Frontier Research Institute for Interdisciplinary Sciences
Tohoku University

Structural colored pigments have potential to replace existing chemicals in paint to create a sustainable, non-fading paint. Such structural color pigments can be created by assembling inexpensive materials such as silica and polymer nanoparticles (NPs) within emulsion droplets. The obtained structures depend on the droplet size, number of NPs, and the rate of droplet shrinking. Microfluidics can be employed to produce monodisperse droplets in laminar flow conditions. However, controlled shrinking of droplets that contain NPs to make ordered spherical assemblies (supraparticles) is often ignored within microfluidic devices and performed afterwards in bulk, leading to variations in structures.

In this research, we will design a microfluidic device that enables on-chip supraparticle formation via osmotic water extraction from droplets containing nanoparticles. To achieve this, a device has to be created that produces both aqueous salt droplets and droplets that contain nanoparticles in water. The droplets with salt have a large osmotic pressure. When they are in close proximity to the particle droplets, water flows through the oil phase that separates them (a semi-permeable membrane) from the particle droplets to the salt droplets via osmosis. The droplets containing particles shrink, leading to an organized cluster of nanoparticles with special reflective properties.

Supraparticles with the same size and structure are expected to be used in paints, but also reflective displays, composite materials, and catalysis. In addition, we foresee that this new microfluidic device can be used to create controlled ‘nanoparticle cocktails’, where many different nanoparticles may be combined for synergistic effects.