Dr. Stephen Hall
I am interested in the thermodynamics of liquids and solids, what is sometimes called "soft matter," as opposed to the much more mature field of solid state physics.
In my current project, we are studying the growth of crystals in a class of materials called glasses. Most materials can be cooled below their ordinary freezing temperature and remain in a metastable liquid state, at least for a while. On further cooling below their ordinary freezing temperature, most materials will solidify into a crystal state in which the molecules are arranged in an orderly fashion. However, at a critical temperature called the glass transition temperature, glass formers will instead enter an odd non-crystalline but solid-like state where the molecules are not arranged in an orderly fashion. This is sometimes called an amorphous solid.
The details of how this happens are not well understood, although there is a basic framework for understanding the glass transition. In order for the liquid to crystalize, the molecules of the liquid must be able to move about freely so they can arrange themselves into the proper crystal structure. In glasses, the molecules are prevented from moving about freely and so aren't able to easily get into the crystal structure. Indeed, in glasses the viscosity of the liquid, a measure of how difficult it is for the molecules to move about, increases enormously around the glass transition temperature.
The shape of the molecule presumably plays a role in the slowing down of the molecules. For example, ball-shaped molecules would be expected to more easily form crystal structure than would pancake-shaped molecules, since the former would just need to get to the right place, while the later would also have to get in the right orientation, which might be prevented by other nearby molecules. Indeed, metals, with generally spherical shape, don't form glasses well, while many organic molecules and liquid crystals, which have two-dimensional shapes, do form glasses.
Our project grows out of previous research done at Pacific and elsewhere where a novel crystal morphology was observed in a material called OTP at temperatures just above the glass transition temperature. This newly observed crystal structure may give some clues to the molecular dynamics occuring near the glass transition.
We intend to observe several other glass-forming materials to see if a similar novel crystal can be observed to grow near the glass transition. We hope that information gathered about these crystals, such as their growth velocities, can yield some insight to the glass transition.
In order to visualize the crystal growth process, a very thin layer of material prepared in a liquid state is placed between two microscope slides. The temperature is reduced below the normal freezing point and the crystal growth is video taped. Anlysis afterwards can determine the speed of the crystal growth. the picture above is of such a slide with crystals growing from the left.
The apparatus for measuring crystal growth is shown to the left. The vertical black tube is the CCD camera and lenses. Underneath it is the brass cell that holds the slide containing the OTP material. The temperature of the cell is regulated by pumping temperaure contolled water through the tubes. The water is maintained at a constant temperature and circulated by the machine in front. The temperature is recorded by a variety of thermometers in the cell and measured by the scanner on the right. The images are displayed on the monitor and recorded by the VCR.
Current Student Involvement
Sean Lindsay, physics major (senior)
Past Student Involvement
Judith Reasoner, B.S. in Physics, Pacific University (2008)
Ryan Kuroda, B.S. in Physics, Pacific University (2008)
Kyle Motta, B.S. in Physics, Pacific University (2007)