Visualizing the Dynamics of Nanoparticles in Liquids by Scanning Electron Microscopy

Taking advantage of ionic liquid nonvolatility, the Brownian motions of nanospheres and nanorods in free-standing liquid films were visualized in situ by scanning electron microscopy. Despite the imaging environment's high vacuum, a liquid cell was not needed. For suspensions that are dilute and films that are thick compared to the particle diameter, the translational and rotational diffusion coefficients determined by single-particle tracking agree with theoretical predictions. In thinner films, a striking dynamical pairing of nanospheres was observed, manifesting a balance of capillary and hydrodynamic interactions, the latter strongly accentuated by the two-dimensional film geometry. Nanospheres at high concentration displayed subdiffusive caged motion. Concentrated nanorods in the thinner films transiently assembled into finite stacks but did not achieve high tetratic order. The illustrated imaging protocol will broadly apply to the study of soft matter structure and dynamics with great potential impact.