Look bubbles!

Ah bubbles, what wondrous things. As children (and even now as a young adults) bubbles have fascinated us all and brought youthful joy to our lives. But now a special type of bubble could revive polluted lakes, clean tankers and computer chips and even kill cancer cells (I won’t be talking about the cancer cells in this post unfortunately). It is called the ‘nanobubble’ and for all intents and purposes it should not exist. So to get to the heart of the conundrum, how exactly does a bubble ‘work’?

The extremely thin film of liquid that surrounds a soap bubble, for example, is only sustained because the pressure of air inside the bubble is higher than the pressure of air outside the bubble, so the air pushes out against the surface tension of the liquid molecules. As air gradually leaks out through the bubble’s thin, porous walls, this excess pressure is gradually reduced, and when it is reduced enough, the bubble bursts.

This is especially true for small bubbles. The smaller a bubble is, the more tightly curved the film is and the more concentrated the inward force that the pressure has to counterbalance. Below a certain size (which also depends on the liquid enveloping the bubble among other factors) the internal pressure needed to counter the surface tension simply becomes too great; would-be nanobubbles collapse even before they can form. Or do they?

In 1994 in a laboratory in Sweden, John Parker was conducting an experiment measuring the repulsion between two water-resistant surfaces immersed in water. As they were forced together, the repulsive force between them first increased as was expected. Then, at a distance of a few hundred nanometres, it suddenly dropped off. Why?

A few years later Phil Attard provided a semi-plausible explanation. He suggested that if the surfaces were populated by nanoscale bubbles, 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.

Now for some numbers: according to physics, for nanobubbles to even exist they would need internal pressures of around 100 atmospheres! Now although that sounds implausible, in 2001 (with the help of his trusty scanning probe microscope), Attard and his colleague James Tyrrell spotted hemispherical nanoscale structures growing on hydrophobic silicon surfaces immersed in water. Subsequent spectroscopic measurements showed the structures were filled with gas. So it seems that nanobubbles do exist… but no-one can say how.

But the nanobubble doesn’t stop surprising us there, as things got weirder when James Seddon (at the University of Twente) used an atomic force microscope to take a closer look at the structure of the nanobubbles. Now, if they were indeed filled with gas, the pressures inside should force molecules out through the bubble walls at an extreme rate. He wasn’t disappointed as the rapid flow of molecules from the bubble’s apex could be felt pressing against the probing tip of the microscope. The strange part was, as Seddon put, “The bubble has maybe a thousand molecules inside it, and it’s losing approximately 1 billion gas molecules per second.”

But how can this be!? 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. However observing such a flow directly would require getting inside the bubble, which is impossible without popping it. With every possible explanation brought a host of unanswerable questions with it and yet our little nanobubble friend happily persists in its existence.

So what happened to this special bubble performing those amazing tasks, you ask?

Well a team from China, led by Pan Gang, has a simple plan to revive a heavily polluted, oxygen deprived lake by pumping it full of oxygen again. To do this he will use his own patented mechanism which 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. To top it off, the process is energy efficient and non-polluting, involving only native soils from the lake’s own edge.

The results from this mechanism? 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 foul smells) had fallen dramatically. Four months later, underwater vegetation was growing prodigiously and plankton populations were thriving again. Impressive.

Using more or less the same principles, nanobubbles created on an electrified surface could help to keep the surfaces of large ships and small silicon wafers (for computer chips) clean. In the case of cleaning silicon wafers, nanobubbles would be a much more beneficial alternative to the multi-cycle method of cleaning that is currently in use (which also uses environmentally hazardous chemicals).

Now how’s that for something that physics says shouldn’t exist.

Physics loses… for now



by Myles Scott – The Demotivator