Princeton Weekly Bulletin June 22, 1998


 

A state between solid and liquid

By JoAnn Gutin

 


 
Chia-Fu Chou with electron microscope at research facility in Buffalo, N.Y.
 

A team of physicists led by research associate Chia-Fu Chou has discovered a new thermodynamic phase between the solid and liquid states.

The existence of this new phase, which was observed in liquid-crystal films only two molecules thick, dramatically extends scientific understanding of the way materials behave when reduced to what is effectively two dimensions. The discovery also sheds light on previously unexplained aspects of phase transition (the process by which materials change from solid to liquid or liquid to gas) and may eventually affect research on phenomena as disparate as liquid crystal displays, high-temperature superconductors and cell membranes. Chou and his team announced the discovery in the May 29 issue of Science.

The scientists discovered this new phase, called smectic A' or Sm-A', in the course of experiments designed to test the accuracy of a theoretical prediction about the way materials melt. This prediction, made nearly 20 years ago, held that if materials could exist in two dimensions, they would melt differently than they do in the real world of three. Scientists can now create nearly-two-dimensional materials; the current work shows the original prediction to be true, yet incomplete.

''In the three-dimensional world, ''explains Chou,'' a thermal discontinuity occurs when a solid melts into a liquid. For instance, when ice melts, the ice water remains at 0deg.C until all the ice is gone; then, suddenly, the temperature begins to rise. But the ice is absorbing the heat in the room in order to melt; where is that heat going? The answer is that it's being used to jumble the molecules, transforming them from the ordered, crystalline state to the disordered, liquid state. Once all the molecules are disordered, the temperature can start to rise.''

Hexatic phase

According to the predictions of theoretical physics, this suddenness or discontinuity might disappear in the effectively two-dimensional world of thin films--that is, in a two-dimensional world the process of melting or freezing could be more like a smooth ramp than a sudden step. To accommodate this smooth transition, the theory also predicted the existence of a previously unsuspected intermediate stage between liquid and solid.

In work done 17 years ago, other scientists found this first intermediate phase, the so-called ''hexatic'' phase, in which molecules assume hexagonal symmetry in orientations, though the positions of these hexagons is slightly offset from the position they assume in the final, crystalline form. Existence of the hexatic phase was a triumph for the theory; still, the thermal jump or discontinuity persisted, though it was now understood to occur between the liquid and hexatic phases. ''It was a big puzzle,'' says Chou.

But the earlier experiments were done on much thicker films. Reasoning that the jump might disappear if the theory were tested in a closer approximation of two dimensions, Chou and his collaborators began work on very thin films. His subsequent work confirmed the existence of the hexatic phase in these nearly two-dimensional models; still, the puzzling, theory-contradicting thermal discontinuity persisted. But the new work discussed in the Science paper offers the possibility of reconciling the theory and the experimental data.

Flat soap bubble

In the experiments that led to the discovery of this novel intermediate liquid phase, the scientists spread a film over a 1 mm. hole until the film was only two molecules thick; the result was not unlike a flat soap bubble. The researchers then examined the emerging structure of this suspended film by observing the patterns formed when they passed a beam of electrons through the film as it crystallized.

In the new phase, which occurs at about 66.3deg.C and is sandwiched between the completely liquid and the hexatic phases, the molecules in the liquid crystal film begin to cluster into prehexagonal positions -- that is, they are less disordered than in ordinary liquid but not yet hexagonal. It is at the boundary between this newly discovered phase and the ordinary liquid that the discontinuity occurs. ''Because the theory didn't predict the existence of Sm-A','' says Chou, ''the experiments demonstrate the theory to be not wrong, but only incomplete. Now we expect the theoreticians to explain our experimental results.''

Although the work of Chou and his colleagues may seem purely academic, it may have far-reaching implications at the interface of physics, biology and materials science, Chou believes. ''Liquid-crystal displays are a multimillion dollar industry,'' he points out, yet the properties of liquid crystal films are imperfectly understood. DNA, too, forms liquid-crystalline phases under certain physical conditions. And the lipid bilayer on a cell membrane is a system similar in structure to the film Chou and his team studied. The current work refines scientific understanding of all these materials.