Nanotechnologies are an increasingly attractive technology that offers much promise for water treatment.

Rotating microrobots have demonstrated the properties of the chemical surfactants commonly used in water treatment and cleaning applications to remove oils from water. As such, the technology may open the door to clean water without the use of expensive chemicals and their residues.

Developed by researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) and Heinrich-Heine-Universität Düsseldorf (HHU), the microbots have demonstrated similar properties to surfactant molecules, which help dissolve oils in water.

“We have a mixture of microbots, and his phase separation is a physical process which is very similar to how water and oil behave in the presence of surfactants."

In the case of the new microbot research, the scientists developed two types of bots that spin in opposite directions.

As these tiny machines rotate, they separate to form two cohesive groups. Like surfactants, these two groups form defined layers and, when linked to make chains are able to demonstrate similar properties.

“We have essentially a mixture of clockwise rotating and counter-clockwise rotating microbots, and they phase separate to left-handed and right-handed parts,” explains Prof. Dr Michael Engel, chair of Multiscale Simulation of Particulate Systems at FAU.

Challenges ahead

One of the lead scientists behind the microbot research, Engel said: “This phase separation is a physical process, which is very similar to how water and oil behave in the presence of surfactants.”

Although there are obvious applications in water treatment, a number of challenges remain to be overcome before any practical results are anticipated.

Currently, for example, the microrobots are produced using a 3D printer. Around a centimetre across and weighing in at about a gramme, the bots have demonstrated the basic principles of this approach.

Speaking to Aquatech Online, Dr Engel said: “It's on a large scale, so what we see is the physical phenomena that you have in surfactants that they are reproducing on this larger scale.

"The idea is not that you use centimetre-sized robots to work directly as surfactants, but it's a reproduction of the mechanism of surfactants and a non-equilibrium system."

Advancing nanotechnology for water

Despite the promising early results, the bots need to be a lot smaller before they can be deployed as an alternative to chemical surfactants.

"The idea is obviously not to use these 3D printed robots, but if you had a similar mechanism where you have active phase separation on a microscopic level and scale it down several orders of magnitude, then you can imagine that there would be an application in water treatment," says Engel.

“A similar mechanism where you have active phase separation on a microscopic level would be an application in water treatment.”

While there is obviously potential given that the bots could be scaled down to the nanoscale and therefore represents an interesting avenue for further research, nanobot surfactants are still some way off.

"We don't currently have active research considering scaling down this technology, but, of course, it's always a possibility. Our motivation was to look at the physical mechanism and to describe the process of such active phase separation," notes Engel.

“I think it's very hard to predict when a practical application may emerge. I would see this work in the greater scheme of research currently conducted in non-equilibrium systems where they use this non-equilibrium behaviour to drive new phase behaviour.”

Promise for the water treatment sector

Nonetheless, nanotechnologies clearly hold much promise for the water treatment sector for the removal of persistent micromaterials such as pesticides and heavy metals, as well as pathogens and pollutants like oil.

As alternatives to many existing chemical and physical treatment approaches, nanomaterials potentially have high reaction rates and large surface-to-mass ratios, offering attractive possibilities as catalysts and materials like carbon nanotube membranes, for example.

With further research, this microbot technology could be applied to reduce the need for detergents and surfactants to help remove oil pollution from water without chemicals.

And, while not be arriving any time soon, nanotechnologies will inevitably find their way into water treatment systems in the not too distant future.