Magnetocapillary microrobots: hunting, harvesting and transporting objects at fluid interfaces

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A tremendous amount of research was invested recently in the fundamental understanding of biological and artificial microswimmers which has led to a remarkable improvement of our knowledge about the behaviour of biological swimmers such as bacteria or sperm, or artificial systems based for example on colloids which can actively propel due to chemical or thermal gradients.In this project we aim at a fundamental understanding of an alternative class of artificial swimmers which was recently realised experimentally by the group of N. Vandewalle in Liege, Belgium. An assembly of paramagnetic particles trapped at a fluid-fluid interface can be used to create a "magnetocapillary swimmer'': Due to the competition of attractive capillary forces stemming from a deformation of the liquid interface by the gravitation force and repulsive magnetic dipolar interactions in case of an applied static magnetic field, a stable particle assembly emerges. By modulating the external static magnetic field via a smaller linearly-polarized magnetic field, the local equilibirum becomes distorted resulting in a directed motion of the entire particle assembly. We will combine simulations and analytical treatment to study assemblies of paramagnetic particles trapped at a fluid-fluid interface. Our lattice Boltzmann simulations will be based on an already existing solver that can handle two fluids with a well defined surface tension, as well as suspended particles and magnetic interactions. The analytical treatment will be based on the Najafi-Golestanian three-sphere model extended by capillary-, interface- and magnetic forces. A particular attention will be devoted to the influence of the particle shape, size and number, the scalability of the problem between micro- and nano- ranges as well as the design of the magnetic fields on the properties of swimmers. We will explore the controlled swimmer motion for hunting, harvesting and transporting of cargo particles. This opens potential applications of the magneticapillary swimmers as microrobots e.g. for the precise and controllable cleaning of interfaces or the transport and deposition of specific objects. At last we will focus on the interaction of multiple magnetocapillary swimmers: We have already shown that a swimmer reaches its maximum speed at a well defined resonance frequency of the external magentic field. The specific value of this frequency depends not only on the fluid- but also on the particle properties. By varying the size of the particles comprising different swimmers, we will be able to create swarms of microrobots with very complex interactions,but precisely tunable individual motion.