So, What is Slime?
To understand slime, we first have to understand viscosity, which can be defined as how much a liquid can resist flow. For example, maple syrup and honey are highly viscous, because they flow much more slowly than substances like water or soda pop. In this book, you will experiment with slimes of differing viscosities, from runny mucus-like slime, to gelatinous substances that hold together well, like bouncy balls.
The poise (labeled as “p”) is the official unit for viscosity, but many scientific instruments that determine a fluid’s thickness measure in centipoise (cps). For example, water is 1 to 5 cps at 21°C (70°F). Because of this small number, you might think water is the least viscous of all liquids, but acetone—which is found in nail polish remover—is a good example of a substance with an even higher rate of flow than water. Acetone is 0.3 cps at 21°C (70°F). Tomato paste and peanut butter have much higher viscosities: 1,000,000 to 2,000,000 cps at 21°C (70°F).
To further understand how viscosity relates to slime, it is best to examine the theories formulated by one of history’s greatest scientists: Sir Isaac Newton. He is said to have been one of the most influential thinkers of all time, since he is famous for having developed numerous concepts in the fields of mathematics and physics, such as calculus and his theories on gravitation. Isaac Newton also experimented with a common liquid called dihydrogen monoxide—or water.
He observed that water—like many other liquids—has a constant flow, or viscosity, and that water’s rate of flow is not affected by pressure, only by temperature. This is why the only other time we see that water doesn’t have a constant flow is when we freeze or evaporate it. Cooling water makes it more viscous, while heating it makes the substance less viscous.
Fluids that don’t follow Isaac Newton’s observations are called non-Newtonian fluids. While Newton’s model helps explain that the viscosity of a liquid is changed only by temperature, you can increase or decrease the thickness of a non-Newtonian fluid by exercising additional factors, such as agitation, pressure, or even an electrical field. Scientists give different names to non-Newtonian fluids with varying properties, and many of these will be mentioned in this book.
There are many characteristics that help identify non-Newtonian fluids. One of those characteristics is called shear stress, which sounds a bit complicated, but it’s really easy to understand once you’ve gone through the science behind it. Although it might sound like the feeling you get before a math test, the definition of “stress” used in this book is actually very different. As you may know, to “shear” means to cut or clip something with a sharp instrument. Imagine that you’re hitting a nail with a hammer. There is a lot of force that is applied onto the head of the nail. The force or “sharp instrument” in this example is the movement of your arm that causes the hammer to deform the nail head. In the case of slime, shear stress is any kind of external force (like your arm hitting the nail) that can cause a fluid to move, such as stirring, spreading, or squeezing.
Inside the nail used in the example, there are small particles that are arranged in the form of a solid. This means that the particles exhibit an unmoving, evenly-sorted pattern that holds its shape until the hammer shifts it with an applied force.
This very same concept can be applied to non-Newtonian fluids. Agitating these kinds of substances will shift their particles in one way or another. For instance, quicksand is a kind of fluid that will increase its viscosity with an increase in pressure. This is why it is hard to get out of quicksand; the more you move, the more pressure you exert, the more solidified it becomes, the more it absorbs you. But if you gently ease your way out of it, the quicksand will become more like a liquid, and you will have a slower (but simpler) escape. Quicksand is an example of what scientists call a rheopectic or shear-thickening fluid for these very reasons. Even more common substances like peanut butter and Silly Putty® are considered to be rheopectic or shear thickening. Rheopectic fluids are even inside your body! Have you ever wondered why your elbows or knees pop? Well, the fluid surrounding your knees and elbows is called synovial fluid, and it thickens when stress is applied. When you pop your knuckles, for instance, you cause the bones of the joint to stretch apart, which allows for bubbles to form and burst in the synovial fluid. Pressure makes the fluid “tight” with more bubbles wanting to escape. The “burst” of the bubbles is what causes the “CRACK!” that makes everybody around you cringe.
So if there’s such thing as shear thickening fluids, then there must be shear thinning fluids, right? Correct!
Shear thinning fluids are also known as thixotropic fluids by the scientific community. With thixotropic substances, if the pressure increases, then the viscosity decreases. If you think about it, thixotropic fluids are all around us. Say you wanted to put some ketchup on a hamburger. You can hold the ketchup bottle upside down, but nothing will happen. When you squeeze the bottle, however, the ketchup flows out like a liquid. In this case, the fluid becomes more like a liquid when stress is applied, not like a solid. Other thixotropic fluids include honey, glue, mustard, shaving cream, hair gels, mayonnaise, butter, and margarine.
If you’ve ever gotten ketchup on your shirt and left it there for a while, you might have noticed that the stain got really crusty since your last meal. However, if you’ve ever gotten peanut butter on your shirt, it most likely became runnier and stickier since the time you last ate the PB&J your mom made you for lunchtime. So, why is this?
Remember that ketchup is thixotropic and that peanut butter is rheotropic. To review: thixotropic substances become runnier with pressure, while rheotropic substances become thicker with pressure. On the contrary, thixotropic fluids become thicker with time, while rheopectic fluids become thinner with time.
So yeah, that’s basically it. Enjoy!