Confined swimming of bio-inspired microrobots in rectangular channels.

Controlled swimming of bio-inspired microrobots in confined spaces needs to be understood well for potential use in medical applications in conduits and vessels inside the body. In this study, experimental and computational studies are performed for analysis of swimming modes of a bio-inspired microrobot in rectangular channels at low Reynolds number. Experiments are performed on smooth and rough surfaces using a magnetic helical swimmer (MHS), having 0.5 mm diameter and 2 mm length, with left-handed helical tail and radially polarized magnetic head within rotating magnetic field obtained by two electromagnetic coil pairs. Experiments indicate three motion modes of the MHS with respect to the rotation frequency: (i) lateral motion under the effect of a perpendicular force such as gravity and the surface traction at low frequencies, (ii) lateral motion under the effect of fluid forces and gravity at transition frequencies, and (iii) circular motion under the effect of fluid forces at high frequencies. Observed modes of motion for the MHS are investigated with computational fluid dynamics simulations by calculating translational and angular velocities and studying the induced flow fields for different radial positions inside the channel. Results indicate the importance of rotation frequency, surface roughness and flow field on the swimming modes and behaviour of the MHS inside the rectangular channel.