The Science of All Things Squishy

Students enter the laboratory to find a collection of mysterious mixtures and substances. They are instructed to examine the materials using their hands or whatever tools necessary. The first sample consists of a white, foamy substance that has been shaped into a tower. They notice the smell seems oddly familiar… 'wait, is that shaving cream?'

Surprisingly, many household items such as ketchup, mayonnaise, toothpaste, hair gel and shaving cream are featured in a seminar class for incoming freshmen at Emory University in Atlanta, Ga. The purpose of the class is to get students excited about science by exposing them to cutting-edge research at the university. The class includes a three-week unit about the properties and behaviors of a group of materials called complex fluids.

Some aspects--students delving into sand boxes and playing with putty-like substances--may seem more appropriate for a kindergarten classroom, but the science behind the activities is actually quite challenging. For example, a mixture of cornstarch and water may feel like clay, but once you stop rolling it around, it flows like a liquid.

So which is it: a solid or a liquid?

According to researcher Eric Weeks, associate physics professor at Emory, it's both. Or better yet, it's "squishy."

That is Weeks' name for materials with both fluid and solid properties. He studies why complex fluids act the way they do, and what causes them to behave differently from simple liquids like water and honey.

His work with squishy substances, along with his commitment to both research and education, earned him a prestigious Faculty Early Career Development (CAREER) award from the National Science Foundation in 2003. CAREER awards are given to the most promising young researchers in science and engineering who also translate their work into significant educational activities.

Encouraging students to question certain aspects of squishy behavior is just one segment of the Emory freshman seminar class. Weeks' colleague, chemistry and biology professor David Lynn, came up with the idea for the class as a method for exposing freshmen to cutting-edge science taking place at the university. The seminar consists of  five modules, each three weeks long and focusing on a different area of science. The topics vary each semester and have included physics, chemistry, biology, anthropology, mathematics, computer science, psychology and medicine. 

Weeks' module is intended to show students how to think about squishy materials scientifically as well as show them actual research done in the laboratory. As the students explore various types of squishy substances, the laboratory experiments designed by Weeks and other colleagues demonstrate not only that science can be fun, but also that common household materials can be useful aids in understanding complex science concepts.

A how-to article about the classroom unit on squishy materials was published in the May 2006 issue of The Physics Teacher journal. Weeks co-wrote the article with David Lynn and Piotr Habdas, an assistant professor in the physics department at Saint Joseph's University in Philadelphia. The resulting paper was one of the journal's three featured items for the month and was offered as a free download from the journal’s Web site.

According to Weeks, the activities described in the paper can be adapted for different age groups. He and his lab group demonstrated how well the lessons work for younger students when the researchers organized three field trips to the Emory physics laboratory for home-schooled children from kindergarten through eighth grade. The children handled the squishy materials and also got to view them through the lens of a microscope.

"For the youngest kids, we're trying to get across the idea that this is science and it's fun," said Weeks. "They come to the fancy lab setting, and I don't think they expect to see common household items as science."

For more advanced students, Weeks provides a series of hands-on activities that allow the students to investigate why squishy materials differ from solids, liquids and gases, and how microscopic structures could explain the unique, complex properties.

One lab assignment teaches students how to identify the properties of squishy materials. Students are given spatulas and beakers filled with different mixtures. They are asked to observe how difficult each material is to stir.

The students notice that the squishy mixtures have unique and often "weird" properties. The first sample is harder for them to mix as it is stirred faster. Another behaves in the exact opposite way; it is easier to mix when stirred faster. Stick a spatula into the next substance and it stays standing up at a slight angle, as if the substance is a solid. But commence stirring, and the material moves almost as easily as water. Despite the mixtures' unusual behaviors,  Weeks noted that the three samples "aren't esoteric: the first is corn starch mixed with water, the second is xantham gum (a common food ingredient), and the third is clay mixed with water."

The varying difficulty of stirring the mixtures demonstrates the property of viscosity, the resistance to flow. By observing how the materials react when force is applied, the mixtures can be classified as specific types of squishy material. 

In addition to examining viscosity and other properties of squishy materials, students also learn that the impacts of research on squishy materials are felt in many facets of everyday life, from improving peanut butter processing and packaging to ensuring that low-fat butter still looks and acts like its fatty counterparts.    

Weeks explains: "It's the chemists' job to put a chemical in [low-fat food such as butter] so it won't kill you and you won't gain weight, but I'm more worried about its physical properties--does it feel like butter? Move like butter?"

Ketchup is another example of a common squishy food product. It only flows when stress is applied to it, and that's why, according to Weeks and others, a delicate combination of power and skill is required to shake ketchup out of a glass bottle. Now, ketchup companies have switched to plastic bottles and the ketchup can be easily squeezed out.

Outside the realm of food and cosmetic products, Weeks suggests that even humans can be considered squishy. "Water is a huge percentage of our bodies," he said, "so how come we're not liquids?"

When he's not getting students to think about squishy materials in new ways, Weeks is trying to answer some of his own questions in the laboratory. He studies a wide variety of materials, but focuses on a specific type called colloidal substances. These substances are made up of microscopic, solid particles in liquid. Colloidal gels, like yogurt, have a small amount of solid particles but manage to maintain their shape. Then there are emulsions, one liquid mixed into a second, like mayonnaise and oil-and-water. 

He uses a confocal microscope to observe the microscopic arrangement, concentration and movement of colloidal particles within substances. For instance, the theory of jamming suggests that the intricate networks of particles within a squishy material allow it to support its own weight. The question of whether jamming could explain the solid-like properties of some squishy materials remains unanswered by physicists.

Weeks' most recent research focuses on how the behaviors of colloidal substances could relate to regular solid materials, like glass. His lab group compared solid-like properties of pastes when confined to small spaces to the transition of glass to a solid state. Their experiments could build on earlier work done with thin polymer films and other glassy materials.

Weeks believes that thinking about this type of complex science is within the understanding of young students. By opening his lab doors to local schoolchildren and providing them with hands-on access to physical phenomena, he is helping young scientists realize that even an ordinary, everyday object can yield great discoveries.