Unraveling modular microswimmers: From self-assembly to ion-exchange-driven motors

Active systems contain self-propelled particles and can spontaneously self-organize into patterns making them attractive candidates for the self-assembly of smart soft materials. One key limitation of our present understanding of these materials hinges on the complexity of the microscopic mechanisms driving its components forward. Here, by combining experiments, analytical theory, and simulations we explore such a mechanism for a class of active system, modular microswimmers, which self-assemble from colloids and ion-exchange resins on charged substrates. Our results unveil the self-assembly processes and the working mechanism of the ion-exchange driven motors underlying modular microswimmers, which have so far been illusive, even qualitatively. We apply these motors to show that modular microswimmers can circumvent corners in complex environments and move uphill. Our work closes a central knowledge gap in modular microswimmers and provides a facile route to extract mechanical energy from ion-exchange processes.

Figure 1

Self-assembly and modular swimming mechanism (side view). (a) Advection: The resin (IEX) exchanges ${\text{K}}^{+}$ ions against more mobile ${\text{H}}^{+}$ ions leading to long-range ionic gradients evoking a spontaneous electric field E. This field acts on the charged fluid inside the substrate double layer inducing fluid flow towards the resin (bottom arrows, blue). (b) Exclusion zone: At large distances colloids (spheres with $-$ symbols) advect towards the resin, but slower than the surrounding solvent due to diffusiophoresis in the charge-neutral ionic gradients created by the resin ($v_a>v_p$). Close to the resin, incompressibility obliges the solvent to stream upwards (dashed arrow), reducing advection along the substrate. Thus, phoresis dominates at short distances so that the colloid settles at a distance where advection and phoresis balance ($v_a=v_p$) defining an “exclusion zone” (hatched rectangle). (c) Self-propulsion: The action of the chemical gradients on the counterions in the Debye layer of the charged and stalled colloid creates an osmotic flow towards the resin. The resin sees an enhanced solvent flow coming from the direction of the colloid leading to its advection.

Figure 2

Experiments: (a) colloids moving towards the IEX along a glass plate. (b) When approaching the resin, the colloids leave an exclusion zone (arrow) to the surface of the resin and form a self-propelling modular-swimmer. (c) Trajectory of a modular swimmer collecting colloids along its path. (d) Downhill swimming: gravity induces downhill swimming (gravitaxis). (e) Self-assembly of a modular swimmer on a tilted substrate. (f), (g) Microgrooves guide modular swimmers around corners (f) and uphill (g).

Figure 3

Simulated self-assembly dynamics of a modular swimmer based on Eqs. (3, 4, 5, 6, 7). As in our experiments, colloids (small spheres, black) advect towards the IEX (large spheres, red) and bind to it leaving an exclusion zone to the resin surface. The complex then starts self-propelling; further colloids bind to the resin, forming rows at the back of the resin, until at some point, colloids can no longer follow the moving resin (ellipse, green). Model parameters $b_c=20\mu {\text{m}}^2/\text{s};b_s=100\mu {\text{m}}^2/\text{s}$; $R_{\mathrm{IEX}}=22.5\mu \text{m},R_c=7.6\mu \text{m}$.

Figure 4

Swim speed in experiments (circles) and simulations (squares with lines) without (bottom symbols and line; black) and with gravity (other symbols and lines, colors); substrate tilt angle given in the key. Experimental values are averages over 60–90 complexes (standard deviation up to $0.2\mu \text{m}/\text{s}$, indicated by circle sizes). Parameters as in Fig. 3 and ${\tilde{m}}_{\mathrm{IEX}}=18.7$ (fitted for zero colloids attached) and ${\tilde{m}}_c=0.097$ (estimated based on mass and size ratio of colloids and IEX).