Rheology is fun!

02.23.2018

Experiment Leaders: Shweta and Athena

This was our first trip to our Venture Academy 7th grade class for the spring semester. To switch things up a little bit from what the students are currently learning in class, we decided to demonstrate to them several experiments on rheology, which is the study of how materials flow. Rheology is a topic most students would not be exposed to in school (or even in university for that matter!), so some of them were pleased to learn something totally new.

To start off, we gave a brief explanation about rheology and several topics associated with it such as viscoelasticity, shear thickening and thinning, Newtonian and Non-Newtonian fluids, and crosslinking. We had the students split into six groups with two groups performing the same experiment. One of the experiments was learning about shear thickening phenomena using Oobleck, which is a cornstarch-water mixture. They learned that depending on how quickly they poked the Oobleck with a popsicle stick, the material would respond differently. At high shear rates (rapid pokes), it feels thicker and the stick wasn’t able to poke through. At low shear rates (slow pokes), the stick was able to easily slide through the Oobleck. This is a result of the shear thickening properties of cornstarch solutions. They also watched a video showing kids running through a pool of cornstarch (and one getting stuck because she didn’t move fast enough!).

The second experiment was making a “magnetic fluid” using Silly Putty and iron filings. We explained that Silly Putty is an example of a viscoelastic material, meaning at high deformation rates (i.e. pulling it apart fast), it behaves like a solid whereas at low deformation rates (i.e. pulling it apart slowly), it behaves like a liquid. They mixed in the iron filings into the Silly Putty and used a magnet to move around a small portion of the magnetic Silly Putty. The students were fascinated (and a little freaked out) that the Silly Putty looked “alive” while doing this. The ability to move around the Silly Putty could only occur because of its viscoelastic properties and the iron filings.

The final experiment involved learning about gels by making “slimy spaghetti and meatballs.” A solution of sodium alginate colored with food coloring was pipetted and/or dropped into another solution of calcium lactate. The calcium lactate causes the sodium alginate, a polymer, to crosslink and form a gel, which is more solid-like than its original solution form. Because this particular polymer likes water, it will uptake water from the solution, which is what makes gels soft and squishy instead of a rigid solid.