What happens when a stream of liquid falls through a screen? As the above video shows, water creates a beautiful flower-like burst of fluid when it hits a screen. Adding a little polymer to the water makes it non-Newtonian and more viscous. When hitting the screen, this slows it down but doesn’t prevent the fluid from flowing.
Add enough polymer, though, and the fluid becomes what’s known as a yield-stress fluid. These fluids behave much like a solid–they don’t flow–until you apply a certain amount of stress. Then they’ll flow. If you’ve ever tried to get ketchup out of a glass bottle, then you’re familiar with how these yield-stress fluids act. When dropped onto a screen, the yield-stress fluid just forms a pile–unless the impact speed is high enough to create the necessary force to get the fluid to flow! (Video credit: B. Blackwell et al.)