What makes baseball’s “magic mud” so special?: It has just the right mix of spreadability, stickiness, and friction to give pitchers a better grip on the ball. (Jennifer Ouellette, Nov 7, 2024, Ars Technica)
In the first experiment, the authors smeared the mud between two plates and then rotated them, measuring the changes in viscosity with a rheometer. In the second, they used an atomic force microscope to peer at the atomic structure of the material to learn more about what makes it sticky. The third experiment required a bit of ingenuity in terms of building the apparatus. They mounted pieces of mudded baseball leather on acrylic base plates and then lowered a ball to contact the surface. At first, they used a steel ball, but it didn’t have the same elastic properties as human skin. So they made their own ball out of PDMS, carefully tuned to the same elasticity, and coated it with synthetic squalene to mimic the secretion of sebum on the fingers by human oil glands.
Pradeep et al. found that magic mud’s particles are primarily silt and clay, with a bit of sand and organic material. The stickiness comes from the clay, silt, and organic matter, while the sand makes it gritty. So the mud “has the properties of skin cream,” they wrote. “This allows it to be held in the hand like a solid but also spread easily to penetrate pores and make a very thin coating on the baseball.”When the mud dries on the baseball, however, the residue left behind is not like skin cream. That’s due to the angular sand particles bonded to the baseball by the clay, which can increase surface friction by as much as a factor of two. Meanwhile, the finer particles double the adhesion. “The relative proportions of cohesive particulates, frictional sand, and water conspire to make a material that flows like skin cream but grips like sandpaper,” they wrote.