Introduction: Make Rheopectic Slime in Less Than 15 Minutes! (It Is Not Oobleck)
Welcome to my easy slime-making instructable.
Update: I added the translucent and more slimy version. Check it out.
First, I want to thank the kind help of JustAnotherDave who has done an excellent job editing this instructable. Thank you very much :)
I'll show how to make your own high-quality slime (this slime is not Oobleck; it is not made with starch) in less than 15 minutes (the timing starts once you have gathered the materials). Of course, you will learn about two topics related to slime structure and behavior: polymers and non-Newtonian fluids. To learn what "rheopectic" means, keep reading.
First we are going to make slime, then I'll explain the science behind slime.
Step 1: Safety First
This project is very safe.
However, it involves the use of borax which can cause skin and eye irritation, so do not touch it and of course do not eat it. It is toxic if eaten! Wash your hands very well after you finish your project and do not use the spoons and flasks you used in this experiment for food any more.
The resulting slime is also safe for children older than 5 but is not for eating (just imagine what a very viscous polymer would do to any stomach!). If it is accidentally eaten, look for medical assistance immediately! I advise you to watch your kids while they are playing with it to avoid accidents. If you or your children are allergic to anything, by all means use gloves to manipulate the slime. I'm not responsible for any damage or accident caused by making or playing with slime.
Step 2: Materials
To make slime, you will need the following materials. The materials are cheap; you should be able to buy everything for less than $15.
2 crystal flasks
Gouache paint (if you want to make translucent slime, then use food coloring)
Mucilage: This is the soul of the slime. Here in Mexico it is very common and cheap but it is hard to find in other countries. Amazon.com carries an amber mucilage that currently costs less than $2 for 3 ounces. Mine is colorless but I think the amber one will also work. If you can't find it, then you can use white glue but the results are not the same. I tried white glue and you can get a nice slime but it is very different from the one you get using mucilage. To make translucent slime you must use mucilage.
Very important: Do not use glues based on silicone. They are toxic due to the solvents they contain. They are also are flammable and not water soluble. You can get a very messy, slimy polymer from it but I can't recommend that method. The solvent fumes are very strong and the results are poor. If you want to try, do it at your own risk in a well-ventilated area and DO NOT give this slime to children. It is not safe.
Now that you have gathered the materials, let's make slime!
Step 3: Borax Solution
Pour about 100 to 150 ml of distilled water in a flask.
Add a spoon of borax and stir well until all the borax has dissolved.
Now you should have a clear solution. Be patient. Borax takes time to dissolve. If it does not dissolve completely, then add a little more water.
Now that you have your solution, continue to the next step.
Step 4: Mucilage Solution
This is really easy to do. Pour some mucilage in a flask. The quantity depends of the quantity of slime you want to make. I recommend you to start with small quantities. Once you have mastered the technique you can make larger quantities of slime if you want.
Add a quantity of water equal to the amount of mucilage. We are aiming for a 50% mucilage solution.
Now add a few drops of gouache paint and stir well until you have a homogenous mix.
Step 5: The Slime
You can see this step in the following video or keep reading
Take half a spoon of your borax solution and add it to your mucilage solution. Stir. You will notice an immediate change in your mucilage solution: it becomes slimy.
Depending of the texture you want (the one I did is thicker), add more or less borax. If you want an elastic slime, do not add too many borax solution; if you want a thicker slime, add more borax. Try different quantities of borax until you get the texture you want.
Once you are done, take your slime out of the flask and mold it with your hands. It will be wet messy and a little sticky, but the more you mold it, the better it mixes. Its texture improves and it becomes firmer.
It tends to dry, so use a Ziploc bag to store it. Keep it in the refrigerator to extend its shelf life. You can also keep it at room temperature if you want, just keep it well packed.
Translucent, more elastic, and "slimier" slime
Making this kind of slime is basically the same as doing a thick slime. There are only two differences: the borax quantity and the coloring. You add less borax and you have to use food coloring instead of paint to get a translucent slime.
The other difference is that you have to make it in batches. You cannot make it in one batch because you cannot add too much borax. This is a really basic way of controlling the polymerization reaction but it works well.
Here is the video:
Now that you are done, read on to see the explanation of what happened while you play with your slime and learn, among other things, what "rheopectic" means.
Step 6: What Happenned? the Scientific Background
Borax is a salt of boric acid, also known as disodium tetraborate. Its formula is Na2(B4O5(OH)4) Making a borax solution produces the dissociated form of the salt: Na+ and ion tetraborate: B(OH4)-
When you add borax to a solution of mucilage (which is made of polyvinyl acetate), the tetraborate ions bind the large polyvinyl acetate chains with hydrogen bonds forming a more complex polymer that is like a tridimensional net. Imagine binding many rings or chain links to form a net.
This net has trapped water molecules which tend to escape by simple evaporation. This is why the slime tends to dry out and become harder. When this happens it loses its slimy characteristics, changing to a soft and plastic but rigid structure. (I think you can use your slime to make figurines, because when it is fluid, you can pour it in a mold and let it dry. I have to try this!)
You can control the texture of the slime by adding more or less borax. Borax acts as the limiting reagent in this reaction. If there are more tetraborate ions in the system, they can form more bonds. The formed "net" becomes much thicker. This affects the texture and rheology of the slime.
Now that you have seen what happens with the structure of the mucilage and how it becomes slimy, it's time to explain its rheology.
The slime we made is a non-Newtonian fluid. It is rheopectic, which means it shows an increase in its apparent viscosity with time under a constantly applied pressure (it is called shear force).
In other words, the more you play with it, the more viscous it becomes. You can even make a ball with it and it bounces. When you stop (when the shearing force is gone), it starts flowing. In the video, first you see it flow when no shear force is present. (You can also leave it on a table and see how it flows.) Then I apply a shear force on it and make it into a ball that bounces.
Differences in behaviour between a thicker and a more flexibe slime
By controlling the amount of borax in the solution, I get two different types of slime: one thick and one more fluid. The question is: Why are they different? They are made using the same ingredients, after all.
The answer to this question is that both slimes are rheopectic but their behavior is different. This is due its chemical structure:
If you make a slime using a small quantity of borax, you obtain a very soft and elastic slime that flows faster than a thicker one. You can make a ball of it, but it is not going to bounce because it starts losing its form very fast and starts flowing again. This kind of slime likes to flow more than a thicker one. However it is much more elastic than a thicker one, and it forms filaments easily (remember it has less borate bindings). The soft version looks more "slimy," as you can see if you compare the video below with the one above.
What you see in the videos and pictures are only a few simple examples of what can you do with your slime. Experiment with it and have fun!
What you saw is only a simple example of what can you do with your slime, so experiment with and have fun at the same time.
Step 7: Differences With Other Non-Newtonian Fluids (Oobleck Vs Slime )
Is this slime the same as Oobleck? The answer is NO.
Oobleck and this slime are quite different. Both are non-Newtonian fluids that share some properties but not all. The chemical and structural reasons for their behavior are different.
Here are the differences:
Oobleck is a simple colloidal dispersion of starch in water.
Slime is a cross-linked polymer made from another polymer, polyvinyl acetate.
Starch is a polysaccharide, a natural polymer present in plants.
Polyvinyl acetate is obtained by chemical synthesis. It is a synthetic polymer.
You can explain the properties of slime by referring to its chemical structure. (refer to step 6)
However, Oobleck is only a physical dispersion, so there is not chemical reactions involved. I think its odd properties are due to a physical phenomenon called adsorption. (NOTE: This is not the same as "absorption." See the glossary in the next step.)
I'll try to explain how I reached this conclusion:
I read a lot of things about Oobleck on the net. 99% of the sites focus on its non-Newtonian properties but no one tries to explain the phenomena that produce it. I found this link (which is in Spanish; sorry) which tried to explain the origins of Oobleck's behavior, but none of these theories satisfied me. So I formulated my own theory:
First I'll explain something about starch and its properties at different temperatures. Commercial native starch is formed by grains which have a low humidity. Starch is not soluble in cold water, it only disperses and adsorbs a few water molecules. With water at room temperature, starch grains can absorb a little water, but it is really a minimal amount. To make starch grains absorb water, you need to heat the water to at least 70 degrees C (158 degrees F). When the starch grains start to absorb water they swell; you get a clear and viscous starch solution at this point. At higher temperatures (90 C/194 F), the starch grains finally break and at 95 C/203 F they form a solution. When this solution is cooled the solution becomes very viscous until it forms a soft gel.
Knowing that, my conclusion is that at room temperature the predominant phenomena between water and starch grains is adsorption. So the Oobleck behavior is due to adsorption. I think when Oobleck is fluid, the starch grains and water have a normal adsorption rate and the space between starch grains is bigger. But when you apply a shear force on it, the starch grains are forced to adsorb more water. The higher the shear force, the more the starch grains adsorb water. Because it is a really saturated dispersion and the grains are spherical there is a large amount of surface area for the water to be adsorbed onto. Most of the water is adsorbed; the grains pack together; and Oobleck becomes solid like a rock. But this kind of adsorption is highly unstable so when the shear force is stopped, the starch grains automatically expulse the adsorbed water and in a matter of seconds the Oobleck becomes liquid again. The more you play with Oobleck the more viscous it becomes, but it is still liquid. I think this happens because of the slight water absorption that occurs at this temperature. You are heating the Oobleck with your hands, so you are helping the grains to absorb water a little bit; the water binds to the starch grains, thus the solution becomes more viscous.
For practical applications there are other differences:
Making Oobleck is a really dirty activity, because it tends to leave everything that it touches covered with a thin white layer of dried starch.
Slime is less dirty and when it is done is not sticky any more. You can pour it on the table, your skin, your clothes, your toys, etc., and you can remove it easily.
Oobleck is starch. If a kid tries to eat it, is less dangerous than if a kid tries to eat slime.
I tried and tried and tried, but my Oobleck never bounced, if you make a thick slime ball it can bounce.
Oobleck's solid form only lasts a few seconds. Slime flows slowly so you can use your creativity to use it for Halloween decoration, for example.
Oobleck is opaque. Slime is very shiny.
You cannot change Oobleck's texture. You can control the slime's synthesis and make it thicker or thinner if you want.
Using the correct shear force, Oobleck can become as hard as a rock. You cannot do this with slime.
Oobleck's shelf life is shorter than slime's because it is more susceptible to microorganism attacks. If you keep your slime well protected from air, its shelf life is longer.
In my opinion both fluids have interesting properties. You should try both of them.
I took the starch images from thissiteCheck it out if you are interested in learning about starch
Step 8: Glossary
Here is a brief glossary of the terms used in the explanation:
(Of course these definitions are very simple. If you want to go deep, you will notice it becomes very complex and that there are many kinds of polymers and fluids)
Adsorption is a reversible process by which a fluid molecule is fixed onto a solid matrix, typically a surface or a porous material, in other words the molecules are fixed only in the surface of the solid. Adsorption has many industrial applications, a good example of adsorption is using charcoal for adsorb gases in gas masks.
Absorption is the drawing of a gas or liquid into the pores of a permeable solid, a example of absorption is the formation of solutions, in which a liquid diffuses into a solid forming the solution.
Borax: A salt of boric acid, it has many industrial applications
Limiting reagent (or limiting reactant) Is the reactant that is completely consumed in a chemical reaction and it determines (or limits) the amount of product formed.
Non - Newtonian fluids: A fluid whose apparent viscosity changes when you apply a shear force to them. . A Newtonian fluid has constant viscosity under shear forces. Water is a Newtonian fluid; custard is a non-Newtonian fluid.
Polymer: is a macromolecule formed by many parts that are equal (like links in chains). Its structure and properties depend on how the chains that forms it are arranged. For example, it can be solid or liquid, transparent or opaque, etc.
Polymers surround us, and are the basis of common materials in our world. There are natural polymers like cellulose as well as synthesized polymers like Nylon, Teflon, etc.
Polyvinyl acetate: A kind of polymer
Rheology: In a few words, is the study of the change in form and flow of materials. For example, Rheology would be concerned with how water behaves when you stir it or pour it.
Rheopectic fluid: A kind of Non Newtonian fluid whose apparent viscosity increases when is under a shear force.
Thixotropic fluid: A kind of Non Newtonian fluid whose apparent viscosity decreases when is under a shear force.
Viscosity: Is the resistance of a fluid to flow. For example, glycerin is more viscous than water. This means glycerin has more opposition to flow than water.
Step 9: To Learn More
There are plenty books of polymers and fluids, here are some:
Polymer chemistry: an introduction
Polymer and composite Rheology, Second edition
Fundamentals of fluid mechanicals
Understanding rheology (Topics in Chemical Engineering)
Thank you for looking and enjoy playing with your slime