Helical swimming is adopted by microswimming robots since it is an efficient mechanism and commonly observed among microorganisms swimming at low Reynolds numbers. However, manufacturing of micro-helices made of sub-micron magnetic thin layers is neither straightforward nor well-established, advanced materials and methods are necessary to obtain such structures as reported in the literature. In this paper, a topological patterning method utilizing basic microfabrication methods is presented for the self-assembly of magnetic micro-helices made of a sandwiched nickel thin film (50-150 nm) between two silicon nitride layers. Strain mismatch between the thin films and the geometric anisotropy introduced by the slanted patterns on the top nitride layer result in self-rolled-up helical microribbons. Moreover, inspired by the actual release process during the wet-etching of the microribbon from the substrate, moving boundary conditions are incorporated in a numerical model to simulate the self-rolling of trilayer ribbons. The simulation results are compared and validated by experimental data within 7% error for all cases, including the geometries that do not result in a helical shape. The swimming performance of the magnetized micro-helix is demonstrated inside a capillary glass tube experimentally and cross-validated with a numerical model.
Keywords: artificial microswimmers; finite-element modelling; microfabrication; self-scrolling.
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