Interfacial nanoparticle layers: Nanorheology and phase separation monitored by scanning electron microscopy.

Recent advances in electron microscopy enable single-particle monitoring of dense, nanoparticle-laden fluid interfaces. Applying scanning electron microscopy to these interfaces, real-time, in situ nanorheology was performed to understand connections between nanoparticle properties, nanoscale packing and dynamics, and bulk interfacial properties. Such systems display unique behaviors due to the finite ratio of stabilizing ligand size to particle size, affecting the softness of interparticle interactions. To conduct nanorheology experiments in the microscope, new liquid devices and new nanoparticle systems were devised to facilitate quantitative visualization while macroscopic properties (e.g., surface pressure, moduli) were measured. Modification of ligands (e.g., chemistry, length) allowed the tuning of both wetting dynamics and two-dimensional interparticle enthaplic and entropic interactions. When different components (e.g., geometically dissimilar nanoparticles, similar nanoparticles with dissilimar ligands, nanoparticles and polymers) were mixed, two-dimensional phase separation was observed at the nanoscale.