Like little rings of Saturn: How electricity pulls a drop of liquid apart
What happens when you put a drop of liquid in an electric field? That’s no idle inquiry to occupy boffins on their lunch break. The outcome has very real implications for a range of important applications, from nanotechnology to spacecraft design. It’s a question that has been open since at least the 16th century, when the English naturalist William Gilbert noticed that a piece of charged amber could make a water droplet change its shape. Some of the answers are today used everywhere from nanotechnology to spacecraft.
Until recently, scientists thought they had the question more or less sorted: depending on a few variables, the drop might change its shape or spin around or, most dramatically it might spray electrically charged droplets out the ends.
Now, however, a pair of American researchers have achieved a surprising new result – one that pulls a liquid drop apart, making it extrude tiny rings from its equator that in turn break up into micro-droplets. The formation of these “Saturn rings” has never been seen before. “This might seem like a rather intuitive way [for a drop] to decay,” says one of the researchers, Quentin Brosseau, “but this is to our knowledge the only occurrence.”
Since the 1960s, scientists have known that, when a drop of an electrically conductive liquid is placed in a strong enough electric field, the drop develops pointy tips and spray out jets of tiny electrically-charged droplets.
This “electrohydrodynamic tip streaming” happens to raindrops in electrical storms, for instance, and has found applications in devices from air purifiers to low-thrust, high-efficiency rocket thrusters used to manoeuvre satellites.
Tip streaming occurs when the fluid drop is suspended in some other fluid that is a worse conductor of electricity. Brosseau and Vlahovska wanted to see what happened if they put the fluid drop in a bath of something that conducted electricity better than the droplet.
To find out, they put drops of silicone oil in a bath of castor oil and applied a strong electric field.
They thought they would most likely see the drop start spinning around, in a well-known effect of electric fields called Quincke rotation. This was, at first, what they did indeed see; but then they added electrolytes to the castor oil to make it an even better conductor of electricity, and were astonished to see tiny rings bubble out.
What was happening was the tip-streaming process turned inside out: rather than fluid streaming to the poles, forming points and jetting off, fluid instead streamed to the equator, the drop flattening into a lens shape, with fluid flying out in circles like Saturn’s rings. As these rings moved away from the drop, they broke up into tiny droplets a few tens of micrometres across.
The micro-droplets are very even in size, and the size can be fine-tuned by adjusting the viscosity of the liquids. The micro-droplets are also electrically neutral, which means the technique could lend itself to bulk production of micro-droplets for an “electroemulsification” process.
This would be of interest for “any industry where micron-sized particles of uniform size are needed,” adds Vlahovska. “Pharmaceuticals, where you need to have pills with the same dosage. Or photonics – in colloidal crystals, for example opals.” Microfluidic techniques are currently the state of the art to make such tiny, uniform particles, but electroemulsification could be quicker.
Useful applications aside, the researchers say there is still a lot of work to do to understand exactly why the ring streaming happens in such a neat, symmetric way. “Electroemulsification might be for now the only obvious application,” Brosseau says. “But there are probably a few fundamental notions to be learned from this effect.”