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Jan. 22, 2010

Gooey Gak

by Austen Saltz

 
Science Friday brings you a science fair project for the recession: combine cornstarch and water and make a fluid with bizarre physical properties, on the cheap. Physicists Robert Deegan, of the University of Michigan, and Harry Swinney, of the University of Texas at Austin, explain why the mixture comes to life under the right conditions.
 
 
Grade Level: 6th – 8th grade
Subject Matter: Physical Science
National Standards: NS.5-8.1, NS.5-8.2
 
Overview
Complex fluids are special kinds of mixtures that have characteristics of more than one phase of matter. In this video, the combination of cornstarch and water resulted in a substance that exhibited the properties of a solid and a liquid depending on the amount of pressure or force applied to it. These types of fluids that don’t behave like what we think of as “normal” fluids are called non-Newtonian fluids. Many non-Newtonian fluids are made of polymers, long chains of repeating molecules that give the fluid unusual physical properties such as flexibility and strength.
 
In this activity, students experiment with another type of non-Newtonian fluid called Gak. In the SciFri Video, students will be able to observe the physical properties of the cornstarch and water mixture and learn the definition of a non-Newtonian fluid. Students also will define polymers and compare and contrast the properties of a linear polymer (glue) and the properties of a cross-linked polymer (Gak).
 
Activity Materials
Borax powder
Water
White glue
Food coloring (optional)
Spoons
Cups
 
Vocabulary
Molecule: the smallest particle of a substance that retains all the properties of the substance. A molecule is composed of one or more atoms.
Non-Newtonian Fluid: a fluid that has the characteristics of both a solid and a liquid, depending on the amount of force applied to it.
Polymer: a long-chained molecule consisting of smaller repeating molecule units (known as monomers).
Linear Polymer: a polymer in which the molecular units are linked in a linear fashion.
Cross-linked Polymer: a polymer in which adjacent linear molecular chains are connected together.
 
 
What To Do
  1. Start the lesson by having the students watch the SciFri Video. Lead a discussion with the students on the properties of the cornstarch and water mixture seen in the video. What were the unique reactions of the mixture in the video when force or stress was applied to it? Explain to students that non-Newtonian fluids are fluids that don’t behave like what they consider normal fluids. Tell students that they will experiment with another type of non-Newtonian fluid called Gak.
  2. Have each student make a saturated Borax solution by stirring one tablespoon of Borax powder into one cup of water. Explain to students that a solution is saturated when the substance that they are mixing in no longer dissolves. Students should continue to add Borax powder until it no longer dissolves in the water. Set the solution aside.
  3. Have students pour one teaspoon of glue in a separate cup. Ask students to describe the characteristics of the glue. Was the glue easy to pour? Explain to students that glue is a type of polymer consisting of long chains of repeating molecules. Ask students to predict what will happen if they add water into the glue. Have students add one teaspoon of water and a drop of food coloring (optional). How does the mixture feel when they stir it?
  4. Ask students to predict what will happen if they combine the glue/water mixture with the saturated Borax solution. Have students stir one teaspoon of the saturated Borax solution into the glue/water mixture. How did the mixture change? Have students touch the mixture and describe what it feels like. If the mixture feels sticky, try adding a little more Borax solution. If the mixture feels very wet and slippery (and no longer runny), remove it from the cup and knead it until it is a stretchy, gooey substance. If the mixture is stringy and not malleable, try adding a little more glue.
  5. Have students perform various actions on the Gak (bounce it, tear it, squeeze it, hold it gently, etc.) and record their observations. Does the Gak have the characteristics of a solid or a liquid?
What's Happening?
White glue is a linear polymer, made up of millions of long strands of molecules linked together like chains. The chains of glue molecules are able to slip and slide freely over one another like strands of cooked spaghetti; however, they are so long that they tangle together, which gives glue its thick consistency. When added to the glue, Borax links the long strands of glue molecules together like a net to form a cross-linked polymer commonly known as Gak. Scientists believe that it is the interaction between these chains that causes this non-Newtonian fluid to react like a solid to hard or fast pressure and like a liquid to slow, even pressure.
 
Topics for Science Class Discussion
  • What will happen to the Gak if you leave it exposed to air for a few days? What are the processes that led to its physical change? Can the Gak be restored to its original state?
  • Compare and contrast the properties of the cornstarch and water mixture from the video to the properties of Gak. In what ways are they similar or different?
  • Why are these fluids specifically called non-Newtonian?
  • What are other non-Newtonian fluids in our everyday life?
Extended Activities and Links
  • Investigate the molecular structures of polymers by building a linear, branched and cross-linked polymer model using paperclips.
  • Depending on its molecular makeup, a polymer can be soft or hard, flexible or stiff, durable or fragile. Natural polymers include wool, hair, silk, rubber, and sand; synthetic polymers include nylon, Teflon, Formica, and rubber silicone. Have students look around their school or home and make a list of all the things they can find that are made of polymers.
  • Try this balloon challenge to demonstrate one of the properties of polymers – elasticity. (N.B., Students will be handling latex, so skip this activity if your class includes anyone who is allergic.)
  • Hand out a wooden skewer and air-blown balloon to each student. Ask students to predict what would happen if they poked the skewer straight through the balloon. Is there a way to pass the skewer into the balloon without popping the balloon? The balloon will not pop if the skewer is gently inserted in through the end of the balloon by the knot and out through the top center of the balloon. Tell students that latex is a type of polymer, and discuss why this would prevent the balloon from popping if done correctly. What is a physical characteristic of latex that will help it seal upon itself?
  • Find out more information about polymers and how they relate to plastics through online games and resources.
  • Explore polymers and their properties by making shrinky dinks from pieces of plastic you can’t recycle.
About Austen Saltz

The views expressed are those of the author and are not necessarily those of Science Friday.

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