Frontiers

Between the Boundaries of the Known The Molecular World between Solid and Liquid

April 2004

Most high school physics teachers describe three states of matter: solid, liquid, and gas. But according to Tom Lubensky, the Mary Amanda Wood Professor of Physics and chair of the Department of Physics and Astronomy, there are probably hundreds of matter states that are neither liquid nor solid, but something in between. Substances that inhabit these in-between states, known as liquid crystals, flow like liquids yet have optical and other properties more characteristic of crystals. Liquid crystals are members of a class of materials called “soft matter” or “soft condensed matter.”

As the name implies, soft matter is squishy. Jell-O, soap bubbles,
and meringue are all soft matter. So are gels, foams, powders, glues, emulsions, and living cells. Without soft matter there would be no life as we know it: no swaying trees, no scurrying animals; just the hard, unyielding rock of a dead planet.

The idea of more than three states of matter was first suggested in 1888 by Austrian botanist Friedrich Reinitzer. He was investigating an organic substance related to cholesterol and observed that the material turned into a cloudy liquid at high temperatures; as it cooled, it became clear, suddenly turned blue, then crystallized. His contemporary, German physics professor Otto Lehman, dubbed the mysterious blue phase that appeared before crystallization a “liquid crystal.” By 1922, liquid crystals had been identified as unique states of matter.

Modern interest in liquid crystals began in the 1960s with work by the French physicist Pierre-Giles de Gennes and others that led in the 1970s to liquid-crystal displays—and to that ’70s icon, the Mood Ring. De Gennes went on to win a Nobel Prize for his groundbreaking work in the field of soft matter physics.

In 1969, a young postdoc, Tom Lubensky, joined the French physicist’s group in Paris. The son of an American diplomat, Lubensky lived overseas for most of his youth. When he was in junior high school in Spain, he made a pact with a friend to meet again in college at Caltech, which his friend said was the best science school in the world. Lubensky kept his part of the bargain, entering Caltech in 1960. He went on to get a Ph.D. in physics at Harvard, writing a thesis on magnetism.

Lubensky is a classical guitarist, as is de Gennes. When he visited France during his doctoral studies, the two hit it off musically as well as scientifically. As a post-doc, the young physicist returned to France and renewed the friendship, which led to dinner invitations at the
de Gennes home in a pear orchard.

“ I got in on the ground floor at an exciting time,” recalls Lubensky, who developed into a leading figure in the field of soft matter research. By the early 1970s, new tools such as X-ray scanners allowed for more precise imaging of molecular structures. After a year in Paris, Lubensky’s research took him back to the U.S., where in 1971 he joined the physics department in the School of Arts and Sciences.

A theoretical physicist, Lubensky uses his imagination, his computer, and an ordinary organic-chemistry model kit to puzzle out the molecular architecture of soft matter. Explaining the whys and hows of these diverse matter states is the work of a wide range of applied and theoretical scientists. Together with a cross-disciplinary team in fields as broad-ranging as biology, engineering, and optics, Lubensky carries out the conceptual research that expands the inner horizons of the known physical world.

In soft matter, the molecules are more structured than liquids but more random than crystals. The molecular structure determines the physical properties. Some phases reveal a braided structure. Others have strings of molecules twined into helices, and some have flat layers like baklava that can slide back and forth. Both jelly and rubber, for example, qualify for the name “soft matter,” but their different textures, elasticity, and other properties are a result of how the molecules are configured. These differences explain why SuperBalls bounce and Play-Doh hits the floor with a dead thud.

Research on the layered, twisted, and braided forms of soft matter has yielded many important inventions. Latex paint, foam rubber, liquid-crystal displays, toothpaste, and mayonnaise are all made from soft matter. So is Silly Putty. “Invented” as a by-product of research at General Electric, it combines the soft, pliable qualities of dough with the elastic properties of rubber. Silly Putty is strange, fascinating, fun—and a gold mine. Though commercial applications of Lubensky’s discoveries are yet to be seen, his research provides the sort of fundamental understanding that sets the stage for such applications.

Physicists, he notes, are more intent upon understanding how nature has put together its molecules than on what they might be made to do. “We physicists are most interested in the big picture. We’re working toward a general understanding of the principles that underlie the natural world. Other researchers are interested
in uncovering trends and developing applications. Science needs both approaches.”

Lisa Jo Rudy is a freelance writer and consultant who lives in Elkins Park, PA.