Consider ketchup. You may need to tap the bottom of the bottle or shake it loose to get it to come out. In doing so, you are applying a shear stress to the ketchup, causing it to become less viscous. Fluids that become less viscous when shaken or stirred are known as shear-thinning fluids. Other shear-thinning fluids are shaving cream, toothpaste and paint. If you rub shaving cream between your hands, it will become thin and runny, as its viscosity decreases.
The next time you brush your teeth, if you use an electric toothbrush, you will notice that as the bristles spin, agitating the toothpaste, you will begin to see the toothpaste twist and flow, becoming less viscous.
Another shear-thinning fluid, popularized in movies, is quicksand. If trapped in quicksand, the more you struggle, the faster you sink. Rather than helping you to escape, all of your thrashing about makes the quicksand less viscous, causing you to sink faster. But don’t worry—quicksand is seldom deep enough for you to sink in over your head. And since its density is about twice that of a person, if you relax you will float to the top.
Other types of fluids respond in the opposite way to a shear stress. If a shear stress is applied, they become more viscous. These fluids are termed shear-thickening. A mixture of corn starch and water is an excellent example of a shear-thickening fluid. When squeezed it appears to solidify. You can run across the surface of a swimming pool filled with a mixture of corn starch and water. When making gravy, which uses corn starch, it becomes thicker when you stir it.
Shear-thickening fluids can be found in your body. The synovial fluid that coats the joints in your knees and elbows is shear-thickening. Usually, this fluid is not very viscous, so as to allow for free movement of the joints. But if you bump your knee or elbow on a sharp corner of a table the synovial fluid comes to the rescue, instantly becoming more viscous and cushioning your joint from the painful blow.
D3O® is another type of shear-thickening fluid. It is composed of a polymer substance suspended in an oily-type liquid lubricant. This mixture is classified as a colloid, a mixture in which the dispersed substance is permanently suspended in a dispersing medium. Other common colloids include fog, whipped cream, and milk.
When a stress is applied slowly to a shear-thickening fluid, the polymer chains have time to move out of the way and rearrange themselves, thus the viscosity is not affected. But if a quick stress is applied, the polymer chains do not have time to rearrange and they become entangled, assuming a solid-like consistency as the viscosity greatly increases.
Imagine many cars trying to quickly leave through one exit in a parking lot. If everyone is in a hurry, the cars will become ensnarled in a traffic jam. But if the traffic exits slowly, there will be time for each car to leave in an orderly fashion.
Shear-thinning fluids behave in precisely the opposite way as shear-thickening fluids. Whereas a sudden stress will cause a shear-thickening fluid to harden instantly, a more prolonged force is required to thin a shear-thinning fluid.