An appetite for liquid-crystal spaghetti.

An appetite for liquid-crystal spaghetti

Ordinarily, when a liquid cools it becomes a solid. Some substances, however, go through an intermediate, liquid-crystal phase, settling into an arrangement lying somewhere between the regular order found in crystallinge solids and the disorder in liquids. A new material shows how dramatic and unusual the change of state from a liquid to a liquid crystal can be.

Slowly cooling the pure-liquid form of this particular material initially produces tiny filaments visible under an optical microscope. Each filament grows longer and longer by adding material not to its ends by everywhere along its length. As the pace of growth accelerates, the filaments lengthen rapidly and fold into convoluted patterns. Within a minute, the entire field of view fills with a spaghetti-like tangle.

Then another curious thing happens. Suddenly, one or more small, compact lumps resembling flattened meatballs appear among the strands. The lumps quickly suck up the strands, clearing space for new filaments to form and grow. More and more lumps develop until the whole material finally ends up in a single, conventional liquid-crystal phase.

"When I first saw it, I found it quite unbelievable," says Peter Palffy-Muhoray of Kent (Ohio) State University. "I think it's the most complicated phase change in a pure material that I have ever heard of. It looks like a biological system." Palffy-Muhoray described the discovery in St. Louis this week at a meeting of the American Physical Society.

He and his collaborators are studying the new material as part of a general effort to gain a better understanding of how phase changes occur and how patterns form from liquids that themselves appear to have no structure.

Like many liquid-crystal materials, the new substance consists of long, linear organic molecules. Each molecule has a body of three benzene rings along with several other groups of atoms and a hydrocarbon tail containing 10 carbon atoms in a chain. Other members of the family, having the same body but fewer carbon atoms in their tails, show nothing of the new sibling's bizarre behavior. They all settle into a liquid-crystal phase known as smectic A, in which the molecules line up to form orderly layers, standing erect in each layer.

Slow cooling of the new material appears to encourage formation of filaments, in which a number of molecular layers wrap themselves around to form a liquid-crystalline tube with a small volume of liquid trapped at its core. As cooling continues, more material "freezes" onto the outside of a filament, but the newly arrived molecules get pulled into the filaments's interior and then diffuse along the layers making up the tube. Instead of getting thicker, the filaments get longer. Researchers are less certain about the formation, structure and behavior of the voracious lumps.

"I think there is a great deal of exciting physics to be done in this area," Palffy-Muhoray says. "We need detailed measurements of the dynamics of these interfacial phenomena. We also need to assess the effects of impurities."