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Oil drop rolls round and round

Oil drops

Researchers have been making oil drops move all by themselves (Image: iStockphoto)

A droplet of oil has been made to continuously orbit a glass petri dish by Japanese researchers who say their experiment will inspire work on molecular motors.

They say this is a unique case of transforming chemical energy directly into mechanical energy, something that current technology cannot do.

Yutaka Sumino and team from Kyoto University report their research in the 18 February issue of the journal Physical Review Letters.

The oil drop's movement is based on the attraction and repulsion forces between molecules.

The researchers put the oil drop in water containing a surfactant, a type of wetting agent with a positive charge at one end and a negative charge at the other.

The oil droplet was pulled along by its attraction to the charged tails of the surfactant, which formed a coating on the glass dish.

As the oil drop moved along, it mopped up the surfactant and left a trail of clean glass that repelled the oil droplet and propelled it forward.

Surfactant molecules floating around in the water restored the coating on the glass so the whole process could keep going and the oil droplet keep moving.

The researchers report that after some initial random movements, the drop started orbiting continuously around the dish.

They did the same experiment in a long thin container as wide as the drop itself and the drop simply paced back and forth from one end to the other.

After about 20 to 50 cycles the drop became saturated with surfactant and stopped moving.

Team member Dr Kenichi Yoshikawa thinks the discovery might help to develop nanomachines that would move through the human bloodstream to make repairs.

"Perhaps in the future you can make small chemical robots that walk in a narrow space by utilising energy from the environment, for example, glucose," says Yoshikawa.

But Australian researcher Professor Richard Sadus, director of the Centre for Molecular Simulation at Melbourne's Swinburne University of Technology, is not so sure how the experiment will lead to useful applications.

Sadus says there are several naturally occurring molecular motors, including the enzyme adenosine triphosphate synthase (ATPase).

In the natural environment, for example, ATPase converts the chemical energy of ATP to mechanical energy, which is used to rotate the molecule.

Some scientists have moved protein molecules around by attaching them to a rotating ATPase molecule.

But, says Sadus, the ultimate goal is to create a totally artificial molecular motor that can be used to purposefully propel artificial nanodevices.

A type of molecular motor?

Sadus says the oil drop experiment is not a molecular motor.

"It's just a controlled manipulation of oil droplets," he says. "I can't see of any way that you could attach anything onto these oil droplets."

He says another feature that enables molecular motors to do useful work is that they are anchored and he cannot see how it would be possible to anchor the oil drops.

But Sadus says the research is interesting because it is looking at a non-biological system whereas other researchers have tended to look only at biological systems.

Tags: biotechnology, engineering