To understand slime, we first have to understand viscosity. Viscosity is how much a liquid resists flow.
For example, maple syrup and honey are highly viscous. They flow much more slowly than water or soda pop. On this website, you will experiment with different kinds of slime. Their viscosities range from runny slime to more compact slime.
The poise (labeled as “p”) is the official unit for viscosity. However, many scientific instruments measure in centipoise (cps). For instance, water is 1 to 5 cps at 21°C (70°F). This means water isn’t very viscous. Acetone—a chemical in nail polish remover—is 0.3 cps at 21°C (70°F). Therefore, it flows even faster than water. Slime usually has a high viscosity.
To further understand the relationship between viscosity and slime, let’s examine some theories by Sir Isaac Newton, one of history’s greatest scientists. Many say he was one of the world’s most influential thinkers. He developed concepts like calculus and gravitational theory.
Newton also experimented with water.
He observed that water—like many other liquids—has a constant flow, or viscosity. Also, temperature, not pressure, affects water’s flow rate. This makes sense: when we freeze or evaporate water, it doesn’t have constant flow. Cooling water makes it more viscous, while heat makes it less viscous.
We call fluids that don’t follow Isaac Newton’s observations “non-Newtonian fluids.” Slime falls under this category. Remember, Newton’s model explains how temperature affects viscosity. But, in addition, factors like agitation, pressure, and electricity affect non-Newtonian fluids. But how?
First of all, we can identify a non-Newtonian fluid by examining several characteristics. One of them is called shear stress.To “shear” means to cut or clip something with a sharp instrument.
Imagine hitting a nail with a hammer. As you know, the hammer applies a lot of force onto the nail head. The force or “sharp instrument” is the movement of your arm, which causes the hammer to deform the nail head. With slime, shear stress is any external force (like your arm hitting the nail) that causes fluid to move, such as stirring, spreading, or squeezing.
In our example, the nail contains small particles arranged into a solid. Thus, the particles form a stationary, even pattern. They hold shape until the hammer shifts them with an applied force.
Non-Newtonian fluids act similarly. Agitating them shifts their particles one way or another.
For example, quicksand increases its viscosity with increasing pressure. That’s why escaping it is so difficult. The more you move, the more pressure you exert, the more solidified the quicksand becomes, the more it absorbs you. But if you gently ease your way out, it becomes more liquid. And you will have a slower (but simpler) escape.
Quicksand is a rheopectic or shear-thickening fluid. Its viscosity increases with increasing pressure. Other rheopectic fluids include peanut butter and Silly Putty®.
Interestingly, Rheopectic fluids are inside your body! Do you ever wonder why your elbows or knees pop? Well, the fluid surrounding your knees and elbows is synovial fluid, and it thickens with stress. When popping your knuckles, the bones in your joints stretch apart. This allows bubbles to form and burst in the synovial fluid. Pressure makes the fluid “tight,” and more bubbles want to escape. These bubbles “burst” and cause a loud “CRACK!”
So if there are shear thickening fluids, are there shear thinning fluids? Yes!
Shear thinning fluids are known as thixotropic fluids. Thixotropic viscosity decreases when pressure increases.
Thixotropic fluids are all around us.
When you hold a ketchup bottle upside down, nothing happens. When you squeeze the bottle, however, ketchup flows out like a liquid. In this case, the fluid becomes more liquid with increasing stress, not solid. Other thixotropic fluids include honey, glue, mustard, shaving cream, hair gels, mayonnaise, butter, and margarine.
A ketchup stain gets crusty after a while. On the other hand, peanut butter gets runnier and stickier with time. Remember, ketchup is thixotropic and that peanut butter is rheopectic.
To review: thixotropic substances become runnier with pressure, while rheopectic substances become thicker with pressure. On the contrary, thixotropic fluids become thicker with time, while rheopectic fluids become thinner with time.
I hope you enjoyed the science behind slime! To make slime, check out the “SLIME” category in the menu above. Or click one of these links: