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Soft matter and liquids

Soft matter and liquids

How to make liquid drops with rings like Saturn

21 Jul 2017 Hamish Johnston
Liquid planet: Saturn-like droplets

A droplet of oil can be transformed into a Saturn-like ringed structure by placing it in a strong electric field – according to Quentin Brosseau and Petia Vlahovska, who did their experiments at Brown University in the US.

The fantastical effect is driven by a process called electrodynamic flow, whereby an external electric field causes the movement of electric charges at the surface of a liquid drop. In turn, this motion causes the liquid within the drop to circulate in cells – and this can distort the shape of the drop.

Brosseau and Vlahovska studied droplets of silicone oil suspended in castor oil and exposed to an electric field. Exactly how the drops distort is a function of the electrical properties of the two liquids, and this was adjusted by doping the castor oil with organic electrolytes.

Concentric rings

In one experiment, the researchers were able to flatten a millimetre-sized drop to create a lens-like disc with a relatively sharp edge. The edge is unstable, and a thin sheet of liquid begins to flow radially away from the edge. As the sheet flows outward, it breaks up into a set of concentric rings. Then, the rings themselves break up into a plethora of droplets, each about 10 μm in size. If the electric field is switched off before the process is complete, the large drop will become spherical again and the system will resemble the planet Saturn (see figure).

Brosseau and Vlahvoska found that this droplet-shedding process lasted for a few tens of seconds before the original drop was transformed into thousands of uniform droplets. Writing in Physical Review Letters, the researchers say that the phenomenon could be used for the large-scale production of tiny droplets of uniform size – something that could find a range of industrial and medical applications.

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