Capillary-Assisted Printing of Droplets at a Solid-Like Liquid-Liquid Interface

TL;DR

This paper demonstrates a novel method for assembling and manipulating droplets at solid-like oil-water interfaces using capillary forces, nanoparticle surfactants, and plasmon-assisted optofluidics, enabling advanced material fabrication.

Contribution

It introduces a new approach to stabilize and control droplet assembly at liquid-liquid interfaces with nanoparticle surfactants and plasmonic techniques, expanding capabilities in material design.

Findings

Droplets attract each other over millimeter scales at solid-like interfaces.

Nanoparticle surfactants stabilize water droplets for extended periods.

Plasmon-assisted optofluidics enables temperature-controlled manipulation.

Abstract

Capillary forces guide the motion of biomolecular condensates, water-borne insects, and breakfast cereal. These surface-mediated interactions can be harnessed to build units into materials with exotic properties deriving from mesoscale structure. Droplets are promising building blocks for these materials, finding applications in tissue engineering, adaptive optics, and structural colour. However, the instability of water droplets at many liquid-liquid interfaces hampers the use of capillarity for the assembly of droplet-based materials. Here, we use nanoparticle surfactants to form solid-like oil-water interfaces at which aqueous droplets sit for extended periods. We find that microlitre-sized droplets at these interfaces attract each other over millimetric scales. We rationalize this interaction with a modified theory of capillarity. Applying printing methods allows us to finely control initial droplet positions, from which they self-assemble into cellular materials. Finally, by functionalising the interface with gold nanoparticles, we use plasmon-assisted optofluidics to manipulate these droplet-based materials with temperature gradients.