup, pointing the end of the tubing away from container walls (see Figure 8).
7. Experiment with a small squirt of fluid first. What happens to the colored fluid in the corn syrup compared to what happens to it in the water layer? Make a sketch on the worksheet.
8. Try a bigger squirt.
9. Sketch and/or photograph the colored fluid, filling out the worksheet. If cameras are available, place the camera directly in front of the glass container on the white sheet of paper. Take close-ups, activating any close-up (macro) feature on the camera. Also take wider angle images that include the top waterline in the container.
10. Complete the worksheet.
11. Clean the station.

Figure 8. Buoyant plume of dyed alcohol rising through corn syrup and water. click for copyright

Negative Buoyancy Station

1. Fill the glass container half full with warm water.
2. Drip food coloring directly into the water, first one droplet, then several together. Observe what happens (see Figure 9).
3. Experiment with dripping from different heights: ½ inch (1.3 cm), one inch (2.5 cm), 2 inches (5 cm), 4 inches (10 cm).
4. Sketch and/or photograph the colored fluid, filling out the worksheet. If cameras are available, place the camera directly in front of the glass container on the white sheet of paper. Take close-ups, activating any close-up (macro) feature on the camera. Also take wider angle pictures that include the top waterline in the container.
5. Complete the worksheet.
6. Clean the station.

Figure 9. Negative buoyancy experiment. click for copyright

• Warn students that the food coloring can stain their clothes and fingers.
• Warn students that alcohol stings if comes in contact with cuts or eyes. It is also not for drinking.

Have students investigate other fluids in motion. Have them look for patterns of clouds outdoors or other examples listed in the Post-Activity Assessment discussion answer. Or, have them answer the question, what causes rain to fall? [Answer: The sun warms water, making water vapor, which mixes with warm air and rises. As it rises, it cools. When it cools enough, the vapor condenses to water again and therefore becomes denser than the air around it. In a cloud, air is moving upward fast enough to keep tiny water or ice droplets in the air, but if the droplets grow big enough they fall as rain or other precipitation.]

Conduct the Density Rainbow and The Great Viscosity Race activity to learn more about the densities and viscosities of fluids.