Visualizing the Interfacial Jamming of Nanoparticles on a Liquid Surface*

Two-dimensional assemblies of nanoparticles (NPs) are model systems for studying jamming and vitrification, with slowed dynamics, rate dependence, and dynamic heterogeneities. We developed a scanning electron microscopy (SEM) technique to image NP assemblies on a nonvolatile ionic liquid (IL) droplet where areal density, φ, is well-controlled. SEM precisely images the real time location of each NP on the droplet surface, allowing NP dynamics to be determined as a function of φ. The packing structure and interactions between NPs can be rigorously measured as assemblies transition from the liquid state to the glassy or jammed state. Here, monodisperse PEGylated silica NPs and gold NP tracers (100-250 nm) were cast on the surface of 1-ethyl-3-methylimidazolium ethyl sulfate IL and imaged. By measuring the mean square displacement as a function of time, NP diffusion coefficients were found and compared with phenomenological models. Assembled structures were also analyzed at each φ by order parameters <ψ₆> and T*, Voronoi tessellation, and pair correlation functions. The results show that as φ increases, NPs diffuse more slowly and their assembly transitions from a liquid-like structure to a crystalline structure.