If I were asked to define what a fluid is, I would most likely use the definition I learned in grade school: a state of matter that flows and takes the form of the container it is in. Similarly, a solid is a state of matter that will hold its shape, and have a defined elastic response. Now these definitions are perfectly sound, but what about materials that don't quite fit into either definition? Silly putty seems solid, if you roll it into a ball it can bounce off a surface, however if you place it on a flat surface it will flow and spread out like like a fluid... Similarly if you pull it slowly it will act like a viscous fluid, become thin like a string and finally fall apart, however if you pull it quickly and with enough force it will exhibit something akin to brittle solid fracture—similar to steel if you pull it hard enough. So what is a material of like this to be classified as, fluid or solid?
Materials of this type are known as visco-elastic materials, that is they display both viscous (fluid-like) and elastic (solid-like) properties. Visco-elastic materials are part of a larger class of fluids known as non-Newtonian fluids. These are fluids whose viscosity changes with an applied shear force—how fast they are being stirred/pushed. Water, a Newtonian fluid, will have the same viscosity no matter how fast or slow you stir it. A shear-thinning fluid is one where the material becomes thinner the faster you stir or push it; milk exhibits this behavior. The converse of this behavior is a shear-thickening fluid, where the fluid becomes thicker the harder you stir or push. Corn starch in water is often used to illustrate this type of behavior, one can slowly stir it as you would expect, but if you hit it hard enough it will behave like a pliable solid and break. This mixture is sometimes called Oobleck, from the Dr. Seuss story "Bartholomew and the Oobleck".
A group of researchers at the University of Texas at Austin discovered a new and interesting phenomenon that occurs when you shake a mixture of cornstarch and water. Their experimental setup was a shallow, flat dish that contained the mixture and connected to an oscillating mechanism. This allowed them to vertically oscillate this mixture at varying frequencies and peak accelerations. What they found was that under certain conditions they were able to poke holes in the fluid that would remain stable! Think about this is relation to water: if you poke you finger into a glass of water and remove it, the surrounding water will rapidly fill the void left by the removal of your finger. The research team found that the holes were stable for very long times and that they did not interact with each other as long as they were over two diameters from one another. This meant that the holes did not order themselves into a regular arrangement.
The research team created a phase diagram for this system as a function of acceleration and frequency. They found four distinct regions: unstable, meta-stable, stable, and delocalized. Unstable is just like it sounds—the holes collapse quickly after formation. In the meta-stable region holes would form, however they would collapse after some amount of time (<100,000 cycles of the mechanism). The stable region is where the holes would exist for very long times and not collapse. The final region—delocalized—is very interesting. It was found that the rims of the holes in this region would experience a build up of material. This build up of material (the finger) could grow as high as 2cm, before falling back down and creating a new hole where this process would be repeated. This would become a chain reaction that would spread across the surface, turning it into a writhing mass.
For those thinking, "What's so new about this? People have known that corn starch solutions have odd behavior for years," the research team also showed that the same phenomena occurs for solutions of small glass beads, 1-20 microns in diameter. While the exact mechanism of why this happens is unknown at this time, the researchers believe it be due to the shear thickening behavior of the fluid. Shear thickening fluids have a "critical shear rate," where they transition from a thick fluid to a very thick fluid, it is theorized that the shear rate at the edges of the holes is very close to this critical shear rate allowing the holes to be stable.
The researchers have a website where you can not only download the paper they wrote on this topic, but a movie of the various behaviors as well. The site has some very interesting pictures of both the cornstarch experiment and the glass sphere experiment. The movie can be downloaded from the main site, and is mirrored here.
~ Writeup by Matt "zeotherm" Ford