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A US chemical engineer is calling on analytical chemists to take advantage of a new class of fluids he has developed that can alter their viscosity in response to light. Such fluids could prove of a great use in capillary electrophoresis (CE), especially microchip CE. 'The beauty of our system is that anyone who is interested in exploring applications with these fluids can recreate them in their lab,' says Srinivasa Raghavan, an assistant professor at the University of Maryland. 'The recipe is right there in our paper and the chemicals are cheap. In fact, I hope people do indeed explore new applications.' Fluids that can alter their viscosity in response to electric or magnetic fields were first developed over 50 years ago and have now found use in a wide variety of applications. For example, the car industry utilise them in clutches, brakes and shock absorbers. Fluids that can alter their viscosity on exposure to light, known as photorheological (PR) fluids, are a more recent development. They offer the potential for even greater viscosity control, as light can be precisely directed at small areas, but the complexity and cost of these PR fluids has restricted their use. Raghavan thought that there must be simpler ways to create PR fluids and, in 2004, obtained a research grant that let him begin searching for one. 'There were concerns raised about the feasibility of my ideas at that stage, but the review panel felt that it was worth giving it a shot,' he says. Together with a team of students, Raghavan then started to trawl through numerous photoresponsive compounds, finally hitting on the right combination of molecules in 2005. This combination comprises the cationic surfactant cetyl trimethylammonium bromide (CTAB) and the photoresponsive organic derivative ortho-methoxycinnamic acid (OMCA), both of which are fairly inexpensive and commonly found in academic laboratories. OMCA is an example of a cis-trans isomer, in that it can exist in two different structural forms that revolve around a central atom or bond. Normally, OMCA adopts the trans configuration, but on exposure to ultra-violet (UV) light it shifts to the cis configuration. Raghavan found that combining CTAB and OMCA produces a thick, viscous gel, which will not flow out of a tilted vial. But exposing this gel to UV light causes its viscosity to fall by up to a factor of 10,000, eventually (after around 37 minutes) transforming it into a free-flowing liquid with the same consistency as water. The speed of this transformation increases for smaller amounts of fluid, with Raghavan and his team finding that the viscosity of a small sample can drop significantly in only a few seconds. Studying this transformation process in more detail, using techniques such as single-angle neutron scattering, Raghavan found that the gel consists of long worm-like micelles (molecular clusters), which are all tangled together. The free-flowing liquid, on the other hand, consists of much shorter micelles, which don't become entangled. It appears that the trans-OMCA molecules can link together the short CTAB micelles into the long worm-live versions, whereas the cis-OMCA molecules are unable to do this. So as UV light switches the OMCA molecules from the trans configuration to the cis configuration, the fluid changes from a thick gel into a watery liquid. Although impressive, this specific PR fluid would probably not be of much use for CE. Much more beneficial would be a PR fluid that changes from a liquid into gel, which would offer an easy way to fill CE tubes, especially microscopic ones. However, Raghavan claims that other PR fluids, in which the viscosity change is reversible, should be obtainable by simply using different photoresponsive organic molecules and this is something he is now working on. Related links:
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. |
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