Trajectories of magnetically-actuated helical swimmers in cylindrical channels at low Reynolds numbers

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Official URL: https://dx.doi.org/10.1016/j.jfluidstructs.2019.06.005

Artificial microswimmers have a huge potential in various microfluidic and medical applications. Most of the swimming scenarios involve some kind of confinement such as arteries or microchannels for both natural and artificial swimmers. It is shown both experimentally and numerically that confinement influences the swimmer trajectories significantly at low Reynolds numbers. Magnetically-actuated helical microswimmers follow helical or straight trajectories depending on whether the tail is pushing or pulling the swimmer. To improve the understanding of this behavior, we use a direct numerical approach that couples a simple kinematic model with a computational fluid dynamics (CFD) model to solve steady Stokes equations for low Reynolds number swimming. The kinematic model updates the position and the orientation of the swimmer by using the linear and angular velocities from the CFD model at each instant. In addition to the viscous force, the external magnetic torque, the gravity force and the normal contact force on the swimmer are also considered in the CFD model. Simulations are used to study the effects of the Mason number, which is the ratio of viscous and magnetic torques, the diameter ratio of the swimmer and the channel, the tail length and the channel flow. In addition to improving the fundamental understanding, these results bear strong relevance to the development of control systems for microswimming robots.