September 2013

# Sharing chemistry with the community: “Walking on Oobleck”

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Haley Barrier*, Duke University, Durham NC

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So, quick! Before the day gets light,

Go, magic smoke! Go high! Go high!

Go rise into the kingdom’s sky!

Go make the oobleck tumble down

On every street, in every town!

Go make the wondrous oobleck fall!

Oh, bring down oobleck on us all!

-Bartholomew and the Oobleck by Dr. Seuss

Oobleck, a mixture of cornstarch and water, derives its name from the story “Bartholomew and the Oobleck” by Dr. Seuss (Random House), in which the king, tired of the rain, snow, and sleet falling from the sky, wishes for something different. Soon his wish is granted; green, sticky, gooey oobleck falls making a mess of his kingdom, leading the king to regret his wish. Bartholomew saves the day by getting the king to say the magic words, “I’m sorry.”

Oobleck is an example of a non-Newtonian fluid. Under sudden stress or strain its viscosity increases significantly resulting in it acting more like a solid, but when the stress or strain is removed its viscosity decreases returning to a fluid, liquid-like state. When mixed with water, the cornstarch does not actually dissolve; rather, a suspension of solid cornstarch in water is formed. Each particle of cornstarch is surrounded by water allowing the particles to move relatively easily past one another. However, when the particles are subjected to a sudden, rapid stress, they are forced closer together “squeezing out” the water. The interactions between the particles increase making it more difficult for the particles to move past one another.

Unlike the king in Dr. Seuss’s story, people of all ages never seem to tire of oobleck. No matter whether it is a small cup full or a whole swimming pool of oobleck, audiences quickly become fully engaged as they explore its properties. Depending upon the quantity desired, it is relatively easy to prepare, safe to use, and easy to dispose of. While it can be rather messy, the benefits are well worth the time needed for cleanup.

## Concepts

- Solids versus liquids
- Viscosity
- Newtonian versus non-Newtonian fluids
- Molecular interactions

## Materials

- Cornstarch (~2 parts by volume)
- Water (~1 part by volume)

## Preparation

The basic recipe for making oobleck is approximately two parts cornstarch to one part water, by volume. The amount of cornstarch depends upon the volume of the container. We used 150 lbs of cornstarch to fill a 5 ft. diameter plastic swimming pool and 40 lbs when filling a 35” x 15” x 5” plastic bin. The amount of time needed to prepare depends upon the quantity needed. Remember that this is a suspension. The cornstarch will settle out over time, so once made, the oobleck needs to be frequently mixed in order to maintain its consistency. We suggest making smaller batches in tubs, buckets, or other suitable containers first and then combining them together in the larger container that will be used for walking on the oobleck. If the mixture is too dry, add water in very small amounts, a little at a time, until the desired consistency is obtained. If the mixture is too fluid, add small amounts of cornstarch.

We provide tubs of water and towels for our guests to wash and dry afterwards.

If staging this activity indoors, we recommend covering the area with tarps that are securely held down using duct tape to prevent people from slipping or falling, and to aid in cleanup afterwards.

## Disposal and Cleanup

Do not dispose of the oobleck in the sink! We place the oobleck mixture into plastic trash bags and carry to the dumpster. If indoors, we have found that allowing the area to air dry first makes it easier to sweep up the cornstarch. If outside and a garden hose is available, hose down the area sufficiently with water.

## Safety

While there are no particular safety issues regarding the cornstarch and water mixture itself, there are several concerns associated with walking on the oobleck, specifically regarding falls. We use a child’s step stool to assist the younger children in accessing the pool of oobleck. We also hold their hands as they run across it and exit the other end. Young children should never be left unsupervised. If perchance a child should fall facedown into the oobleck, he or she could panic and not be able to get free.

Oobleck tends to grow mold easily and therefore should not be stored for reuse. It has been suggested that a small amount of bleach be added to help prevent molding, but we have not actually done this.

We first experienced “walking on oobleck” at the USA Science & Engineering Festival held in Washington, DC, October 2010. Haley Barrier, a member of Duke Chemistry’s Festival Team, was greatly intrigued by it, and, as a direct result, she instigated “walking on oobleck” as part of Duke Chemistry Outreach’s repertoire. The following are her reflections on her first time walking on oobleck and her subsequent experiences of providing opportunities for others to experience walking on it as well.

I will never forget the first time I “walked on oobleck.” I read Dr. Seuss’s Bartholomew and the Oobleck countless times as a child, and had helped numerous children mix up small batches of the cornstarch and water mixture for hands-on polymer activities. However walking on (and then skipping over, and then sinking into) the non-Newtonian fluid allowed me to experience the substance on a much grander scale. In Washington, we stopped by Michigan Technical University’s large booth, shed our shoes and socks, and ran across a roughly four foot long vat of oobleck. From the initial surprise of being able to prance quickly across the substance to the frustration of extricating my feet once I allowed myself to sink ankle-deep, I became absolutely fixated with “walking on oobleck.” I wished for others to share in the tangible nature of oobleck, to challenge their notions of solids and liquids and the traditional laws of physics. After listening to me plead the case for a large-scale oobleck event during the five-hour long trip home, Dr. Lyle agreed, and Duke’s Alumni Weekend Science Event set the stage for our first “walking on oobleck.”

While we considered constructing a long wooden chute for our oobleck event, we worried about the safety of the participants. Duke’s Alumni Weekend event invited former students of all ages, and their families, back to campus, and many of our audience members were to be younger children. Not wanting them to slip and hit their head against a wooden container wall, we found a round, inflatable children’s pool (about five feet in diameter) to be the perfect option. The inflatable pool was cost-effective, easily rinsed and stored, safe, and was roomy enough for multiple people to experience the oobleck at a time. A small stool was placed on one side of the pool so the children did not have to hop over the side. When selecting a location for this initial event, we chose an open, central inside area with easy access to the lab room where the oobleck was mixed. Large tarps were placed underneath the pool and taped down on all sides to prevent participants from slipping on slick floors, and vats of water and clean towels were provided for feet rinsing. To allow even the youngest of audience members to safely participate, we also prepared two small buckets for hands-on play. Young toddlers, children, and adults who did not want to remove their shoes were still able to experience the oobleck, punching it with clinched fists and allowing it to drip between their fingers.

Most importantly, volunteers were recruited to arrive early and help mix the cornstarch and water. Instead of mixing all of the cornstarch and water at once, we found that making smaller batches enabled us to achieve the desired consistency. Two hours prior to the event, around six people mixed about 150 pounds of cornstarch with buckets of water, and we filled the pool just in the nick of time. Initially, we as presenters tested out the oobleck. As the alumni and their families arrived, seeing us college students laughing, jumping, and tiptoeing around the pink inflatable pool definitely attracted an audience. At first, I attempted to establish a line, instructing the audience volunteers to remove their shoes and socks, step up on the stool one at a time, and then run across the oobleck. To ensure safety, several presenters were stationed at the oobleck pool; some of us focused on line control while providing an overview of oobleck’s properties, others discussed the science in greater depth with parents, held the hands of the younger children in the pool, and helped to dry off rinsed feet. Eventually, line control became an issue. Running across the oobleck is only a small portion of the experience — the non-Newtonian fluid can be punched and sank into, rolled into balls and allowed to drip between fingers. These activities enable the audience to better understand the liquid and solid properties of oobleck under varying degrees of pressure, but they also resulted in six to seven people occupying the pool at a time.

Throughout the event, I noted the varying levels of eagerness among the audience members. Some children literally threw off their shoes and hopped into the pool, while others looked at the oobleck with hesitancy. Their parents also displayed varying levels of interest. I could sense that several adults viewed the oobleck tub as a mere “fun activity”, literally dropping their child off in the line and returning much later to hurry them along. However, the majority appeared to appreciate the learning experience, and encouraged their children to try running on top of and sinking into the oobleck. More than just observing their children, these engaged parents asked me questions, prompted their children to describe what the oobleck felt like, and were excited to experience the large pool of oobleck for themselves. These alumni truly impressed me; they set an example of participation for their families and their genuine smiles revealed their enjoyment of the activity.

During the event, we paused the activity momentarily to re-mix the oobleck. After thousands of footsteps, some of the cornstarch had settled out, leaving a two-inch layer of water that was prone to splashing. To encourage audience participation, we enlisted the assistance of the children as we mixed, discussing different properties of the substance and drawing real-life parallels with quicksand. After two hours, the audience waned and it was time to clean a colossal mess. The once off-white fluid was now a dingy gray, with some hair, dirt, and other small objects floating on the top of the surface. Since we were set up inside, we removed the oobleck by pouring as much as we could into buckets and carting them off into an outdoor dumpster. The pool was rinsed and laid out to dry; however I could never have anticipated the film of cornstarch that coated the floor and walls, connecting hallways, and lab. Hundreds of footprints lined the hall and Dr. Lyle returned after the event to try and remove the mess before our housekeeping staff came into work.

Overall, we received great feedback from our audience, who commented on how enjoyable and cool the activity was for them and their children. For subsequent events, we made the decision to pare down the scale of our tub, however. Instead of a large pool, we decided to use a long, flat, rectangular Rubbermaid container. About forty pounds of cornstarch was required to make the four inches of oobleck, but despite the smaller size, both adults and children could still stand in the tub and take a step or two across the substance. Achieving the desired oobleck consistency was more feasible on a smaller scale, however the shorter tub meant that the more intrepid oobleck-runners would jump right out of the tub, over a taped down tarp, and onto a very slick floor. This tub was used for both an indoor and an outdoor event, with rave reviews following the outdoor endeavor. Making large oobleck tubs indoors leads to slick, powdery floors, and the outdoor event expedited clean up as we rinsed the oobleck off of the concrete and into the grass. In addition, we were able to add an “Oobleck Egg Drop” from the stairs outside to further engage the audience and better demon-strate how the behavior of the fluid varies at different pressures.

While many of our audience members had created cups of oobleck at home and in the classroom, walking on oobleck allowed them to experience the science on a grander scale. I found my own enthusiasm mirrored in the shocked expressions of children who discovered that slow movements allowed them to remove a submerged foot and the grins of adults who joked about “walking on water.” Provided that ample time and materials are available, creating a “walking on oobleck” station enables audience members to get a hands-on, feet-in, immersive experience.

*Haley Barrier completed her undergraduate degree in English, with a minor in chemistry December 2012, Duke University. She will begin her study of medicine this fall.

**Dr. Kenneth Lyle, a lecturing-fellow in the Department of Chemistry at Duke University, serves as the lecture-demonstrator and chemistry outreach coordinator.

GlaxoSmithKline–RTP, Biogen Idec–RTP, and the Powell Family Foundation, graciously supports the Duke Chemistry Outreach program.

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