Soft matter mechanics of baseball's Rubbing Mud

Abstract

Researchers looking for sustainable materials with optimal mechanical properties may draw inspiration from a baseball tradition. For nearly 100 y, a mysterious mud harvested from an undisclosed river site in New Jersey (USA) has been the agent of choice in the USA's Major League Baseball for "de-glossing" new baseballs. It is unclear, however, what makes this "Rubbing Mud" work. Here, we perform a multiscale investigation of the rheology and tribology of this mud material under baseball-relevant conditions and identify three mechanisms by which the mud alters the surface properties of the baseball. First, the mud creates a more uniform baseball surface by filling in pores in the leather; this is possible because of its relatively high cohesion (clays and organics) making the material remarkably shear thinning. Second, the residue of cohesive particles coating the baseball effectively doubles contact adhesion. Third, a sparse population of angular sand grains are bonded to the baseball by clay-sized particles, leaving a studded surface that enhances friction. The proportions of cohesive, frictional, and viscous elements in Rubbing Mud conspire to create a soft material with an unusual mix of properties, that could find other applications in the development of sustainable geomaterials. Our improved understanding of the flow and friction of natural muds may also find use in modeling natural hazards such as mudslides and for locomotion in muddy environments.

Keywords: adhesion; baseball mud; geomaterials; rheology; soft tribology.

Fig. 1.

Rubbing Mud composition and microstructure. (A) The microstructural visualization of a clean baseball surface (Top) and a mudded baseball (Bottom), using confocal laser scanning microscopy (CLSM) (green images), and scanning electron microscopy (SEM) (gray images). (B) Particle size distribution of Rubbing Mud. The dashed line indicates the suspected mesh size used for the processing of Rubbing Mud; the gray box indicates the range of particles considered “fines” (≤62 μm). (C) Energy dispersive X-ray spectroscopy (EDS) shows a qualitative map of the elements present in Rubbing Mud smeared on a baseball; the Inset shows SEM image of the same area as the EDS map. SEM images of clay sheets and silt particles that contribute to the (D) tail and (E) the peak of the particle size distribution. (F) Optical microscopy of angular sand grains and silt-size particles with plant detritus that correspond to the coarse fraction.

Fig. 2.

Rheological characterization of Rubbing Mud. (A) Picture of the process of rubbing Rubbing Mud onto the baseball surface (Top) and a schematic of the parallel-plate experimental setup (and corresponding flow directions) used to measure flow properties of Rubbing Mud. The spin rate of the top plate determines the velocity V, and the shear rate $\dot{\gamma }\equiv \mathrm{\nabla }V\approx V/h$, where h is the gap thickness. (B) The steady-shear flow curve, where the viscosity and shear stress are plotted as a function of applied shear rate, for Rubbing Mud. The slope $\approx -1$, implies that $\eta \sim {\dot{\gamma }}^{n-1}$, where n ≈ 0 at low to medium shear rate values. (C) Linear viscoelastic properties, elastic ($G^{\prime }$) and viscous ($G^{″}$) moduli, as a function of applied oscillatory frequency (f). Data show that Rubbing Mud is a soft material with viscoelastic behavior.

Fig. 3.

Soft tribological characterization of Rubbing Mud friction on baseball leather. (A) Photo of a human hand gripping the baseball surface (Left) and the constituent interfaces at play (Right). (B) The rheo-tribometer customized to mimic the hand gripping a baseball surface. Experiments are conducted at a constant applied normal load, and the rotational speed of the PDMS sphere at the top is varied to generate different sliding speeds. The ratio of the sliding shear force and the applied normal force is then calculated as the frictional dissipation of the system. (C) The top view of the setup with an illustration of the cut-out slabs of baseball leather that are glued to the bottom acrylic plates, which are then attached to the bottom geometry. (D) The change in friction coefficient (μ) as a function of applied sliding speeds (u), for a squalene-dipped PDMS sphere in contact with clean (pink) and mudded (blue) baseball leather under an imposed $F_N=10$ N. The clean baseball has a constant low μ at all sliding speeds, while the mudded baseball shows enhanced μ that increases with u up to $u\approx {10}^4\mathrm{\mu }$m/s. Beyond that speed, the friction drops to the value of a clean ball; the confocal images (at an angle of ∼20^° from the x–y plane of reconstructed 3D image) show the change in microstructure between a clean and mudded baseball, and the removal of larger particles from the “scrubbed” baseball surface at high-speed shear. (E) The vertical z profile of the reconstructed 3D images of samples (left to right) that show the changes in lengthscales from bare to mudded to scrubbed. (Scale bar, 500 μm.).

Fig. 4.

Adhesive properties of Rubbing Mud at the nanoscale. (A) Illustration of the approach (in red) and retraction (in green) of the AFM tip on the baseball surface. (B) Representative force curve from an AFM experiment [same color scheme as (A)]. The adhesive force is measured as the peak retraction force. Optical microscope images of the probing sample with the AFM cantilever (Top), and representative force maps (Bottom) on a sample size of 1 μm × 1 μm, on (C) a clean baseball surface and (D) a mudded baseball surface. On average, the adhesive force on a clean baseball is half that of a mudded baseball. The optical images show the contrast between the surface pores on a clean baseball surface and the sand particles (shiny substance) dispersed across the clay matrix (dark substance) on a mudded baseball surface. In the latter case, AFM measurements are performed only on the dark substance, which primarily contains clay and silt particles.