For many fluids, the relationship between force and deformation is not simple. The catch-all name for these materials is non-Newtonian fluids. In a recent episode, the Hydraulic Press Channel did some experiments extruding a couple non-Newtonian fluids: oobleck and a temperature-sensitive putty. What they demonstrated is that a fluid’s response to the forces it experiences can change depending on the rate at which force is applied.
Take their putty example from the latter half of the video. When the hydraulic press pushes the putty slowly, it extrudes in a smooth, semi-solid string. When they increase the pressure driving the hydraulic press, it pushes the putty more quickly, causing it to spray out of the die in a shredded mess. What they actually did here is surpass a threshold for what’s known in manufacturing as the sharkskin instability. This behavior occurs due to long-chain polymer molecules in the fluid. Inside the die, flow near the walls is slowed down by friction but moves freely in the middle of the pipe. When the walls are suddenly gone, flow at the outside accelerates to match the inside of the stream, which stretches the polymers until they can snap free of the die. The result is the rough, saw-tooth-like pattern seen here. (Video and image credit: Hydraulic Press Channel, source)
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