# Directional flows using capillary assembly of photo-deformable colloidal particles at water-air interfaces

## TL;DR

Researchers developed light-sensitive polymer particles that change shape with polarized light, creating controlled flows and structures at water-air interfaces.

## Contribution

The use of photo-deformable particles to dynamically control capillary interactions and generate sustained interfacial flows.

## Key findings

- Particles deformed by polarized light assemble and create interfacial flows up to 90 μm/s.
- Flow patterns can be designed independently of illumination intensity, such as shear flows along a single stripe.
- The method enables dynamic manipulation and assembly of soft matter at liquid interfaces.

## Abstract

Colloidal particles at liquid interfaces experience long-ranged capillary interactions, whose magnitude and directionality depend on the particle shapes. When particle shapes are determined by fabrication or synthesis, the resulting shape-mediated interactions are predefined and often lead to the formation of persistent interfacial structures. Here, we introduce polymer particles at water-air interfaces whose shape and, therefore, interactions can be altered by illumination with polarized light. Specifically, we selectively trigger capillary self-assembly by anisotropically deforming the particles at the interface. Intriguingly, further deformation of already assembled particles induces sustained interfacial flows with velocities of up to 90 μm/s. Benefitting from polarization-defined deformation directions, we create flow-patterns that do not simply follow the illumination intensity pattern, such as shear flows along a single rectangular illumination stripe. We anticipate that this interplay between photo-deformation and capillary interactions of particles will enable various forms of mixing, manipulation, and assembly of soft matter at liquid interfaces.

Colloidal particles experience capillary interactions at liquid interfaces, but modifying these interactions is challenging as shape change is required. Here, the authors report polymer particles that change shape with polarised light, and therefore create flow patterns with unusual paths.