FYFD

Tag: sharkskin instability

  • Wild Extrusions

    Wild Extrusions

    In their continuing quest to squish all the things, the Hydraulic Press channel recently debuted a tool with a series of small holes they can extrude various substances through. The video features several great extrusions, including oobleck, temperature-sensitive putty, cheese, and crayons (above). Most of these substances are non-Newtonian fluids of some kind, and the extreme forces the hydraulic press causes makes for some wild effects.

    Many of the substances, including the crayons above, display signs of the sharkskin instability in their rough edges. When non-Newtonian fluids (like the paraffin wax in crayons) get extruded quickly, the material at the edges experiences a lot of friction and shear when trying to flow along the wall of the hole. When the fluid finally breaks free, the region along the outside accelerates to match the speed of fluid at the center of the extrusion. Parts of the mixture may resist that acceleration, resulting in the uneven edges seen above. (Video credit: Hydraulic Press Channel; GIF via Colossal)

  • Pressing Non-Newtonian Fluids

    Pressing Non-Newtonian Fluids

    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)

  • Sharkskin Instability

    Sharkskin Instability

    Homemade spaghetti noodles exhibit a roughened surface that’s the result of viscoelastic behavior known as the sharkskin instability. It’s usually observed in the industrial extrusion of polymer plastics. In the case of spaghetti, the long, complex polymer molecules necessary for the instability come from the proteins in eggs. The characteristically rough surface of the extruded material is caused by the transition from flow through the die to air. Inside the die, friction from the walls exerts a strong shear force on the outer part of the fluid while the inner portion flows freely. When the material exits the die, the sudden lack of friction on the outer portion of the fluid causes it to accelerate to the same velocity as the middle of the flow. This acceleration stretches the polymers until they snap free of the die; after the strained polymers relax, the material keeps a rough, saw-tooth pattern. In industry, the sharkskin instability can be prevented by regulating temperature or flow speed. In the case of spaghetti, though, Modernist Cuisine suggests the roughness is desirable because it helps trap the pasta sauce. Bon appetit!  (Image credit: Modernist Cuisine)