Polygonal micro-whirlpools induced in ferrofluids.

We report on the observation of the polygonal whirlpools in the thin layer of ferrofluid under illumination with a laser beam carrying optical vortex and in the presence of a vertical magnetic field. This kind of structures have attracted attention after discovering a hexagonal storm in Saturn's atmosphere. Our polygonal whirlpools were created in a closed system (no free surfaces) in micro-scale (whirlpool diameter <20 µm) by the use of holographic optical tweezers. The polygonal shape was changed by varying the magnetic field strength or value of the optical vortex topological charge. [ABSTRACT FROM AUTHOR]