Fig. 5.

Rolling motion in acoustic and magnetic fields. a The 1–2 µm superparamagnetic particles self-assemble and form spinning aggregates rotating in the clockwise direction in the presence of a magnetic field. b The aggregate migrates towards the channel wall due to the radiation force of an acoustic field. c Image sequence demonstrates the rolling-type propulsion along the sidewall of the microfluidic channel (Supplementary Movie 4). d The superparamagnetic particles disassociate as soon as the magnetic field is turned off. Scale bar of b–d is 50 µm. e Image sequence demonstrates the counter-clockwise motion of an aggregate and executing rolling when reached the wall under a magnetic field strength and rotation frequency of 20 mT and 20 Hz, respectively, at an acoustic driving voltage of 20 V_PP (Supplementary Movie 6). Scale bar, 25 µm. f The plot of the rolling velocity against acoustic driving voltage at magnetic rotational frequencies (m _f) of 7.5, 15 and 22 Hz, respectively, at 15 mT, of 2.9 µm superparamagnetic particles. Each data point represents the average rolling velocity analysed from 3–5 swimmers except data points at 10 V_PP. The error bar represents the standard deviation (s.d.). g An aggregate is shown weak to no rolling at low-acoustic driving voltages of 10 V_PP under a magnetic field strength and rotation frequency of 15 mT and 7.5 Hz, respectively (Supplementary Movie 7). Scale bar, 25 µm