To determine the viscosity of common liquids by measuring the transit time of marbles through the liquids.

Introduction

On a cold winter morning, have you ever tried to squeeze some honey out of the honey bear onto your toast? It’s pretty tough, huh? Honey is one of those liquids that is very sensitive to temperature. As the temperature goes down, the viscosity, or resistance to flow, goes way up and you can squeeze and squeeze all you want, but very little honey comes out. If you set the honey bear in a pan of warm water for a few minutes and try again, what happens? One little squeeze and honey comes gushing out all over your toast. The viscosity, or resistance to flow, goes way down as the temperature goes up.

As a measure of a liquid’s resistance to flow, viscosity can be thought of as friction inside the liquid. If, for example, you try to ride your bike with the hand brakes on (a form of friction), it is difficult to roll the bike forward. The resistance to motion is high. Likewise, in highly viscous liquids (those with high internal friction), the resistance to flow is high.

Viscosity is a very important quality of liquids that scientists, engineers, and even doctors are frequently trying to measure and change. It is difficult, for example, to transport highly viscous crude oil through offshore pipelines, so scientists and engineers use a variety of methods to try and lower the oil’s resistance to flow through the pipelines. Likewise, in medicine, doctors try to keep blood viscosity in the correct range. If blood is “too thick,” or viscous, a patient can develop blood clots. If blood is “too thin,”or lacks viscosity, however, then the patient is at risk for bruising or bleeding events. Blood viscosity, like most things in medicine, has a happy medium.

Volcanologists (people who study volcanoes) have a big interest in viscosity, too. The viscosity of molten rock or magma determines how easily a volcano will erupt, and what shape the lava flows and resulting mountains will take on. A very thin and fluid magma erupts more easily and forms gentle mountain slopes, while a very thick magma erupts explosively and forms a fat lava flow and steep mountain slopes. So, if you see a mountain formed from a volcano, you can estimate the viscosity of the magma that formed it just by looking at the angle of its slope!

Common liquids around your house (thankfully) don’t form mountain slopes though, so to measure their viscosities, you have to use some other method. One of the oldest methods is the dropped-sphere method—a glass marble or sphere of some other material is dropped into a column of a liquid. If the liquid is very viscous (imagine cold honey), it will take a long time for the marble to drop to the bottom of the column. Dropping the marble into a less viscous liquid (like water) will take much less time.

Viscosity of a liquid can be calculated from the time elapsed, provided that you know the height of the column and the densities of the sphere and the liquid. Density is a measure of how “compact” something is. It is the ratio of mass to volume, and is a measure of how much matter is packed into a space. Think of a 1-inch cube of bread. Then think of a 1-inch cube of potato. The potato is denser than the bread (there is more “stuff” in the same space). You can calculate density yourself for an object by using a scale to find out the object’s mass and then dividing that by the object’s volume. You can also look up the densities of many common substances, like glass, stainless steel, water, seawater, oils, etc. in materials tables.

Knowing the time it took to travel through the column of liquid, the height of the column, the density of the sphere, and the density of the liquid, you can then calculate the viscosity of the liquid with the viscosity equation:

Equation 1:

where:

So, now it’s time to race some marbles and see if common liquids in your home are thick or thin!

Terms, Concepts and Questions to start Background Reasearch

• Viscosity
• Friction
• Density
• Terminal velocity
• Inverse relationship
• Direct relationship

Questions

• What is liquid viscosity?
• How does viscosity change (in general) with temperature?
• Why is it important to understand viscosity?

Bibliography

This source discusses what viscosity is, its importance to understanding volcanology, and how to measure viscosity in the laboratory:

• Hawai’i Space Grant College, Hawai’i Institute of Geophysics and Planetology, University of Hawai’i. (1996). Viscosity Teacher Page. Retrieved September 6, 2008, from http://www.spacegrant.hawaii.edu/class_acts/ViscosityTe.html

Materials and Equipment

• Tall, slender drinking glasses or glass jars with straight sides, equally sized (4)
• Marker, water soluble (1)
• Ruler, preferably metric
• Glass marbles, equally sized (5)
• Safe liquids to test (4), approximately 1/2 gallon of each, such as
  • Corn syrup
  • Honey
  • Cooking or vegetable oils
  • Glycerin
  • Seawater
  • Milk
  • Water
  • Molasses
• Stopwatch
• Strainer
• Liquid dish soap
• Dish towel
• Access to sink
• Graduated cylinder, at least 1,000-mL size (1); available at science supply stores, such as Carolina Biological: www.carolina.com. Note: the larger the graduated cylinder, the easier it is to do testing.
• Lab notebook
• Helper

Experimental Procedure

Preparing Your Glasses for the Marble Race

1. Measure down about 2 cm from the top of each glass with the ruler, and mark the 2-cm location with the water-soluble marker.
2. Fill each glass with a different test liquid, all the way up to the 2-cm mark.

Racing Your Marbles

1. Have a helper hold two marbles level with the tops of two glasses. You hold two marbles level with the tops of two glasses also.
2. Say, “Ready, Set, Go!” and then you and your helper should drop your marbles at the same time and see which marble hits the bottom first and which one hits the bottom last. Record your observations in your lab notebook. If you have trouble figuring out a clear winner between two liquids, race the marbles with just those liquids a second and third time after first completing the next set of steps: Marble Retrieval and Cleanup.

Figure 1. This drawing shows the setup of glasses and example liquids for the marble race.

Marble Retrieval and Cleanup

1. Place the strainer inside a sink or over a large bowl set inside a sink.
2. Pour the contents of each glass (the liquid and the marble) slowly into the strainer.
3. Retrieve the marbles from the strainer and wash and dry both the marbles and the glasses.

Preparing the Graduated Cylinder to Measure the Viscosity of Each Liquid

1. Fill the graduated cylinder up with one of the liquids to a level 5 cm below the top of the cylinder, as shown in Figure 2.
2. Measure down at least 2 cm below the surface of the liquid (as shown in Figure 2) and mark a starting line on the graduated cylinder with the marker. The starting line needs to be lower than the surface of the liquid to allow time for your marble to reach its terminal velocity before you start taking measurements.
3. Measure up from the bottom of the graduated cylinder, approximately 5 cm, and mark an ending line on the graduated cylinder with the marker. You don’t want the ending line to be at the bottom of the cylinder because the marble will slow down as it approaches the bottom, due to interactions with this boundary.
4. Measure the distance between the starting point and the ending point. Record this distance in your lab notebook. This is the distance that you will use to calculate the speed of the marble as it travels through the liquid. Remember, the average speed is equal to the distance traveled, divided by the time it took to travel that distance. Now you’re ready to test and get some travel times.

Figure 2. This drawing shows how to prepare the graduated cylinder for testing.

Testing Your Liquids

1. You or the helper should hold a marble at the surface of one of the liquids.
2. The other person should zero out the stopwatch.
3. The person holding the stopwatch should say “Go!” and have the other person drop the marble. As the marble passes the starting point, which was marked in the previous section, the person holding the stopwatch should start the stopwatch. As the marble passes the ending point, which was marked in the previous section, the person holding the stopwatch should stop it.
4. Record the time elapsed in a data table.
5. Repeat steps 1-4 of this section, with the same liquid and graduated cylinder, four more times with four other marbles.

Measured Time Data Table

Marble Retrieval and Cleanup

1. Place the strainer inside a sink or over a large bowl set inside a sink.
2. Pour the contents of the graduated cylinder (the liquid and the marbles) slowly into the strainer.
3. Retrieve the marbles from the strainer and wash and dry the marbles and the graduated cylinder.
4. Repeat the experiment up to this step, starting from Preparing the Graduated Cylinder to Measure the Viscosity of Each Liquid, for the remaining test liquids.

Analyze your data chart

1. Calculate the average for the five time trials for each liquid and enter it in your data table.
2. Calculate the average velocity for each liquid by dividing the distance, measured in step 4 of Preparing the Graduated Cylinder to Measure the Viscosity of Each Liquid, by the average time it took to travel that distance. Record your calculation in a second data table.
3. Remember Delta P from the viscosity equation, Equation 1? Calculate Delta P, the difference in densities between the marble and each liquid, by using the table below. Record each Delta P calculation in your data table. Note: If you tested a liquid that is not in this table, you will have to look up its density online, or calculate the density yourself, using a scale.

Common Approximate Densities (kg/m³)

1. If you know the diameter of your marbles, divide the diameter by 2 to get the radius of the marbles. If you don’t know the diameter of your marbles, measure the diameter with a ruler, and then divide by 2 to get the radius. Record the radius of your marbles in your lab notebook.
2. Calculate the viscosity of each liquid using Equation 1 from the Introduction. Record the calculations in a data table.

Viscosity Data Table

1. Compare your viscosity test results with the marble race results. Do they match up and make sense? Did the liquids with the lowest viscosities win the race? Plot the viscosity of each liquid on the y-axis and the calculated average velocity on the x-axis. Is the relationship between viscosity and velocity inverse or direct?