Grier awarded $500,000 Packard fellowship

Grier, Assistant Professor in Physics and the James Franck Institute, studies phase transitions -- how liquids freeze to form solids and how materials change from one form into another, such as from water into ice or from graphite into diamonds.

Grier said the physics of how phase transitions occur is very poorly understood. To follow a phase transition's process, researchers must be able to see how the atoms move in relation to each other, he explained.

"You can't look at phase transition in ordinary materials, because the atoms are far too small and they move too fast," he said. "Some people use X-ray scattering in their research, but there you get an average over the whole material -- it's impossible to look at individual atoms."

Instead, Grier uses model systems composed of colloidal microspheres ("little plastic balls," Grier explained), each about half a micron in diameter -- 200 times smaller than the width of a human hair, but more than 500 times larger than an atom.

"With these model systems, you can actually see individual particles and watch what happens to them," he said.

By suspending the electrostatically charged plastic balls in water and viewing them under a light microscope as they are squeezed together under pressure, Grier can watch the material change from a liquid, where the particles move freely, to highly ordered solids. Under pressure, the charged particles are forced together, but they adopt a configuration that allows each particle to be as far from its neighbor as possible.

Grier can also watch the material change from one highly ordered form to another -- the equivalent of changing graphite into diamonds. He watches the interactions on a TV monitor and records them on videotape.

"All the current theories say these transitions should be abrupt," Grier said, "but I have yet to see a definite point where I've been able to say for sure, OK, now it's one form, now it's another. The way it actually happens is that there's a pocket over here, then one over there, and the form changes gradually."

Grier said his preliminary results are very intriguing but don't fit into any of the theoretical models proposed to date. "It's clear that we and the theorists have been looking at this problem in the wrong way. It's as if we've been trying to define night and day without knowing anything about light. When a lot of doors open and close, that must be day. When that's not the case, we think it's night. But we're missing the essential ingredient.

"I hope that what we see will feed back into the theory and lead us to a much deeper understanding of how phase transitions happen," he said.

Along the way, Grier has already made waves by discovering a method for measuring how the colloidal particles interact with one another. "Measuring the force of the interactions is like measuring the force of gravitational attraction between two people standing a kilometer apart," Grier said. "But we found a way to study it. Using optical tweezers -- focused beams of light that can pick up small particles -- we pick up individual particles and bring them together and watch how they move apart. Up until now, the form of the inter-actions had just been assumed, but now we actually understand them."

Understanding the interactions of colloidal particles has applications in surprisingly diverse arenas. Ink, concrete, ice cream and paint, to name just a few examples, are all suspensions of colloidal particles. Grier said he has been getting calls from people in industry who want him to use his optical tweezers to study the interactions of colloidal particles in whichever particular suspension material they manufacture.

"A lot of the recipes for colloidal suspensions are empirical -- they know they have to add a certain amount of one ingredient based on experience, but if they change the recipe, they have to adjust the ratios all over again by trial and error," he said. "Nobody really understood before how these particles were interacting."

Grier's research was published in the July issue of Physical Review Letters. A patent application is in the works.

Grier received his A.B. in 1984 from Harvard and his Ph.D. in 1989 from the University of Michigan. He worked as a postdoctoral fellow at AT&T Bell Laboratories before joining the University faculty in 1992.

He said his Packard fellowship will make it easier to advance his research.

"In the days before riches, everything was strung together with sticky tape and good intentions," he said, laughing, pointing to a cutout piece of a file folder being used as a beam chopper for a microscope. "With this fellowship, I'll be able to buy some new equipment, and I'll also be able to stop filling out grant applications and get back into the lab."