# Dynamic self-assembly of charged colloidal strings and walls in simple fluid flows

## TL;DR

This study demonstrates how charged colloidal particles self-assemble into 1D strings and 2D walls in fluid flows, revealing the balance of hydrodynamic and electrostatic forces through experiments and modeling.

## Contribution

It introduces a simple microfluidic method to observe and analyze non-equilibrium self-assembly of colloids into ordered structures, including the formation of strings and walls.

## Key findings

- Colloidal strings form near solid boundaries under flow conditions.
- Hydrodynamic attraction and electrostatic repulsion balance to create stable structures.
- Electric fields can induce formation of 2D colloidal walls.

## Abstract

Colloidal particles can self-assemble into various ordered structures in fluid flows that have potential applications in biomedicine, materials synthesis and encryption. These dynamic processes are also of fundamental interest for probing the general principles of self-assembly in non-equilibrium conditions. Here, we report a simple microfluidic experiment, where charged colloidal particles self-assemble into flow-aligned 1D strings with regular particle spacing near a solid boundary. Using high-speed confocal microscopy, we systematically investigate the influence of flow rates, electrostatics and particle polydispersity on the observed string structures. By studying the detailed dynamics of stable flow-driven particle pairs, we quantitatively characterize interparticle interactions. Based on the results, we construct a simple model that explains the intriguing non-equilibrium self-assembly process. Our study shows that the colloidal strings arise from a delicate balance between attractive hydrodynamic coupling and repulsive electrostatic interaction between particles. Finally, we demonstrate that, with the assistance of transverse electric fields, a similar mechanism also leads to the formation of 2D colloidal walls.