Tomographic PIV Measurement of a Magnetically Actuated, Bio-Inspired, Four-Legged Soft Robot Swimmer

The unsteady hydrodynamics of an untethered, centimeter-scale soft robot executing breaststroke-like swimming were investigated using the tomographic particle image velocimetry (tomo-PIV) technique. The robot was fabricated with four axisymmetrically arranged legs. Proximal leg segments were cast from a magnetically polarized PDMS-based composite, while distal segments and a central hub were molded from non-magnetic elastomer. In total, nine mechanically continuous material blocks were created. A square, alternating, 96 V in amplitude signal operating at 5 Hz induces a magnetic field via a pair of Helmholtz coils using a lithium polymer battery pack. The alternating magnetic field, with a peak amplitude of ±35 mT, induces synced, periodic flexion–extension of the legs. Volumetric flow fields around the upward-swimming robot were recorded at 250 frames per second. Kinematics of the leg-tip trajectories, flexion angles, and tip velocities were extracted from the coordinated images. Three-dimensional distributions of velocity, vorticity, and viscous dissipation rate were calculated from the volumetric flow measurements. During the power stroke, a coherent starting jet is observed beneath the swimmer. Following this, four azimuthally spaced vortex rings form and shed from the leg tips. The jet impulse and vortex circulation were compared with those reported for various jellyfish and breaststroke-swimming zooplankton (Daphnia magna), respectively. The co-occurrence of a central starting jet and leg-tip vortex rings suggests that both body and appendage motions contribute to thrust generation.