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)