A Gibbs monolayer of jammed, or nearly jammed, spherical nanoparticles on the surface of an ionic liquid was imaged in real time by <i>in-situ</i> scanning electron microscopy with a spatial resolution at the single-particle level. At nanoparticle areal fractions above that for the onset of two-dimensional crystallization, structural reorganizations of the mobile polymer-coated particles were visualized after a stepwise areal compression. For small compressions, a slow shearing near dislocations and a reconfiguration of nanoparticle bonding were observed at crystal grain boundaries. At larger scales, domains grew as they rotated into registry by correlated but highly intermittent motions. Simultaneously, the areal density in the middle of the monolayer increased. For large compressions, the jammed monolayers exhibited out-of-plane deformations, such as wrinkling. Due to their large interfacial binding energy, few (if any) of the two-dimensionally mobile nanoparticles returned to the liquid subphase. After compression for several hours and longer, the monolayers transformed into solid nanoparticle films, as evidenced by their cracking and localized rupturing when the interfacial area was increased. These observations provide unprecedented mechanistic insights into the dynamics of a simple model system that undergoes jamming/unjamming in response to mechanical stress.

Dynamic reconfiguration of compressed 2D nanoparticle monolayers

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