Magazine article American Scientist

Pulling on the Shade

Magazine article American Scientist

Pulling on the Shade

Article excerpt

Transparent, liquid-filled films become opaque when stretched

You are surrounded by single-purpose objects: a table that is only a table, a shirt that provides only one level of warmth, a window that always admits the same amount of light. But there is no reason fabricated materials should not be able to switch their properties on and off the way biological materials often do, such as the eye adjusting its sensitivity to light or hairs standing up on the skin in the cold. "To make materials that are dynamic and responsive is the future of materials science," says Joanna Aizenberg, a materials engineer at Harvard University.

Aizenberg started down this path while brainstorming one day with a group of colleagues. As they were talking about future directions for their field, they jotted down an idea: Is it possible to create a tent out of a waterrepellant fabric that could be transparent on rainy days but reflective when it is sunny? As Aizenberg and her colleagues reported in the journal Nature Materials, they have created a hybrid liquid-solid material that can perform this optical shift simply by being stretched.

The researchers began by working with a woven polymer material, which can stretch and then return to its original shape. They coated this fabric with a thin layer of lubricant that has a particular affinity for the fabric, so it does not rub off or evaporate easily. The lubricant also repels water, ice, dust, oil, and many other contaminants.

On the unstretched fabric, the lubricant forms a coating that is perfect down to the atomic level. "It's kind of counterintuitive but we use rough surfaces to hold the liquid and create molecularly smooth surfaces," Aizenberg says. "There are no defects to hold onto, and liquid after all is movable anyway. Everything that touches these surfaces just slides off." In addition, any incoming light has nothing to scatter off of, so the material is transparent.

When the fabric is stretched, the pore size between fibers expands and some of the liquid recedes into them. At that point, the liquid surface becomes uneven; light scatters from it, turning it opaque; and the material no longer repels water. When the fabric is released, it becomes smooth again and its previous properties return.

To scale up their test-size material to something like a tent, real-world conditions have to be considered. …

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