We induce and investigate the coarsening and melting dynamics of initially static nanoparticle colloidal monolayers at an ionic liquid–vacuum interface, driven by a focused, scanning electron beam. Coarsening occurs through grain interface migration and larger-scale motions such as grain rotations, often facilitated by sliding dislocations. The progressive decrease in area fraction that drives melting of the monolayer is explained using an electrowetting model whereby particles at the interface are solvated once their accumulating charge recruits sufficient counterions to subsume the particle. This new driving mechanism for colloidal systems, whose dynamical timescales we show can be controlled with the accelerating voltage, opens the possibility to manipulate particle interactions dynamically without need to vary particle intrinsic properties or surface treatments. The decrease in particle size availed by electron imaging presents opportunities to observe force and time scales in a lesser-explored regime intermediate between typical colloidal and molecular systems. By furthermore exploring binary particle mixtures of different size ratios, we explore compositional phase separation precipitated by a crossover from jamming for the smaller particles. By directly visualizing the particle dynamics, we confirm that enhanced small particle mobility at the largest size ratios provides a mechanism for percolation and separation into distinct macrophase regions.

Charging-driven coarsening, melting, jamming, and percolation in colloidal monolayers at an ionic liquid-vacuum interface

ACS Spring 2022

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