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September 12, 2005
Squishy stuff

BY Eric RAngus

Associate Professor of Physics Eric Weeks is not impressed by the lofty terminology of his discipline. He has a variety of research interests but they all spring from core investigation into a specific family of materials known as “soft condensed matter.”

These types of material can’t decide what they are, solid or liquid. They take on characteristics of both. Butter, for instance, is a sort-of solid, but it consists of several types of liquid. Why does ketchup stay in the bottle? How can foam, which is made up of liquid and air, support weight? Why do these materials behave like they do?

These are questions Weeks has pondered and tried to explain for years, and it’s work that has a great deal of practical application. In a lot of ways, it’s fun. Some of the fun goes away, though, when the term “soft condensed material” is brought up.

He prefers a different description: squishy stuff.

“What does it take to make a squishy material act like a solid?” Weeks said. “A good example is shaving cream. Shaving cream is made up of soapy water and air, and neither of these things are solids. You put them together and you can support a little bit of weight. It can support its own weight, you could put a piece of paper on it and it would support that. So how much weight can it support?”

“What if I add a little bit of water?” he continued. “Then add a little more. At some point the shaving cream stops being a foam that can support weight and starts being a liquid with bubbles in it that can’t support weight. That transition happens a lot in different systems. The question is, how does it happen and does it happen in the same way in different systems?”

Glass is another interesting case, Weeks said. “We understand a lot about atoms, but we don’t know why window glass is solid,” he said. “Take molten glass—a liquid. As you cool it down, the viscosity grows and it flows slower. But it is a smooth transition. And at some point we are tired of waiting for it to flow, so we call it a solid. But that’s an arbitrary point—a human time scale.

“It’s much different than water freezing,” he continued. “We know it’s either water or it’s ice; there is no ambiguity, no question about it. But the glass transition is an open question. Is it really different? Or is it just superficially different?”

The status of glass as a solid is generally accepted without question by non-physicists. It’s the physicist, though, who asks the question why, since the reasons have yet to be uncovered. And those questions lead to others, which are uniformly relevant—once you think about them.

“Is the reason liquid turns into glass the same reason a bubbly liquid can turn into a foam if you change something about it?” Weeks said. “Does that explain why Jell-O is a solid? Does that explain why people are solids, even though we are almost all water? Leaving out the bones, our muscles are water and cells—bags of goo. Bags of squishy material, but that gives a lot of solidity to us.”

Weeks has sought answers to these questions since he came to Emory in 2001, following the completion of a postdoc at Harvard (he earned his Ph.D. at the University of Texas-Austin in 1997). Along with his research (which has won him a Presidential Early Career Award for Scientists and Engineers), Weeks brought with him an innovative teaching technique called “peer instruction.”

Developed by Harvard physics Professor Eric Mazur, peer instruction mixes multiple-choice questions with lecture material. “And the question, if it’s well written, is something that is not too trivial, but closely related to what you’ve been talking about,” said Weeks, who is the physics department’s director of undergraduate education.

“You have the students vote on the answer, after they think about it for a few seconds,” he continued. “If it’s a well-written question, the class is often split on their answers. And it’s good because it forces them to think, ‘Hey, maybe this is a little more subtle than I thought.’”

The following question is one Weeks has used in class. Like many of his questions—though not all—a graphic is also used. This particular example pictures the parabolic paths of two balls. The professor has a question about them.

I arrange launchers to shoot two balls simultaneously, which follow the parabolic paths shown (the graphic depicts a gray ball that flies half as far as a black one, but goes up twice as high). Which ball lands first? (The answer is at the end of this story)

a. The gray one.
b. The black one.
c. They land simultaneously.
d. Not enough information is known.

Class members are given time to consider their answer. “Then you throw it back to them: ‘I’m not going to tell you the answer, but I want you to discuss with your neighbor what you think the answer is,’” Weeks said.

“Usually what will happen is that people with correct explanations are more likely to convince their neighbors than those with incorrect explanations.”

That’s where the “peer instruction” part comes in. Rather than the professor merely standing at the head of the class, the entire learning process becomes interactive. “Then you have the class revote, and they often move closer to the correct answer,” Weeks continued.

That interaction is a crucial portion of peer instruction and one of the reasons it has been effective in large classes. Weeks has utilized peer instruction in introductory physics classes that can number 180 people. It’s an atmosphere where stimulating class discussion was next to impossible, and peer instruction has opened the door.

There are additional benefits to this method of teaching, Weeks said. Not only are students are more inclined to participate in class discussion, but when they speak up they are more confident in their answers, and they also learn to work together. Better than simply introducing students to another way to think, Weeks said, peer instruction makes them think, period.

“Normally, students are just copying down notes and, depending on how good I am that day or if I’m talking slowly enough, they can ask questions,” Weeks said. “This is a chance to really force them to think in class—to not be in the copying-down-notes mode. It gets them active in the classroom.”

Peer instruction has caught on in the physics department—several faculty (Senior Lecturer Bob Coleman and Associate Professor Tad Day among them) use it when teaching introductory classes. It’s not always the best tool for advanced classes, which are smaller, usually have a lot of discussion and where suitable multiple-choice questions are not easy to create.

Coleman, Weeks said, has taken the process one step further. Instead of asking students to raise their hands to answer questions, he handed out flash cards, which they held up at the appropriate time. That way the professor can see all the answers, while the students cannot. This prevents unsure students from hesitating and voting with the majority.

Weeks adopted the flash card method himself and tried it out on his Physics 152 class. He had the same group the previous semester for 151. They preferred the flash cards.

There are further advances available to professors who use peer instruction, including the use of handheld infrared devices. Students press a button and the answers flash on a screen. The “clickers” are bundled with textbooks, making them easy to implement. Coleman, who lobbied for their use, and both use
them now.

Weeks said that one of the reasons he believes peer instruction is effective is that it points out that—while formulas are common in the discipline—physics is not math. “An important part of physics is the concepts,” he said. “This really points out that physics is related to everyday activities.”

So, which ball landed first? B, the black one, which flew farther but lower. Horizontal motion and vertical motion are independent, so the one that goes up less high takes less time to come down. It just happens to go farther because it has more horizontal velocity.

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