Viscosity is a measurement of a fluid’s resistance to change or deformation, or more simply put, how thick it is. A fluid can be a gas or a liquid, and it is easy to see that liquids are thicker than gases, and therefore have higher viscosities. Of course, some liquids are more viscous than others. For example, honey is more viscous than water. Can you imagine trying to swim in a pool of honey instead of water?
Changes and deformations are caused by stress, which is a force. Viscosity is a result of friction between molecules in a fluid that are moving at different speeds. Typically, fluids move faster away from the walls of the container, and fluids move more slowly the closer they are to the wall. This is called the “no-slip” condition.
Which fluids do you think are the most viscous?
- Tall graduated cylinder
- Notebook and Pen
- Cooking oil
- Liquid glue
- Hand sanitizer
- Any other liquid you want to test!
- Fill the graduated cylinder with one of the sample liquids. Leave a couple of centimeters at the top so it does not overflow. Be sure to fill each the cylinder up to the same height each time. Why is this important?
- Hold the marble at the opening of the graduated cylinder in one hand and the stop watch in the other hand.
- Simultaneously drop the marble and start the stopwatch.
- Stop the timer when the marble touches the bottom of the cylinder.
- Record the name of the liquid you tested, the original height of the liquid, and how long it took for the marble to fall in seconds.
- Repeat the experiment 2 more times for each liquid so there is enough data to take an average. Calculate the average time.
- Calculate the average velocity of the marble through the liquid. The distance is the height of the liquid, and the time will be the average time calculated in Step 6.
- Repeat the experiment testing other liquids. How can you tell which liquids are thicker and which are thinner using the velocity?
Viscosity from greatest to least: Liquid glue, honey, hand sanitizer, glycerin, syrup, cooking oil, water.
Note: your viscosity lab might have produced some variance in this order depending on what type of each product you use. Some brands or types may be more or less viscous than others.
To compare the relative viscosities of liquids, it is easy to use the calculated velocities. Liquids in which the marble had the slowest velocity had the highest viscosity. Filling the graduated cylinder up with the same amount of liquid each time is not necessarily essential to calculating the velocity properly, but it makes calculations easier if you use the same number for each distance value.
The units of viscosity used in engineering are Pascal-seconds (Pa·s) or centipoise (cP). 1 Pascal-second is equal to 1 kilogram/(meter*second). If you want to calculate the actual viscosity of each of the liquids tested, weigh the marble in kilograms. You can then calculate the viscosity by using the equation below:
Whereµis the viscosity of the liquid in Pa·s.
Friction between the molecules of a fluid resists fluid change and deformation. The weight of the marble, which is the gravitational force, also causes stress on the liquid. High viscosity fluids like honey and glue resist the changes caused by these forces the best.
Viscosity of liquids is often very temperature sensitive, with most liquids and gases becoming less viscous (thinner) as they heat up. You can imagine this with hot glue or melted chocolate. To take this experiment further, you may want to try microwaving the liquid for a short period of time to see if the marble drops through faster when the liquid is warmed.
Try to use what you’ve learned to guess which materials are more viscous than others: ketchup, chocolate syrup, blood, peanut butter, lava. Use the web to find out!