Modular Micro-Swimmers

Swimming at the micrometer scale is a fascinating and hot topic in condensed matter physics. The constraint of low Reynolds numbers, where inertial effects are absent requires completely new strategies to move objects. In biologic systems asymmetric organelles push or pull freely moving bacteria. In artificial systems, this is either mimicked or alternative propulsion mechanisms are sought based on thermo-phoresis, electro-phoresis or diffusio-phoresis (R. Kapral, J. Chem. Phys. 138, 020901 (2013); A. Sen, M. Ibele, Y. Honga, D. Velegol, Faraday Discuss. 143, 15–27 (2009)). Most experiments with artificial micro-swimmers are performed using individual swimming objects, some more recent are concerned with their collective behaviour. A very recent review highlights several approaches taken in this rapidly evolving and expanding field (Sen, Nano Today 8, 531-554 (2013)).

Our approach differs from these main stream approaches by addressing modular micro-swimmers, composed of different parts taking different functionalities. The underlying mechanism is of electro-osmotic nature and has already been exploited to assemble autonomous micro-swimmers of different complexity as well as stationary aggregates (A. Reinmüller, H. J. Schöpe, T. Palberg, Langmuir 29, 1738-1742 (2013); A. Reinmüller, E. Oguz, R. Messina, H. Löwen, H. J. Schöpe and T. Palberg, J. Chem. Phys. 136, 164505 (2012)).
We are interested in the interplay between fuel reservoir particles (RP), substrate surface charges generating the driving flow, additional particles acting as gearing or steering, and cargo particles to be collected and released. To illustrate the flexibility of our modular approach, we show some examples and highlights below and some videos here.

Microswimmer examples
Fig. 1: Demonstration of different experimental situations in modular micro-swimming. All images taken at the same magnification, a 100µm scale bar is shown in f). a) Minimal phoretic drifter: single, electro-osmotically propelled HCl-releasing reservoir particle (RP), drifting on a negatively charged quartz substrate (speed vMAX ≈ 0.5µm/s). b) school of similarly sized RPs creating a combined roughly circular symmetric flow field coupling them to a long time stable, stationary ensemble (vMAX ≈ 0µm/s, time averaged school diameter d ≈ 130µm); c) transition to directed propulsion after a school has met a single significantly larger ERP and the flow field changed its symmetry (vMAX ≈ 3µm/s; d  190µm); d) left: minimal swimming complex: spindle type RP with a single, large colloidal particle coupled by the flow field and propelling in slightly curved trajectories (vMAX ≈ 12µm/s). The second colloidal sphere at its back is a firmly attached cargo particle, right: the same RP coupled to a crystalline ensemble of several mono-sized large colloids (vMAX ≈ 3µm/s). The resulting trajectory is very straight. e) Characteristic trail formed under conditions of larger colloid concentration. Due to restricted storage capacity of the convective flow, surplus NAPs are left behind allowing accurate tracing of the (again straight) complex´ path. f) Smaller colloids (2a=2.6µm) assemble in the RP back, but also leave the substrate plane and form a convection cell, seen by the blurred, defocused particles around the ERP. Upon encounter of stationary complexes, a scattering event occurred.
Shaped ion exchange resins
Fig. 2: Different shapes of well defined RPs cut from a mm-sized Ion Exchange resin bead by Microtom and Focused Ion Beam in a collaboration with the MPI-P (Image courtesy M. Müller).

Within the SPP 1726 we persue a project aiming at optimized performance based on a thorough characterization and mechanistic understanding. There we will utilize a recently developed optical cell (A. Reinmüller, H. J. Schöpe, T. Palberg, Rev. Sci. Instrum. 84, 063907 (2013)) and our long standing expertise on electro-kinetic phenomena (T. Palberg, T. Köller, B. Sieber, H. Schweinfurth, H. Reiber, and G. Nägele. J. Phys.: Condens. Matter 24, 464109 (19p) (2012)). In 2014 we will further submit applications for two subprojects within the local SFB on “Molecularly controlled Non-Equilibrium”. Several Master and Batchelor thesis topics are waiting for interested students. One Post-doc and three PhD-positions are available within these projects, the MAINZ graduate school and the MPGC.