Welcome, one and all, to #RealTimeChem week. Thousands of scientists (mainly chemists) this week around are going to be tweeting pictures of their labs and showing the rest of us what they do as the week goes by. I don’t work in a lab anymore, but you don’t have to. Chemistry – and the wonders therein – are all around us. I, for one, will try to update my blog a few times this week to support the hashtag.
To start us off, I want to discuss some awesome chemistry (and chemical engineering) you can do in your kitchen. All you need is a little water and some cornstarch. When you mix 225-300 grams of cornstarch with 250 grams of water, you’ll notice that the harder you try and stir, the more and more difficult it becomes. Congratulations, you’ve made a non-Newtonian fluid: oobleck.
Now, I cannot find the point in history when “oobleck” first came to mean “a suspension of cornstarch in water.” So here’s a little contest. Normally I’d put this at the end, but it’s important to me: Who coined the term “oobleck” to mean “cornstarch in water?” To the person who finds the first instance, I will send 1-dozen chocolate chip cookies OR one 100-mL volumetric flask, your choice.
Let’s play with our oobleck a bit. If you try and gently pick it up with your fingers, it’ll ooze like a fluid. If you get a good handful and try and roll it into a ball, it will act like a solid. What?! Yes. You’ve made something cool and gross – now, for the science of it all.
What is a non-Newtonian Fluid?
A Newtonian fluid is a fluid which behaves exactly as you’d expect – the viscosity (resistance to flow) is constant regardless of the forces applied. A non-Newtonian fluid changes viscosity when a force is applied. Ketchup is a shear-thinning fluid; it’s hard to get moving, but once you get it to start coming out of the bottle is explodes everywhere and all over the table. Oobleck is just the opposite (a shear-thickening fluid) – it likes to move very slowly, but if you try and force it to move, it gets hard like a rock. There are lots of other types of non-Newtonian fluids – like honey, caramel, Flubber (a mixture of borax and glue, basically), and silly putty. Actually, Silly Putty is the reason I picked the college I eventually got my degree at!
Why does Oobleck like to misbehave and pretend to be a solid?
It turns out that oobleck is simply a suspension of cornstarch particles in water. Let me show you a picture.
The cornstarch particles are happily floating around in the water. Given enough time, they’ll all settle to the bottom. When I smack the oobleck (e.g., apply a force):
The water molecules run away faster and the cornstarch particles get stuck to one another, forming a hard solid-like surface. In 2012, Waitukaitis and Jaeger confirmed this explanation in 2012 and had a paper published in Nature.
Interestingly, this solid surface acts weird when very, very large forces are applied. In 2013, a group at Princeton University, lead by Professor Roché, decided to see how oobleck breaks under strain. They filmed a thin layer of oobleck being hit with a 300-gram tungsten carbide rod. While the properties of oobleck would suggest it would undergo failure like a metal (and tear), the oobleck shattered like glass, forming many jagged edged cracks. Within a few moments, these cracks were filled like magic. You can actually find a cool video of one of their experiments here. If all this wasn’t cool enough, in 2013, a group lead by Professor Crawford at the Colorado School of Mines set out to determine if the ratio at which the cornstarch and water were mixed would affect the viscosity under stress. As it turns out, to form a suspension with enough viscosity to hold a running adult, 52.5% of the slurry must be cornstarch. The more you know.
Try this at home! Seriously! It’s great fun, makes a mess, and kids love it. If you do it because I inspired you to try it, send in your pictures and I will share them with everyone else. It’s rare that you can experience engineering in the home and this is a great example.
I hope you all have an excellent #RealTimeChem week! I know I will.