Locomotion at low Reynolds number is a key physical mechanism [1] for microscopic living organisms, but has lots of technological applications like nanomachines and microrobots [2]. Our last paper [3] proposes a tricky experiment for generating low Reynolds swimming of microscopic particles along a liquid-air interface.

First, identical soft ferromagnetic particles are placed on a liquid-air interface. Capillary attraction is prevented by applying a vertical magnetic field which provides a repulsive interaction between the particles. The balance of capillary attraction and magnetic repulsion creates a self-assembly [4] as shown in the front picture. These structures are then perturbed by applying an oscillating horizontal field. The resulting cooperative dance of the particles provides a net propulsion of the particles along the liquid surface (see trajectories on the picture below). The self-assembly is swimming ! See the movies below :

• Movie 1 - An efficient swimmer of N=3 beads. 
• Movie 2 - The horizontal field is switched off. The swimmer is seen to stop its motion. Then the field is switched on but in another direction. The motion resumes but in a different pulsating mode (with a different swimming speed).
• Movie 3 - The complex motion of a swimming (pentagonal) assembly made of N=6 beads.

This work opens new perspectives in mesoscopic physics. The most important feature of our system is that both self-assembling and periodic deformations can be rescaled to smaller sizes.

References

[1] E.Lauga, Soft Matter 7, 3060 (2011)
[2] P.Tierno, R.Golestanian, I.Pagonabarraga, and F.Sagués, Phys. Rev. Lett. 101, 218304 (2008)
[3] G.Lumay, N.Obara, F.Weyer, N.Vandewalle, Soft Matter 9, 2420 (2013) - PDF
[4] N.Vandewalle et al., Phys. Rev. E 85, 041402 (2012) - PDF - Post