Utilizing active suspensions: Creating micro-machines and electric circuits
Theoretical Condensed Matter Physics
Final Report Abstract
Within this project several new realizations as well as applications for active particles have been discovered and studied. Herefore the numeric calculations of active suspension have been performed using graphical units, which allows it to reduce the time efficiency of the simulations compared to conventional simulations. The most important result could be obtained by considering a new class of active particles – ferromagnetic rollers. Here micron sized spherical ferromagnetic Nickel particles have been exposed to an oscillating external magnetic field. The torque induced by the field on the magnetic particles leads to an rotation and therefore generates a rolling sphere if the particle in in contact with a surface. The study of the collective motion of such a system has been selected as a Science Highlight by the Office of Science of the US Department of Energy and has been published in Science Advances. These particles show the same dynamic patters, such as the formation of flocks, that are known for living organisms. Furthermore we could connect the onset of flocking with the synchronization of the ferromagnetic rollers. Confined in an harmonic potential ferromagnetic rollers self-assemble into a vortex whose steady rotation in one direction allows to stir the fluid that surrounds the rollers. Furthermore we took the first step to study the possibility to use an active suspension as ink for printing purposes. Such an active ink has in contrast to passive ink the advantage that the submersed swimmers effectively reduce the viscosity of the ink which would make flushing the ink out of a nozzle much easier. So far we could show using theory and simulations what the optimal geometry for both the used nozzle and active particles would be. During the study we could also show that rhotaxis is possible in a converging flow war away from any walls and the high shear regime. Beyond all that the behaviour of swimming magnetic particles has been studied in simulations. In contrast to rolling magnetic particles it is possible for swimmers to maintain the clusters that selfassemble in case of passive particles. However these clusters remain stable just for small selfpropulsion velocities and large velocities destabilize these structures. As a result various fission events can be observed which depend on the hydrodynamic characteristic of the individual swimmers. Within this project the concepts of fission and fusion of particle clusters, well known various systems on various length scales from subnuclear to astronomic - has been studied for the first time in an active system on the micronscale. Moreover it could been shown that even system that are frequently studied in the field of complex and dusty plasmas can show an emerging activity and behaves as an active fluid.
Publications
- Emerging activity in bilayered dispersions with wake-mediated interactions. Journal of Chemical Physics 144, 224901 (2016)
Jörg Bartnick, Andreas Kaiser, Hartmut Löwen, Alexei V. Ivlev
(See online at https://doi.org/10.1063/1.4953225) - Fission and fusion scenarios for magnetic microswimmer clusters. Nature Communications 7, 13519 (2016)
Francisca Guzmán-Lastra, Andreas Kaiser, Hartmut Löwen
(See online at https://doi.org/10.1038/ncomms13519) - Flocking ferromagnetic particles. Science Advances 3, e1601469 (2017)
Andreas Kaiser, Alexey Snezhko, Igor S. Aranson
(See online at https://doi.org/10.1126/sciadv.1601469) - Focusing of Active Particles in a Converging Flow. New Journal of Physics 19, 115005 (2017)
Mykhailo Potomkin, Andreas Kaiser, Leonid Berlyand, Igor S. Aranson
(See online at https://doi.org/10.1088/1367-2630/aa94fd)
