# Tunable optical matter: electrostatic repulsion modulates near- and far-field gold nanoparticle arrangements

## TL;DR

This study shows how salt concentration affects the arrangement of gold nanoparticles through electrostatic repulsion in optical binding.

## Contribution

The paper demonstrates that electrostatic forces can be tuned to control optical binding configurations of gold nanoparticles.

## Key findings

- Reduced electrostatic interaction promotes near-field optical binding along the polarization direction.
- Low salt concentration induces far-field optical binding perpendicular to the polarization.
- A theoretical model confirms the dominant role of electrostatic forces in these configurations.

## Abstract

The dynamics and equilibrium configurations of immersed optically-bound particles are complex phenomena involving several physical mechanisms such as optical forces, electrostatic interactions, and fluid dynamics. In this work, we unravel, using experiments and numerical simulations, the key role played by short-range electrostatic forces. The repulsive interaction among gold nanoparticles is adjusted by changing the salt concentration. When the electrostatic interaction is reduced, near-field optical binding with particles oriented along the polarization direction is promoted, while, for low values of the salt concentration, inter-particle repulsion induces far-field (FF) optical binding configurations oriented perpendicular to the polarization. The importance of electrostatic force is confirmed by a theoretical model in which the repulsive effect is explicitly tuned. The numerical results reproduce the measured particle configurations and highlight the dominant role of electrostatic interactions, particularly in FF optical binding configurations.

Experiments and simulations reveal that salt-dependent electrostatic repulsion modulates near- and far-field optical binding of gold nanoparticles, with a simple model reproducing observed configurations and highlighting electrostatics' key role.