Desorption-Mediated Motion of Nanoparticles at the Liquid-Solid Interface

Nanoparticles (NPs) confined in thin layers of liquid within liquid cells used for in situ transmission electron microscopy (TEM) move very slowly, in contrast to free particles in bulk liquid. The reason is still poorly understood. Here, we tracked gold NPs moving in water at the liquid solid interface with in situ TEM at rates of 100 frames per second. The recorded motion exhibited three key features: (1) it was made up of sustained sequences of "sticky" motion where NPs only moved a few nanometers each time; (2) sporadic long "flights" where the NPs traveled tens to hundreds of nanometers between frames; and (3) "flights" are accompanied by intermittent, fast pivoted rotations. Trajectory analysis shows that the displacements follow a truncated Levy distribution, pointing to desorption mediated motion of NPs at the liquid solid interface. We further associate pivoted rotations with a transient "weakly adsorbed" state between desorption and adsorption of NPs. The frequency of desorption was also controlled by electron flux and solution chemistry. We propose that the pattern of motion is the result of an inhomogeneous distribution of surface charges on silicon nitride (SiNx). Such insight into the interactions between NPs and solid surfaces in liquids is useful for understanding dynamics at liquid solid interfaces and has general implications for microfluidics, nanotribology, sensing, and self-assembly.