Nanoparticle Jamming at Liquid Interfaces Studied by In Situ Scanning Electron Microscopy*

Functionalized with polymer ligands, nanoparticles can segregate to liquid interfaces, where their two-dimensional organization and dynamics depend on nanoparticle areal fraction. Here, these organization and dynamics were imaged for PEGylated silica nanospheres on ionic liquids, the latter’s nonvolatility facilitating high resolution scanning electron microscopy at vacuum. An in situ liquid cell varied surface area and thereby areal fraction, and particle interactions were well approximated as those of hard spheres. A focus was high areal fraction nanoparticle mixtures approaching and/or exceeding the threshold for jamming; such mixtures were prepared for nanospheres of different diameter and nanospheres mixed with nanorods, and in both cases, composition was varied. By image analysis, quality of mixing and nanoparticle organization were assessed through orientational and translational order parameters. For uniform nanospheres, when compressed beyond jamming, surface wrinkling and buckling were observed, and when held jammed for large times before decompression, cracking was noted; throughout the imaging, individual nanospheres could be tracked to the resolution of a few nanometers.