Triboelectrification During Coffee Grinding and its Effect on Espresso Flow Dynamics*

Granular materials accumulate surface charges through triboelectrification (charging resulting from friction) and fractoelectrification (charging resulting from material fracture). These processes occur during coffee grinding, and it serves as an ideal platform to study the fundamental aspects of electrical charging, while also having implications for brewing coffee at both the end-user and industrial length scale. By sourcing commercially roasted coffee, as well as roasting our own, we find that roast color and grind coarseness impact the charging; fine, darker roasts acquire charge-to-mass ratios comparable to those inferred from particles in volcanic plumes and thunderclouds. Furthermore, we elucidate the influence of residual internal moisture on electrification, concluding that moisture can tune both the magnitude and polarity of charge. In addition to possible technological applications, we demonstrate that the addition of external water simultaneously suppresses surface charging and clumping of ground coffee, and results in notably different flow dynamics in espresso formats, likely yielding markedly different taste profiles.