Assembly and speed control in ion exchange based modular phoretic micro-swimmers

TL;DR

This study experimentally investigates ion-exchange based modular micro-swimmers, demonstrating how assembly and speed are influenced by various factors, and proposing a geometric model to explain the self-assembly process.

Contribution

It provides the first detailed experimental analysis of assembly and speed control in ion-exchange micro-swimmers, introducing a geometric model for their self-assembly.

Findings

Micro-swimmers achieve speeds of several microns per second.

Assembly shape develops regularly with increasing cargo.

Speed increases stepwise with cargo number and saturates.

Abstract

We report an experimental study on ion-exchange based modular micro-swimmers in low-salt water. Cationic ion-exchange particles and passive cargo particles assemble into self-propelling complexes, showing self-propulsion at speeds of several microns per second over extended distances and times. We quantify the assembly and speed of the complexes for different combinations of ion exchange particles and cargo particles, substrate types, salt types and concentrations, and cell geometries. Irrespective of experimental boundary conditions, we observe a regular development of the assembly shape with increasing number of cargo. Moreover, the swimming speed increases stepwise upon increasing the number of cargo and then saturates at a maximum speed, indicating an active role of cargo in modular swimming. We propose a geometric model of self-assembly to describe the experimental observations in a qualitative way. Our study also provides some constraints for future theoretical modelling and simulation.