FROM a distance, it is an idyllic scene. The small boat floats idly, its oars aloft. Sheltered from the sun by a blue-and-white striped awning, passengers busy themselves around a large silver tea-urn.
Move a little closer, though, and things don鈥檛 look 鈥� or indeed smell 鈥� so rosy. The waters of the lake are fluorescent green, and the stench of decay wafts from its surface. Pleasure trips have been a rarity on Lake Tai since 2007, when the Chinese government declared the lake, some 100 kilometres west of Shanghai, a disaster area. A year of unusual drought following decades of unchecked pollution .
These are no pleasure-seekers on the boat. Nor is the urn for making tea. On closer inspection it is connected via hoses to the oars, which are themselves sprayer arms dousing the lake鈥檚 surface with a frothy slurry. and his team from the Chinese Academy of Sciences in Beijing are attempting to revive the lake.
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That was in 2007. Just a few years earlier, they would have been told they were on a fool鈥檚 errand. Quite apart from anything else, basic physics said that the 鈥渘anobubbles鈥� at the heart of Pan鈥檚 magic brew simply don鈥檛 exist. Yet he and others scattered around the world are proving something very different. Not only are nanobubbles there, they seem to be everywhere. Basic physics was wrong.
Consider this next time you are busy blowing bubbles: each of these pockets of air represents a significant, if fleeting, victory against the forces of physical correctness. The spherical film of liquid surrounding a soap bubble, for example, is sustained only because gas pressure inside the bubble is higher than on the outside, so the gas pushes out against the surface tension of the liquid molecules. As gas gradually leaks out through a bubble鈥檚 thin, porous walls, this excess pressure is gradually reduced, and when it drops enough, the bubble bursts. That higher internal pressure also does for bubbles in a glass of fizzy drink when they reach the top of the liquid and meet the lower pressure of the surrounding air. Bubbles, sadly, just aren鈥檛 built to last (see diagram).
That鈥檚 especially true of small bubbles. The smaller a bubble is, the more tightly curved its skin is and the more concentrated the inward force that the pressure has to counterbalance. Below a certain size, which depends on the liquid enveloping the bubble and other factors, the internal pressure needed to counter that surface tension simply becomes too great. Would-be nanobubbles collapse even before they can form.
Or do they? An inkling that things aren鈥檛 that straightforward came in 1994, when John Parker at the Institute for Surface Chemistry in Stockholm, Sweden, was measuring the repulsion between two water-resistant surfaces immersed in water. As Parker forced them together, the repulsive force between them first increased as expected. Then, at a distance of a few hundred nanometres, it suddenly dropped off.
It was a few years before , then at the University of South Australia in Mawson Lakes, provided a semi-plausible explanation. If the surfaces were populated by nanoscale bubbles, he suggested, these would join forces to minimise their surface tension as the surfaces neared each other, just as two soap bubbles blown in air merge. This effect would draw together surfaces that would otherwise repel each other.
For bubbles that small to exist, the internal pressures would have to be around 100 atmospheres 鈥� 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 (). Subsequent spectroscopic measurements showed the structures were filled with gas. Nanobubbles, it seemed, did exist.
No one can say how. Perhaps in some situations the presence of contaminants might create particularly strong bubble walls able to counteract high pressures. Or perhaps imperfections on the surfaces the bubbles cling to could somehow change the dynamics of the situation. So far none of the proposed explanations quite has the ring of truth. 鈥淭here are now a few lines of argument as to why nanobubbles exist,鈥� says , who researches the phenomenon at the Australian National University in Canberra. 鈥淎nd I have a problem with all of them.鈥�
Things got even stranger last year when and of the University of Twente in the Netherlands used an atomic force microscope to took more closely at the structures. If they were filled with gas, the pressures inside should have been forcing molecules out through the walls at a rate of knots. And indeed they were: a fast flow of molecules from the bubble鈥檚 apex could be felt pressing against the microscope鈥檚 probing tip. 鈥淭he bubble has maybe a thousand molecules inside it, and it鈥檚 losing approximately 1 billion gas molecules per second,鈥� says Seddon.
鈥淭he bubble has maybe 1000 gas molecules in it, yet it鈥檚 losing about a billion molecules per second鈥�
Awkward existence
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 (). 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. 鈥淭here are laws of thermodynamics that you鈥檙e not allowed to break,鈥� Seddon says.
And yet nanobubbles persist 鈥� and not just fleetingly. 鈥淭hey last for as long as you want to look at them for,鈥� says Seddon. So far, five days is the longest anyone鈥檚 bothered to wait. In June, the journal ChemPhysChem published a . There seemed little doubt that nanobubbles do in fact exist, said, but questions about how they form and stay stable remain 鈥渁wkward鈥�.
On his lake in China, Pan has no time for awkwardness. He is deputy chief scientist of a huge project funded by the Chinese government to clean up Lake Tai. 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.
He has a simple plan to revive waters that have been deprived of oxygen by bacteria decaying on their surface: pump the lake full of oxygen again. Conventional ways of doing this are extremely inefficient. 鈥淚t takes lots of energy to pump gas into the water and most of it will escape into the air,鈥� says Pan. This problem becomes even more acute when the water is under pressure at the bottom of a lake.
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鈥檚 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鈥檚 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 鈥� products of the cyanobacterial metabolism and the source of the foul smell 鈥� had fallen dramatically. Four months later, underwater vegetation was growing prodigiously and plankton populations were thriving again (). 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鈥檚 Lake District.
Not bad for something that shouldn鈥檛 exist. But this is not the only context in which an appreciation of nanobubbles鈥� apparent scrubbing properties is making itself felt. Large ships such as oil tankers waste a huge amount of fuel each year because organisms such as barnacles attach themselves to the ship鈥檚 outsides, 鈥渇ouling鈥� the surface and creating drag. 鈥淚f you could paint a large oil tanker with anti-fouling paint, you鈥檇 save a million dollars a year in fuel alone,鈥� says Craig.
Together with Chinese colleagues, Craig is pursuing an alternative: using nanobubbles created on an electrified surface in water to take up biological molecules deposited there, sweeping the surface clean. His idea is to turn a ship鈥檚 hull into a giant electrode and so prevent it from ever becoming fouled (). In lab tests the technique seems to work, with nanobubbles being the key to success, but Craig himself is far from understanding exactly how. 鈥淭he simple answer is I don鈥檛 know,鈥� he says.
The same goes for cleaning the silicon wafers that go into making computer chips. The presence of any nanoscale contaminants on these wafers鈥� surfaces 鈥� things like quartz, iron, stainless steel or aluminium, all common in a manufacturing environment 鈥� is enough to cause defects in the extremely precise etching process used to print circuits on them. Ensuring the right level of cleanliness requires many cycles of cleaning, often using environmentally hazardous chemicals.
Or you can use nanobubbles. By allowing them to form on the wafer surface, contaminants are gently lifted up and washed away. 鈥淲e did some investigation of nanobubbles as a mild method for cleaning, and have some very nice results,鈥� says Jacques van der Donck of the based in Delft in the Netherlands, which is working with , the company that makes most of the machines Intel uses to fabricate its silicon wafers. A half-hour, multi-cycle cleaning routine might be curtailed to just a minute or so, with no harmful chemicals to dispose of later, says van der Donck.
In 鈥渓abs on a chip鈥�, 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鈥� properties comes from a drugs company called based in Tacoma, Washington. In May 2010, the US Food and Drug Administration gave approval for Revalesio to investigate the properties of a 鈥渘ovel anti-inflammatory product鈥� 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. Proteins and other biological molecules can have the kinds of hydrophobic surfaces that nanobubbles like to cling to. Revalesio does not talk explicitly about nanobubbles in its presentations. RNS60, the firm says, contains 鈥渃harge-stabilised nanostructures鈥� generated by 鈥渃ontrolled turbulence鈥�. By pumping high-pressure oxygen into a saline solution flowing in a spiral pattern between two spinning coaxial cylinders, vortices are generated 鈥� and perhaps more. 鈥淭he current research is showing that devices like ours produce nanobubbles,鈥� says , Revalesio鈥檚 director of clinical science.
In March this year, researchers from Revalesio and Rush University in Chicago presented preliminary in-vitro studies at the in Baltimore, Maryland, showing that RNS60 , the support scaffolding for neurons. Last October, Revalesio published results of phase I clinical trials in which inhaling RNS60 in aerosol form 鈥渟ignificantly鈥� 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鈥檚 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. The double layer of charged particles that make up the scaffold predispose the nanobubbles to interact with anything charged, such as the ions that carry signals into and out of biological cells. That might provide a route to new drugs for some neurodegenerative conditions and others in which these signalling channels are thought to be disturbed.
Such claims invite scepticism. 鈥淲hen I first heard about [Revalesio], I thought they might be crazy or quacks,鈥� says Craig. 鈥淏ut I鈥檝e spent some time working with them, and I鈥檓 quite impressed with them as scientists.鈥� Seddon, who has received samples of the company鈥檚 solutions to analyse, is similarly impressed with the company鈥檚 openness 鈥� although it is proving hard to determine what, if anything, lies at the heart of their product. 鈥淲e don鈥檛 have experimental techniques that can look at its internal structure,鈥� he says.
Attard is less convinced that there is anything to explain. He says that nanobubbles can only be survive in an environment that can sustain a continued excess gas pressure, which is not the case in biological fluids, and there is no evidence that the bubbles could influence biological processes such as the opening and closing of ion channels.
Of course, if RNS60 does have some powerful anti-inflammatory property, the researchers鈥� inability to work out whether or not the nanobubbles are there won鈥檛 stop it working. Just like physics forbidding their existence won鈥檛 stop them cleaning lakes, or scrubbing computer chips. But for those who like their physics in good order, the mystery continues to bubble beneath the surface.
