Learning how to erase electronic paper

Developing electronic paper that can be written on and then erased with the touch of a button is a challenge. Sometimes the ink must adhere to the paper and other times bead up.

Getting it just right requires knowing how, on a molecular level, the liquid ink interacts with the solid paper.

Now Jeanne E. Pemberton has clarified why changing the electrical charge on electronic paper affects how well ink will stick.

The finding will further efforts to make a reusable tablet.

“The structure of water is different depending on whether the surface is charged or not,” said Pemberton, the John and Helen Schaefer professor of chemistry at the University of Arizona in Tucson. “People have predicted this change, but no one has ever fully understood its molecular basis. Now we’ve seen it. This finding will help us predict how ink will interact with electronic paper.”

Pemberton is the recipient of the American Chemical Society’s 2004 Award in Analytical Chemistry. At the symposium being given in her honor at the 227th ACS National Meeting in Anaheim, Calif., she’ll discuss her finding about electronic paper and other aspects of her work on liquid-solid interfaces. Her talk, “Chemical Measurement Science at the Interface,” will be given on Monday, March 29 at 1:30 p.m. in Room 207D of the Anaheim Convention Center.

Pemberton’s specialty is studying what happens at the boundary between liquids and solids, an area called interfacial chemistry. She wants to know what’s going on right at the interface, the region where the layer of liquid just six or seven molecules deep interacts with the solid surface.

Knowing more about what happens at the interface will help with a variety of problems, including making better electronic paper, controlling corrosion, or figuring out whether some toxic chemical will stick to the soil or wash into the groundwater.

But studying the molecular interactions at the liquid-solid boundary is hard because the bulk of the liquid gets in the way, Pemberton said.

“Only sampling one-to-two nanometers of stuff is hard to do. That’s been the challenge,” she said, adding that a nanometer is the length of only a couple of molecules.

So she figured out a way to create just the interface, without having the rest of the liquid present. She got the idea from noticing that if a solid object is dipped into water and removed, sometimes some of the water still clings to the object.

The method she and her research team developed, known as “emersion,” applies a drop of liquid onto the end of a rotating cylinder. As the cylinder rotates, the liquid is spread into a thin film only a few molecules thick. Then the scientists use light beams of different energies to determine how the atoms in the liquid molecules are vibrating. The reseachers use that information to determine how the molecules in the boundary layer are different from molecules surrounded by lots of liquid.

“There are lots of other methods to study surfaces and interfaces and none has been as successful as emersion at understanding these solid-liquid interfaces at the molecular level,” she said. Her research group is currently the only one in the world using the emersion method, which was developed in her laboratory.

So far, her team has used the method to figure out how water interacts with a solid that is chemically similar to electronic paper. Such paper is composed of a tiny checkerboard of cells, each of which can be individually charged.

If the cell has no charge, the water molecules are more attracted to each other than the paper, and they bead up. If the cell has an electrical charge, individual water molecules are attracted to the paper and spread out on it rather than sticking so strongly to one another.

Her finding will help make better electronic paper, she said, because knowing how the surface charge affects the structure of ink molecules at the interface is key to figuring out how to repeatedly write on and then completely erase the paper.

Providing a better understanding of a variety of liquid-solid interfaces is the goal of Pemberton’s research.

“You can’t have control at the subtle molecular level you need to make these technologies work without understanding the chemical nature of the interface,” she said. “Mostly we don’t understand that yet.”