Hands-on Activity: Floating and Falling Flows
Educational Standards :
Pre-Req Knowledge (Return to Contents)
Some knowledge of forces is helpful. Also, a basic knowledge of viscosity deepens students' understanding; as provided in the associated activity, Density Rainbow and the Great Viscosity Race.
Learning Objectives (Return to Contents)
After this activity, students should be able to:
Materials List (Return to Contents)
Each group needs:
For Positive Buoyancy Station (see Figure 2):
For Negative Buoyancy Station:
Introduction/Motivation (Return to Contents)
(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.)
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.
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.
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.
Vocabulary/Definitions (Return to Contents)
Procedure (Return to Contents)
Before the Activity
Assemble one buoyancy station per student group.
Positive Buoyancy Station
Negative Buoyancy Station
Attachments (Return to Contents)
Safety Issues (Return to Contents)
Assessment (Return to Contents)
Brainstorming: In small groups, have students engage in open discussion. Remind them that no idea or suggestion is 'silly.' All ideas should be respectfully heard. Ask the students:
Activity Embedded Assessment
Discussion Questions: Solicit, integrate and summarize student responses.
Show and Tell: Create a display of students' images of flow and hang it in the classroom, hallway or the library of your school. Have the students 'show and tell'to another class the flow visualizations they created, explaining their work to other students.
Fluids at the Movies: Many movies begin with images of fluid flow; often natural flows like clouds or waves at a beach. Keep a log of whether a movie starts with some sort of fluid flow for one week (or for the next 10 movies you watch). According to your statistics, what percentage of movies start this way?
Activity Extensions (Return to Contents)
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.
ContributorsCody Taylor, Gala Camacho, Jean Hertzberg, Malinda Schaefer Zarske, Denise Carlson
Copyright© 2006 by Regents of the University of Colorado.
The contents of this digital library curriculum were developed with assistance from the University of Colorado's Undergraduate Research Opportunities Program.
Supporting Program (Return to Contents)Flow Visualization Laboratory, Department of Mechanical Engineering, College of Engineering and Applied Science, University of Colorado at Boulder
Last Modified: March 10, 2014