Effects of geometric parameters and flow on microswimmer motion in circular channels

Micro swimming robots offer many advantages in biomedical applications, such as delivering potent drugs to specific locations in targeted tissues and organs with limited side effects, conducting surgical operations with minimal damage to healthy tissues, treatment of clogged arteries, and collecting biological samples for diagnostic purposes. Reliable navigation techniques for microswimmers need to be developed for navigation, positioning and localization of robots inside the human body in future biomedical applications. In order to develop simple models to estimate trajectories of magnetically actuated microswimmers blood vessels and other conduits, effects of the channel wall must be understood well. In this thesis, experimental and numerical model results are presented on swimming of microswimmers with a magnetic head and a helical tail in laminar flows inside circular channels filled with glycerol. Designed to mimic the swimming behavior of biological organisms at low Reynolds number flows, the microswimmers are manufactured utilizing a 3D printer and a small magnet and consist of a helical tail and a body that encapsulates the magnet. The swimming motion results from the synchronized rotation of the artificial swimmer with the rotating magnetic field induced by three electromagnetic-coil pairs. In order to obtain linear and angular velocities and to analyze the motion of the microswimmer, a computational model is developed to obtain solutions of quasi-steady Stokes equations, which govern the swimming of the microswimmers and the flow inside the channel. Experiments and numerical simulations are carried out for a number of cases with different geometric parameters and flow rates in the channel. Numerical simulation results agree well with experimentally measured velocities of the swimmer validating the experimental results. It is also presented a discussion on the influence of geometric parameters of the tail, such as wavelength, amplitude and length, and the direction of rotation of the swimmer on its trajectory based on the observed behavior in experiments and numerical solutions. Moreover, a computational fluid dynamics (CFD) model for swimming of microorganisms with a single helical flagellum in circular channels is presented. The CFD model is developed to obtain numerical solutions of Stokes equations in three dimensions, validated with experiments reported in literature and used to analyze the effects of geometric parameters, such as the helical radius, wavelength, radii of the channel and the tail and the tail length on forward and lateral swimming velocities, rotation rates and the efficiency of the swimmer. Optimal shapes for the speed and the power efficiency are reported. Effects of Brownian motion and electrostatic interactions are excluded to emphasize the role of hydrodynamic forces on lateral velocities and rotations on the trajectory of swimmers. For thin flagella, as the channel radius decreases, forward velocity and the power efficiency of the swimmer decreases as well; however, for thick flagella, there is an optimal radius of the channel that maximizes the velocity and the efficiency depending on other geometric parameters. Lateral motion of the swimmer is suppressed as the channel is constricted below a critical radius, for which the magnitude of the lateral velocity reaches a maximum. Results contribute significantly to the understanding of the swimming of bacteria in micro channels and capillary tubes