Stabilitiy, control and acoustic manipulation of magnetically actuated helical swimmers

Official URL: https://risc01.sabanciuniv.edu/record=b2486338 _(Table of Contents)

Microswimmers are prospective agents for manipulation in fluid environments at low scales. Potential use cases include targeted drug delivery and microsurgery. Magnetized helical microswimmers are used extensively in the literature as they can be actuated externally with a rotating magnetic field. This dissertation reports on the modes of instability of magnetized helical swimmers and proposes several solutions to enable controlled navigation, which is crucial considering the potential biomedical applications. The modes of instability are characterized with a kinematic model that relies on snap-shot solutions of Stokes equations. Pusher-mode instability occurs in confined environments, resulting in helical trajectories. A novel, magnetic steering control algorithm is proposed to suppress the oscillatory trajectories. Contrary to the state-of-the-art, this method doesn’t require any orientation feedback and performs equally well. On top of magnetic steering, acoustic fields are demonstrated to be beneficial in reducing wobbling. The bio-compatible nature of acoustic fields makes it an ideal complement to the magnetic field. A novel and efficient computational model for the calculation of the acoustic radiation force on helices (which is costly otherwise) is presented where the helix is approximated as a chain of spheres for which simple analytical formulae exist. The sum of forces on spheres is very close to the force acting on the helix. The approach is utilized in simulating the trajectories of helical swimmers under acoustic and magnetic fields with promising results. In experiments, magnetic swimmers made from thin wires are placed under magnetic and acoustic fields. Viscosity reduces acoustic propulsion significantly