Assembly and Actuation of Magnetic Polymers and Membranes by Precessing Magnetic Fields
Abstract: Magnetic colloids are used in drug delivery, tissue scaffolding, image contrast, and tumor reduction applications as well as self-healing membranes. Flexible magnetic filaments can be synthesized by joining superparamagnetic beads with elastic linkers, giving rise to interesting phenomena due to the combinations of their elastic and magnetic properties, which have found diverse applications, such as micro-mechanical sensors and self-propelled swimmers. Flexible superparamagnetic filaments under the influence of fast precessing magnetic fields are studied here using simulations and a continuum approximation analysis. We find that individual filaments can be made to exert controllable tensile forces along the precession axis. These forces are exploited for microscopic actuation. In bulk, the precession frequency affects filament aggregation and conformation by changing the net torques on the filament ends. Using a time-dependent precession angle allows considerable freedom in choosing properties for filament aggregates. Open and closed membranes composed of linked paramagnetic beads are also studied via analytical and numerical methods. We characterize these shapes in terms of the area and material parameters of the membrane, as well as of the strength and precession angle of the magnetic field. In particular, we show how depending on the precession angle open membranes may form either rippled or helicoidal surfaces, whereas closed membranes may elongate or flatten. These membranes might be suitable for actuation and for constructing devices with controllable conformational changes such as artificial muscles.
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