Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "oobleck"

Oobleck, a non-Newtonian fluid made up of water and cornstarch, is a perennial Internet favorite for its ability to dance and the fact that one can run across a pool of it. It’s typically described as a shear-thickening fluid and only exhibits solid-like behavior under impact. Strictly speaking, oobleck is a suspension of solid grains of cornstarch in water. When struck, the initially compressible grains jam together, creating a region more like a solid than a liquid. From this point of impact, a solidification front expands through the suspension, jamming more grains together and enabling the fluid to absorb large amounts of momentum. The process is known as dynamic solidification. (Video credit: University of Chicago; research credit: S. Waitukaitis & H. Jaeger)

Many common fluids—like air and water—are Newtonian fluids, meaning that stress in the fluid is linearly proportional to the rate at which the fluid is deformed. Viscosity is the constant that relates the stress and rate of strain, or deformation. The term non-Newtonian is used to describe any fluid whose properties do not follow this relationship; instead their viscosity is dependent on the rate of strain, viscoelasticity, or even changes with time. A neat common example of a non-Newtonian fluid is oobleck, a mixture of cornstarch and water that is shear-thickening, meaning that it is resistant to fast deformations. Like the cornstarch-based custard in the video above, these fluids react similarly to a solid when struck, resisting changing their shape, but if deformed slowly, they will flow in the manner of any liquid.

We’ve seen the effects of vibration on shear-thickening non-Newtonian fluids here on Earth before in the form of “oobleck fingers" and "cornstarch monsters”, but, to my knowledge, this is the first such video looking at the behavior in space.  The vibrations of the speaker cause shear forces on the cornstarch mixture, which causes the viscosity of the fluid to increase. This is what makes it react like a solid to sudden impacts while still flowing like a liquid when left unperturbed. In microgravity there is one less force working against the rise of the cornstarch fingers, so the formations we see in this video are subtly different from those on Earth.

Shear-thickening non-Newtonian fluids like oobleck become more viscous as force is applied to them. This behavior causes them to form finger-like structures when vibrated, makes it good liquid armor, and even enables people to run across a pool of it without sinking. Now undergraduates at Case Western Reserve University have found a new use for such fluids: pothole filling. They have created a pothole patch that consists of a waterproof bag filled with a dry solution that, when mixed with water, creates a non-Newtonian fluid capable of flowing to take the shape of the pothole but resisting a car tire like a solid. They cover the patch with a layer of black fabric so that drivers don’t avoid the patch. See the video above for a demonstration and ScienceNOW for more. (submitted by aggieastronaut)

Oobleck is a commonly utilized fluid in demonstrations of non-Newtonian behavior. Rather than being linearly viscous with respect to shear, oobleck is shear thickening, meaning that it becomes more viscous the more that it is sheared. This is what causes crazy formations when it’s vibrated, makes it useful as liquid armor, and enables people to run across pools full of it. Yet it flows readily when undisturbed. #

Shaking a fluid surface often results in standing waves known as Faraday waves, but with a non-Newtonian fluid like oobleck, at some frequencies it’s possible to incite other behaviors. Oobleck is shear-thickening, meaning that its viscosity increases when force is applied. This is what allows it to develop finger-like protrusions under high frequency vibrations.

We’ve featured the non-Newtonian fluid oobleck here before, but it bears repeating as a fun and easy exercise for anyone to do at home or at school, especially with kids. For extra fun, try vibrating it, using it as liquid armor, or filling a pool and walking on it.

The Mythbusters walk on "water" using non-Newtonian fluids. I think everyone wants to do this at least once in their life.

The patterns formed when vibrating a liquid on a speaker cone are standing waves known as Faraday waves. With a large enough amplitude, this produces some very cool effects with a shear-thickening non-Newtonian fluid like oobleck. (It would actually be interesting to see what happens when you vibrate a shear-thinning liquid like shampoo…) This video also details how you can set up this demonstration yourself at home.