Residual water from contaminated fluid inlets or chemicals used in solution preparation caused self-nucleation of silver particles throughout the imaging area over several seconds (Supporting Movie 4), attributable to generation of strongly reducing es -and H • radicals. Such widespread, rapid particle growth is consistent with other reported in situ aqueous metal nanoparticle syntheses observed in LP-TEM. Bubbling solvents with inert gas (Supporting Movie 3) or applying multiple simultaneous measures to dry the experimental apparatus (Supporting Movie 5)