the Boundaries of the Known
The Molecular World between Solid and Liquid
by Lisa Jo Rudy
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
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
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.
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
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.
and the Math
In 2003, Tom Lubensky, a member of the National Academy
of Sciences, received the Oliver E. Buckley Condensed Matter
Prize, awarded by the American Physical Society. The prize,
the highest U.S. honor in the field of soft matter physics,
recognized the Penn physicist’s recent prediction of
a new state of matter known as TGB, or “twist-grain
“ Imagine a Manhattan skyscraper,” Lubensky
explains, “layer upon layer of floors. Now imagine
the floors twisting around a Renaissance spiral staircase.
Add a whole array of staircases that hold together the twisted
layers. Now imagine that structure repeating itself as a
Lubensky built his theory of the TGB phase by analogy. De
Gennes had established that the mathematical models describing
superconductors and liquid crystals were similar. Building
on de Gennes’ work, Lubensky predicted the existence
of the TGB phase as the analog in liquid crystals of a phase
in superconductors that forms in an external magnetic field.
And he developed a mathematical model of its structure. Serendipitously,
experimentalists at Bell Labs discovered the TGB phase at
the same time that the Penn physicist conceived his prediction.
While the Buckley prize cited this specific discovery, it
was really “a lifetime achievement award” for
Lubensky’s many contributions to the ever-expanding
field of soft matter theory.