107 Strange Fluids-Mid

Strange Fluids FOCUS Question Why do certain fluids sometimes act like solids and other times act like liquids?

Normal Fluids Think about pouring a glass of water. The liquid quickly flows from the pitcher into the glass. Water is a fluid. A fluid can be a liquid or a gas. Water has low viscosity (vis-KOS-uh-tee). This means the fluid is not very thick. Now think about how honey moves. This fluid flows slowly. It may come out of the jar in globs. Honey has high viscosity. Different fluids have different viscosities. The temperature of a fluid can affect how it moves. If you heat up honey, it flows faster when you pour it. Its viscosity goes down. Water and honey are considered normal fluids. These fluids are affected only by temperature.

Strange Fluids Not all fluids act "normal," Some fluids seem like a liquid one moment and act like a solid the next. These strange fluids are affected by something other than temperature. Their viscosity changes when a force, or stress, is applied. If you hit water with your hand, not much happens. The viscosity of the water doesn't change (see black line in graph). But if you hit a strange fluid, its viscosity does change. Some strange fluids get thinner when stress is applied (see green line in graph). Others actually get thicker (see orange line in graph).

Getting Thinner So what puts stress on a fluid? Strange fluids can respond to shaking, hitting, stirring, or squeezing. The viscosity of some strange fluids goes down when stress is applied Paint hardens if left alone in a jar. But if you shake or stir it, the paint becomes thinner. Ketchup also thins out when you add stress. Getting this fluid out of a glass bottle is difficult at first. But if you hit the bottom with enough force, the ketchup comes pouring out.

Getting Thicker Oobleck is a strange fluid made of cornstarch and water. When you quickly squeeze it together in your hands, it gets thick and feels solid. But open your hands and it flows through your fingers like a liquid. Do You Know? The name "oobleck" comes from the Dr. Seuss book Bartholomew and the Oobleck. You even have a strange fluid inside your own body. This fluid is in your joints, including your knees, elbows, and shoulders When you suddenly put a lot of pressure on a joint, the fluid gets thick. This keeps your bones from rubbing together. When the stress stops, the fluid gets thin again. This allows your joints to move freely.

Stress over Time Some strange fluids change instantly when a force is applied. Others change only when the stress lasts for a longer time. Have you ever made whipped cream? When you start, the cream is a liquid that can easily pour. But after whipping it for a few minutes, the cream becomes thick. If you let it sit for a while, it acts like a liquid again. Other fluids have the opposite response to stress over time. Think about squeezing hair conditioner from a bottle. The liquid is very thick. When you shake it for a while, the conditioner becomes thinner and pours from the bottle. If you stop shaking it, it becomes a thicker fluid again.