  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.
