Stripes or dots?
To provide finer control over a membrane reaction, the researchers used a system in which the chemical that forms the membrane polymer was in an organic solvent, and a separate chemical that triggered this reaction was dissolved in water. Separately, a molecule that inhibits the reaction was placed in the organic solvent, ensuring that the reaction was limited to the interface with water.
To make this a true Turing-style system, the researchers dissolved a large molecule in water. This had the effect of making the water more viscous, which slowed the diffusion of the activator. In addition, the molecule was chosen so that the activator would stick to it, slowing things down even further. The end result was a system similar to the ones defined over a half-century ago.
And it behaved much like Turing’s description. Depending on the precise details of the two solutions involved, the researchers could tweak the system so that it would form a dense array of dots or a thick pattern of stripes. Outside of these areas, the membrane formed as usual, creating an impermeable barrier.
Imaging of the features show that rather than simply thickening the membrane, the membrane retained the same width in these areas; instead, it bulged out to form the structures. That’s critical, as the amount of surface area exposed to a salt solution should influence how much water gets through the membrane. In fact, the researchers confirmed that more water was purified when the new membranes were used (the version with the stripes outperformed the dotted one). Unfortunately, the researchers don’t compare this system to commercially available membranes.
The researchers wanted to confirm that the visible (using microscopy) physical features were responsible for the improved performance and not some unusual chemistry at the polymer level. So they put some nano-sized particles in the water that they pushed through the membrane, which ended up clustered around the bulges in the membrane, confirming that more water was passing across the membrane at these points.
It took 40 years for anyone to follow up on Turing’s hypothesis and another 25 to produce something that’s potentially useful. Science doesn’t necessarily give us a quick return on investment, but improvements in desalination and water purification could be a big help. A scarcity of clean water has already caused many countries to try desalination and water recycling, and the energy savings of a more efficient membrane could be substantial.