Membrane blocks gas when illuminated by purple light

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A membrane developed at Rochester University blocks gas when one colour of light is shone on its surface and permits gas flow when another colour of light is used.

It is said to be the first time that scientists have developed a membrane that can be controlled by light in this way.

Eric Glowacki, a graduate student at the university’s Laboratory for Laser Energetics, and Kenneth Marshall, his adviser, invented the membrane, which is a piece of plastic punctured with tiny holes that are filled with liquid crystals and a dye.

When purple light illuminates the surface of the membrane, the dye molecules straighten and the liquid crystals fall into line, which allows gas to flow through the holes.

When ultraviolet light illuminates the surface, the dye molecules bend into a curved shape and the liquid crystals scatter into random orientations, clogging the tunnel and blocking gas from penetrating.

Controlling a membrane’s permeability with light is preferable to controlling it with heat or electricity for several reasons, according to Glowacki. Light can operate remotely, so 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.

Another advantage is that the colour of the light illuminating the membrane can be changed precisely and quickly. Other methods, such as heating and cooling, take a relatively long time and repeated heating and cooling can damage the membrane.

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

Creating the membrane is a multi-step process. First, a circular hard plastic chip is struck with a beam of neutrons to make the tiny, evenly spaced holes that are about one hundredth of a millimetre in diameter.

The chip is then dipped in a solution of liquid crystals and dye and the mixture fills the holes through capillary action. The final product is spun in a centrifuge to remove the excess liquid crystals from the surface.

According to Rochester University, the membrane could be useful in controlled drug delivery and industrial processing tasks that require the ability to turn the flow of gas on and off, as well as in research applications.

Marshall presented the findings at the annual conference of the International Society for Optics and Photonics (SPIE) in San Diego yesterday.