Bolded words are vocabulary and concepts to highlight with students during the activity.
What is a viscoelastic material? What is a solid? What is a fluid? A solid is a material that has structural rigidity and resistance to change in shape or volume: solids maintain their shapes and do not form to their containers. A fluid, on the other hand, is a material that flows to take the shape of its container. A viscoelastic material is one that has properties of both solids and liquids. For example, silly putty. If you leave silly putty in a container, over time it takes the shape of the container. But, if you pull it apart quickly, it breaks, similar to a solid. Today, students make silly putty with varying amounts of viscoelasticity.
Class Demonstration Supplies
- Silly Putty®
- 2 clear cups
- ice cubes
Each group needs:
- 4 plastic spoons
- 1 bottle of saturated Borax® solution
- 1 piece of blank paper
- 1 bottle/tube food coloring (any color)
- 1 bottle of water
- 1 bottle of 2:1 Elmer's® glue/water solution
- 4 plastic cups
- paper towels
For the class to share:
- 1 box of small plastic zipper bags
Note: to save time, prepare bottles of glue/water and Borax mixtures in advance.
Explain the properties of a viscoelastic material using the demonstration described below. Afterwards, students make their own silly putty with differing properties.
Demonstration – Viscoelasticity
Discuss solids, liquids and viscoelasticity with students. If you fill a glass with ice cubes, the ice is solid, and does not conform to the shape of the glass; spaces in the glass are filled with air. If you add water to the glass, it is a liquid, and therefore fills all of the spaces and conforms to the shape of the glass.
Remind students that viscoelastic materials have properties of both liquids and solids. Show students the silly putty. Put it in another empty cup and show them that it fills the spaces in the cup, but slower than the water did. Pull the putty out of the cup, and slowly stretch the putty to show students that it is stretchy and elastic. Then, quickly yank on the putty and show students that it breaks like a solid.
Making Silly Putty
Instructor note: students make small batches of different mixtures to create a variety of putties. Do not hand out supplies until steps 4 and 5, listed below.
- Show students the four ingredients they will be using for their putty: food coloring, water, Borax solution and glue solution. Tell them they only get one bottle of each ingredient, so they should use it wisely.
- Tell students that each batch of putty they make should consist of exactly four spoons of liquid, the ingredients of which are distributed any way they choose. For example, they could try one spoon of glue, one spoon of Borax, and two spoons of water. Tell students they should use two drops of food coloring per batch.
- Tell students that their goal is to make putty that is very elastic, but not super sticky. (Note: to describe "too sticky," tell students their putty should easily peel from the sides of the cup or even their hands. It should have a consistency similar to cookie dough that easily comes off of waxed paper, not peanut butter that must be scraped off.)
- Pass out paper to each group. Instruct them to draw a line down the center of their papers and label the left side: Trials, and the right side, Results.
- Inform students that the best way to mix the putty is as follows:
- Decide on a recipe to try consisting of four spoonfuls of available ingredients.
- Write down the recipe (the ingredient and the number of spoonfuls) they will be mixing on a blank piece of paper. Have students call their first recipe Trial 1 (followed by Trial 2, Trial 3, etc.)
- Put the two drops of food coloring and pre-selected amount of glue mixture that they determined they will use for each recipe/test into a plastic cup and mix well.
- Next, add water and mix well.
- Add the Borax mixture and mix until a solid forms.
- Scrape the solid off the spoon and put it back into the cup and mix thoroughly again three times. If liquid remains in the bottom of the cup after three mixes, take out the solid part and have them mix it with their hands until it is no longer wet.
- Have students describe the silly putty consistency on their papers (under Results). Specifically, report on each batch's texture: too elastic, perfect, too sticky, etc.
- Repeat steps with a new recipe. Wipe off and reuse the spoons, but get a new cup for each different recipe. They have a maximum of four recipe trials.
- Have students try several recipes, again trying to create a putty that is elastic but not sticky.
- Have students show their most elastic putty to the class, and have them describe their recipes.
- If desired, offer zipper bags to students who wish to save their putty to take home. Remind them to "zip" their bags because the putty dries out quickly.
Wrap Up - Thought Questions
- Which ingredient makes the putty more solid?
- Which recipe made the most elastic putty? Why?
- Which recipe made the least elastic putty? Why?
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Last modified: July 21, 2017