Mechanics of Nanoparticle-Covered Liquid Surfaces Examined by SEM

Polymer ligand-functionalized nanoparticle spheres adsorbed to a nonvolatile liquid surface were imaged by in situ open SEM and tracked as a function of areal fraction. This data was interpreted by particle tracking nanorheology to provide dynamic surface mechanical properties. Nanoparticle stability at the surface changed with chemistry and length of ligands, imparting different interfacial rheologies. Using dilute tracer particles larger dispersed among smaller nanoparticles, dynamics could be monitored at multiple frames per second. Beyond nanorheology, employing a Langmuir trough adapted for the electron microscope, the surface area of nearly jammed particle-covered liquid surfaces was oscillated to enhance nanoparticle mobility such that hexagonal ordering (i.e., 2D crystallization) and phase separation were observed on experimental timescales.