# Direct Assembly of Magnetic Janus Particles at a Droplet Interface

## TL;DR

This paper demonstrates a method to assemble magnetic Janus particles at droplet interfaces using external magnetic fields, enabling tunable and reconfigurable nanostructures for advanced material applications.

## Contribution

It introduces a liquid interface assisted approach for controlled assembly of magnetic Janus particles with a developed energy model explaining the process.

## Key findings

- Magnetic Janus particles can be directed to specific locations on droplet interfaces.
- Reconfigurable hexagonal lattice structures are achievable with magnetic fields.
- Magnetic fields influence particle deposition patterns during droplet evaporation.

## Abstract

Self-assembly of nanoparticles at fluid-fluid interfaces is a promising route to fabricate functional materials from the bottom-up. However, directing and controlling particles into highly tunable and predictable structures -- while essential -- is a challenge. We present a liquid interface assisted approach to fabricate nanoparticle structures with tunable properties. To demonstrate its feasibility, we study magnetic Janus particles adsorbed at the interface of a spherical droplet placed on a substrate. With an external magnetic field turned on, a single particle moves to the location where its position vector relative to the droplet centre is parallel to the direction of the applied field. Multiple magnetic Janus particles arrange into reconfigurable hexagonal lattice structures and can be directed to assemble at desirable locations on the droplet interface by simply varying the magnetic field direction. We develop an interface energy model to explain our observations, finding excellent agreement. Finally, we demonstrate that the external magnetic field allows \revisedtext{one} to tune the particle deposition pattern obtained when the droplet evaporates. Our results have implications for the fabrication of varied nanostructures on substrates for use in nanodevices, organic electronics, or advanced display, printing and coating applications.