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Advanced photons in science

I am currently working with Argonne National Laboratory on a series of articles for the annual report of the ANL's Advanced Photon Source. Elemental Discoveries offers a preview of the advanced science featured in my report. Advanced Photon SourceElemental Discoveries shows how X-rays reveal more of the inner secrets of the world around us as seen under the illumination of ANL's Advanced Photon Source:

Are films ferroelectric?
Discipline for gold nanocrystals
Photosynthetic system
Dissecting the atom
Catalytic clues
SAXS and the water channel
Folding Protein Sensors
X-ray movies

Digging in the dirt

They say each of us will eat a ton of dirt before we die, but how do materials respond to the presence of dirt? In most cases, dirt is detrimental, but for some systems, such as superconductors, impurities enhance the ability of the material to conduct an electric current without energy loss. Other materials affected by dirt include the diverse class of compounds known as liquid crystals, which are not only ubiquitous in modern electronic display devices but are also found in biological systems, such as cell walls.

Real condensed matter systems, i.e. materials, invariably have imperfections, so understanding disorder is essential to a complete understanding of materials in the real-world. However, physicists face a problem in trying to understand real-world systems. In their theoretical frameworks, neat, pure systems work ideally, but the theories have a very hard time dealing with externally imposed randomness; such systems are at the frontier of physics. Finding a way to circumvent this problem is at the core of research being undertaken by Robert Leheny of the Johns Hopkins University and colleagues there and at MIT.

They hope to answer important questions about one of the most fragile states of matter - the smectic liquid crystal. Liquid crystals are fluids, explains Leheny, in which the molecular constituents possess a degree of order intermediate between fully ordered crystalline materials and fully disordered simple liquids. There are many types of liquid crystal, which have all been well characterized and so make an ideal test bed for discovering the effects of imposed randomness.

Smectic liquid crystals normally form a layered arrangement in which their long, rodlike molecules align parallel to each other and stack into layers. But, understanding how they respond to the presence of externally imposed disorder, dirt, in other words, is a rather vexing question as Leheny and his colleagues have discovered. "The short answer is that the smectic doesn't handle the dirt well at all," he says, "The phase is destroyed, which is not surprising." What is left behind, however, is far more intriguing, from disorder, the theory goes, a new state of matter might emerge. Leheny and his colleagues are using a controlled chemical environment to see if they can observe this new state.

The key to Leheny's experiments is to trap the liquid crystals in a gel. The random structure of a gel imparts disorder on the liquid crystals which are already torn by their two-faced nature hanging between crystalline order and liquid disorder. By exploiting beamline IMMY/XOR 8-ID, Leheny and his colleagues have focused on the behavior of smectic liquid crystals based on the organic compound octylcyanobiphenyl (8CB), which they can trap in a colloidal silica gel, an aerosil.

Leheny explains that the liquid crystal molecules interact with the gel surfaces and a competition arises between the liquid crystal's natural tendency to order and the pull of the randomness of the gel. Their x-ray studies are revealing the nature of this conflict in detail and the forces that are involved in disturbing the liquid crystal order.

"We are ultimately reporting a negative result," says Leheny, "The specific motivation for the experiment was to find evidence that, once the smectic phase is destroyed by the externally imposed disorder, a new state of matter called a 'Bragg glass' would emerge in its place." In contrast to theoretical predictions they did not find a Bragg glass. Leheny believes that the jury is still out on whether the theory is correct, and believes the answer may lie in observing different systems with weaker disorder. Perhaps then the new state of matter that is latent in smectic liquid crystals will be revealed.

Liang et al, "Smectic liquid crystals in anisotropic colloidal silica gels," J. Phys.: Condens. Matter 16 (2004).

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