Membrane Makes Light Work of Permeation

Membrane blocks gas from flowing through it when colored light is shined on its surface.

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Scientists at the Laboratory for Laser Energetics of the University of Rochester, Rochester, N.Y., have developed a membrane that blocks gas from flowing through it when one color of light is shined on its surface, and permits gas to flow through when another color of light is used.

Eric Glowacki, a graduate student at the university, and Kenneth Marshall, research engineer, optical materials, invented the membrane, which reportedly is the first one whose permeability is controlled by light.

Applications could include those where gas flow must be cycled on and off but where making electrical or other hardware connections would be difficult. "Think in terms of very small or micro-scale apparatus that would need gas delivery — such as very small hand-held chemical analyzers, medical equipment, complex process equipment, or even nano-scale processes," says Marshall. Another possibility is the separation of straight-chain from branched-chain hydrocarbons, he adds.

The researchers create 400-mm pores in a commercial "Isopore" polycarbonate membrane and fill these with liquid crystals and a dye (Figure 2).

Then, when purple light illuminates the surface of the membrane, the dye molecules straighten out and the liquid crystals fall into line, which allows gas to easily flow through the holes. But when ultraviolet light illuminates the surface, the dye molecules bend into a banana shape and the liquid crystals scatter into random orientations, clogging the tunnel and blocking gas from penetrating. This switching takes about 5 sec, but faster times are possible, notes Marshall. Gas cutoff isn't total; the permeability change is about an order of magnitude, he adds.

"Controlling a membrane's permeability with light is preferable to controlling it with heat or electricity — two readily used alternative methods — for several reasons," Glowacki says. "For starters, light can operate remotely. Instead of attaching electrical lines to the membrane, a lamp or a laser can be directed at the membrane from a distance. This could allow engineers to make much smaller, simpler setups." In addition, heating and cooling take a relatively long time and repeated heating and cooling can damage the membrane.

Also, light does not have the potential to ignite a gas, which could be a crucial benefit when working with hydrocarbons or other flammable gases. Lastly, the amount of light energy needed to switch the membrane on and off is miniscule.

However, challenges remain. Currently, the membrane can't stand high pressures or temperatures because the liquid crystal (LC) material is only held in the pores by capillary action. The researchers currently are working on a new cross-linkable LC material that has the dye bonded to the polymer backbone. It promises to raise the pressure range and extend the temperature limit well above 100°C.

The next step in the development is to look at how the membrane performs with other gases, as only nitrogen has been used to date. "We also want to look at other liquid crystal phases (e.g., smectic phases) which are highly ordered and a bit more viscous than the nematic materials we now use, and thus may provide both larger permeability switching ratios and greater operating pressure differentials across membranes," notes Marshall.

"This technology has a very bright and important future, but it is still in its infancy," says Marshall.

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