Pan Gang and his team from the Chinese Academy of Sciences in Beijing are attempting to revive the lake Tai using nanobubbles. In 2006, a silver tea urn like container was connected via hoses to the oars of boat, which are themselves sprayer arms dousing the lake’s surface with a frothy slurry of nanobubbles. Since his first tests in 2006, his quaint boat with the stripy awning has been replaced by a sleeker, modern version that sprays clay out of deck-mounted water cannons.
Pan’s patented mechanism for solving this problem involves putting a suspension of lakeside clay in chilled water and saturating it with oxygen bubbles. All but the smallest bubbles float away, but microscopic imaging confirms the presence of oxygen bubbles just 10 nanometres in diameter in the clay. Spraying the resulting slurry on the lake’s surface pushes the polluting cyanobacterial blooms to the lake bottom within minutes. The chilled water warms up in the body of the lake, allowing larger oxygen bubbles to form at the interface between the clay and water. These bubbles break free and break down the algae, re-oxygenating the water. The process is energy efficient and non-polluting, involving only native soils from the lake’s own edge.
The results seem impressive. Experiments in a 50,000-square-metre area of the lake cleared the whole centimetre-thick algal bloom in half an hour. The following day, concentrations of ammonia, nitrates and phosphorus compounds in the lake water – products of the cyanobacterial metabolism and the source of the foul smell – had fallen dramatically. Four months later, underwater vegetation was growing prodigiously and plankton populations were thriving again (Ecological Engineering, vol 37, p 302). Pan says that researchers given the job of restoring lakes left behind after tar sand extraction in Canada have come calling, as have people looking to improve water quality in the Baltic Sea and the UK’s Lake District.
Problematic Physics of Nanobubbles
For bubbles that small to exist, the internal pressures would have to be around 100 atmospheres – the sort of pressure you would experience a kilometre down in the ocean. That sounds implausible, and yet in 2001, with the help of a scanning probe microscope, Attard and his colleague James Tyrrell spotted hemispherical nanoscale structures growing like mushroom caps on hydrophobic silicon surfaces immersed in water (Physical Review Letters, vol 87, p 176104). Subsequent spectroscopic measurements showed the structures were filled with gas. Nanobubbles, it seemed, did exist.
The nanobubbles have maybe a thousand molecules inside it, and are losing approximately 1 billion gas molecules per second.
How can that be? The researchers could only suggest that something must be recycling the molecules back into the bubble, perhaps at the join where the bubble wall meets the surface on which it sits (Physical Review Letters, vol 107, p 116101). Observing such a flow directly would require getting inside the bubble, which is impossible without destroying it. And Seddon readily admits there is a problem with the explanation: to counter the higher internal pressure of the bubble something needs to be pushing the gas back in, requiring an energy source that is not obviously present.
Wonder Drugs and other Applications
In “labs on a chip”, nanobubbles could also be attached to the walls of the tiny channels in which multiple reactions take place for genomic analysis or assessing candidate drugs, reducing friction and therefore cutting the energy needed to force the reacting chemicals through. But perhaps the most controversial claim for nanobubbles’ properties comes from a drugs company called Revalesio based in Tacoma, Washington. In May 2010, the US Food and Drug Administration gave approval for Revalesio to investigate the properties of a “novel anti-inflammatory product” that the company calls RNS60.
If nanobubbles do exist, then it is not too implausible that they might pop up in biological contexts, and perhaps also be harnessed to alter biochemistry.
RNS60, the firm says, contains “charge-stabilised nanostructures” generated by “controlled turbulence”. By pumping high-pressure oxygen into a saline solution flowing in a spiral pattern between two spinning coaxial cylinders, vortices are generated – and perhaps more. “The current research is showing that devices like ours produce nanobubbles,” says Richard Watson, Revalesio’s director of clinical science.
In March this year, researchers from Revalesio and Rush University in Chicago presented preliminary in-vitro studies at the annual conference of the American Society for Neurochemistry in Baltimore, Maryland, showing that RNS60 inhibits the production of chemicals that cause inflammation in glial cells, the support scaffolding for neurons. Last October, Revalesio published results of phase I clinical trials in which inhaling RNS60 in aerosol form “significantly” improved the amount of air some people with asthma can exhale. The implication is that it might make an alternative to steroid-based treatments.
That would be controversial, not least because it would require the existence of nanobubbles not just on surfaces, where most of the evidence so far has been found, but freely floating in a liquid. The company’s hypothesis is that since saline is composed of water and dissociated sodium and chlorine ions, those ions form a scaffold containing oxygen bubbles of around 50 nanometres in diameter.