Non-ideal swimming of artificial bacterial flagella near a surface

The artificial bacterial flagellum (ABF), a helical swimming microrobot, has the potential to be used for biomedical applications such as cellular and intracellular manipulation. The velocity and the propulsive force of the ABF can be controlled by the input frequency of the rotating magnetic field. In this paper the swimming behavior of the ABF near a solid surface is reported. Three regions have been observed for the frequency-dependent swimming behavior of the ABF, i.e. the step-out, the linear and the drift-dominated region. At low frequencies it has been found that the desired screw-type motion is replaced by a wobbling swimming movement with a frequency-dependent precession angle. Moreover, the experimental results show that the wobbling motion of the ABF enhances the undesired sidewise drift due to wall effects. Additionally, the cause of the precession motion has been investigated by a hydrodynamic model. Our results imply that the linear range of the input magnetic frequency and the output ABF velocity is not only limited by the applicable torque at high frequencies but also by the wobbling of helical swimming at low frequencies.