Structured self-assembly of like-charged bridging nanoparticles driven by varying separation between two interfaces

In this work, we observed the formation of rings and star-shape clusters formed from like-charged silica nanoparticles bridging a planar aqueous liquid-organic liquid interface and water-in-oil emulsion droplet. It is hypothesized that the confined space between the two liquid-liquid interfaces provides a template for the structured self-assembly of nanoparticles.

The real-time formation of tightly packed rings and star-shape clusters of bridging nanoparticles between two interfaces is observed using wide-field fluorescence microscopy without requiring additives or external magnetic/electric fields. A model considering the interfacial adsorption energy, and pairwise attractive capillary and repulsive electrostatic interactions between bridging nanoparticles is developed. Both the experimental and simulation data for the inter-nanoparticle separation (δ) with respect to distance between the interfaces (i.e., thickness of the confined space, d) are compared to gain insights into the mechanisms driving the formation of the rings and corresponding cluster arms.

The rings containing bridging nanoparticles in close proximity arise due to Young's law and suppression of repulsive forces by dielectric screening from the emulsion. Beyond the ring, the curvature of the emulsion leads to a continuous increase in both d and surface charges exposed to the continuous phase; resulting in greater inter-nanoparticle repulsion and δ in the arms. The d-dependant interaction potential provides a means for continuously tuneable pairwise interaction via controlled thickness of the confined space between interfaces.