How Rubbing Mud makes baseballs grippier

Lena Blackburne Baseball Rubbing Mud has been used since before the 1950s to improve the grip players can get on baseballs. But how the creamy, dark-brown material modifies the surface of baseballs hasn’t been clear. New research homes in on the interactions between the magic mud and the leather ball that may boost friction during gameplay; it also hints at other applications for mud (Proc. Natl. Acad. Sci. U.S.A. 2024, DOI: 10.1073/pnas.2413514121).

The research “gives a nice, scientific explanation at a range of length scales,” says Gareth McKinley, a rheologist at the Massachusetts Institute of Technology who wasn’t involved with the work. By studying how the mud flows and interacts with surfaces, the researchers get at the chemistry and physical properties behind the mud’s behavior at both the nanoscale and the macroscale, he says.

Mud is mostly clay particles, which form a gel-like network, and water. It can also include sand and a small amount of organic matter. Lena Blackburne Baseball Rubbing Mud is sourced from a secret location in New Jersey along a tributary of the Delaware River. The company’s proprietary process includes straining the mud to remove large particles and reducing the amount of water, along with other undisclosed steps.

The research “allows us to connect what pitchers feel and the mechanical properties of a material,” says Shravan Pradeep, a chemical engineer at the University of Pennsylvania. Pradeep and his colleagues mimicked Major League Baseball’s process by smearing Rubbing Mud over some baseballs and allowing them to dry overnight. The mud tends to stay in place, like a face cream might, but thins when a force—such as a hand spreading it—is applied.

Then they tested the friction between clean or mudded leather and an object that mimicked a hand. They mounted slicesof baseball leather on plates. Above them, a ball made of the soft polymer polydimethylsiloxane, or PDMS, rotated—rubbing across the surface of the baseball pieces. To make the PDMS ball even more like a hand, the team coated it in squalene, one of the major components in skin oil. At some sliding speeds, the friction measured for mud-bearing slabs was nearly twice that for the clean leather.

The team has done a good job of replicating the complex interaction between a finger and a surface, says Matt Carré, a mechanical engineer at the University of Sheffield who wasn’t part of the study. But the experiments didn’t measure the friction that resists the start of motion. Sometimes a pitcher may not want their finger to slide across a ball, he says, such as when a pitcher imparts spin for a curveball. Often there is a link between static and dynamic friction, Carré says, but it doesn’t always work that way.

Using scanning electron microscopy, the team saw that the mud fills pores on the baseball’s leather surface. When the mud dries, not all the water evaporates. The team’s previous research found that some of the water forms capillary bridges, necklike links that adhere jagged bits of sand and other particles to the baseball surface. This phenomenon could explain the doubling of adhesive forces the team found.

These larger sandy particles contribute to friction at low sliding speeds in the frictionexperiment but detachfrom the leather at higher sliding speeds. When a player picks up a ball, they feel the stickiness of the smaller clay particles and the friction of the larger sand, Pradeep says. “It grips like a sandpaper,” he says, “but it spreads like a face cream.”

The approach of investigating materials across multiple length scales could provide clues about the behavior of other systems, such as face creams and mudslides, McKinley says. And the researchers are curious about other applications for muds and other natural materials that haven’t received a lot of attention. For instance, with some tweaks to its formulation, this mud may be able to serve as a lubricant, Pradeep says.

Although Lena Blackburne Baseball Rubbing Mud has been in use for decades, “most probably a lot of other mud will perform as well,” Claire Chassagne, a physicist at the Delft University of Technology who was not part of the work, says in an email. “You could argue that it is the fact that the rubbing mud is from a secret location that adds to the myth and the belief that this mud is so special.”