Speaker
Description
Hydrodynamic interactions (HIs), namely solvent-mediated long-range interactions between dispersed, microscopic particles, play a crucial role in the emergent dynamics of many active systems, from swimming bacteria to swarms of propelling microrobots. For example, HIs forces the bacteria E. Coli tend to swim close to a surface, where it performs a circular trajectory nearby. The attraction results from the pusher type force dipole of the bacteria, which creates a flow field able to force the bacteria towards the wall. Similarly, for a puller type swimmer, the generated flow field induces a repulsion from the wall. However, for pullers, the alignment parallel to the wall is not a stable configuration. Inspired by this effect, we design a microscale magnetic rotor able to produce a puller-like force dipole, but with a stable alignment parallel to the wall.
These S-shape lithographic particles are doped with nanoscale magnetic colloids and can be manipulated via external, time-dependent magnetic fields. Under a rotating, circularly polarized magnetic field, these propellers are subjected to a magnetic torque, and rotate exerting a force on the surrounding fluid. Because of the anisotropy of the drag coefficient on the elongated shape, similar to a pusher type bacterium, a pair of forces arise pointing towards the center of mass of the particle. Since the S-shape rotates above a wall, it experiences an upward lift force. We optimize the shape of the S particle to maximize the lift force. Since the lift force decreases with the distance from the wall, we calculate the equilibrium rotation height for different rotation frequencies. We find that an S-shaped particle with the cross-section radius of 3 microns and the length of 120 microns lifts to a height of around 100 microns when rotated at a frequency of 6 Hz in water-glycerol mixture with the viscosity 9.7 mPa s.
