Moisture-controlled Triboelectrification During Coffee Grinding

Granular materials accumulate surface charges through triboelectrification and fractoelectrification-charging resulting from material friction and fracture, respectively. These processes occur during coffee grinding and impact coffee production at both the enthusiast and industrial -length scales. 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 and more concentrated extracts.