Assembly and Speed in Ion-Exchange-Based Modular Phoretic Microswimmers

Abstract

We report an experimental study on ion-exchange-based modular microswimmers in low-salt water. Cationic ion-exchange particles and passive cargo particles assemble into self-propelling complexes, showing self-propulsion at speeds of several micrometers 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 the 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 the 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 modeling and simulation.

Controlled assembly of single colloidal crystals using electro-osmotic micro-pumps

“…Data shown in Fig. 8 are averages over some 120-150 colloid particles of PS4 and PS10 assembled at different splinters of average diameters of (40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50) µm. We also show data for PS2, which does not form a ring.…”

“…It should break down for moderate and large flows, where solvent backflow and the 3D advection of cargo may not be neglected any longer and cargo is assembled close to the IEX [40,50] or even lifted above the substrate [27-36, 41, 45]. Measurements of the assembly radius of a given colloid species around a given IEX type on different substrates and of its size dependence for differently sized colloids can check these assumptions experimentally.…”

“…For charged suspensions, the latter can be well described within the Debye-Hückel theory [50] yielding the following expressions…”

Large scale micro-photometry for high resolution pH-characterization during electro-osmotic pumping and modular micro-swimming

“…Characterizations most often employ optical methods. Swimmer position, orientation, speed and acceleration are conveniently obtained from optical microscopy [1,3,30]. Holographic microscopy in addition was used for three-dimensional imaging of swimmer motion and orientation, as well as mapping of 3D solvent flow fields using buoyancy matched tracers [31,32].…”

Spontaneous membrane formation and self-encapsulation of active rods in an inhomogeneous motility field

“…One feasible realization consists of colloidal Janus rods driven by light [49,50], which can be exposed to almost arbitrary motility landscapes [13]. Similar possibilities exist for self-propelled droplets [11] or modular microswimmers steered by an electrolyte gradient [56]. Moreover, rod-like bacteria at high concentrations [37,53] may serve as another model system to observe smectic ordering in motility landscapes.…”