Monday, December 12, 2011

The Future is Clear

Two new research articles were recently published, both of which have a common theme: taking objects that are normally opaque and (through the magic of chemistry) making them translucent. Both materials are made up a (more or less) continuous matrix which enclose open spaces. However, the approaches to translucency differ and make this an interesting comparison.

The first object to become clear is a crab shell (original article ($), review (free)), which involves immersing a (dead) crab into a serious of baths - HCl, NaOH and ethanol to remove the minerals, proteins, fats and pigments and leaving just the chitin behind. The chitin is then "clarified" after acrylic monomers are injected and reacted. Here's a picture from the abstract of the starting material, after the three baths, and then the final product.
The "clarification" that occurs in the last step occurs is the removal of scattering sites because the refractive index of the acrylic and the chitin are similar. Absent the acrylic, the air that filled the opening left by after the other materials were washed out in the baths was different enough in refractive index to cause scattering.

The second object that is clarified is an open-celled glassy, foam (original article ($), review article (free)). The preparation technique is completely different - a polyurethane sponge is used as a template, into which nanoparticles of silica are added, the pH is adjusted, and then the urethane is burned out leaving the clear glass foam behind. Nanoparticles are generally too small to scatter light, so even with the difference in refractive index between the silica and the air, clarity is maintained.

I'm not sure if this is the start of a trend that we will see more of, but it might very well be since transparency is generally considered "good" and increase visibility (and not just in science as transparency in government and financial institutions are both highly desired). A rheologist that I knew from an earlier portion of my career had replaced the covers on the twin towers of his Rheometrics RDA rheometer with acrylic so that all the electronics inside could be visible. It was quite a bit more interesting to look at than the normal beige - I'm just not sure that any real purpose was served.

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