Oobleck: The Dilatant Fluid

Introduction

An interesting category of fluids are those dubbed Non-Newtonian Fluids. These substances, unlike their counterpart and namesake Newtonian Fluids, do not possess viscosities that are single-value constants. In this demonstration, we will show how to make a non-toxic and inexpensive non-Newtonian fluid, called oobleck, and also show some of its more interesting properties.

Theory

A Newtonian fluid behaves in a manner described by:

Where τ is the shear stress for the fluid, μ is a constant that represents the viscosity of the fluid, and du/dy is the shear rate (or velocity gradient perpendicular to the shear stress).

In Newtonian fluids, the viscosity is a constant that usually only varies with factors like temperature and pressure. Fluids whose shear stress does not vary linearly with deformation rate are called non-Newtonian fluids. The graph below can give the reader an idea how various non-Newtonian fluids behave compared to Newtonian fluids.

Two common types of non-Newtonian fluids should be mentioned. One type is called shear thinning fluids, or pseudoplastics. These fluids have a viscosity that decreases with increasing deformation rate. These fluids tend to settle into a solid/plastic-like state but will become more like a fluid the larger a shear stress is applied. Examples include ketchup, toothpaste and modern paints. The other common type of non-Newtonian fluid, and the type we will be investigating in this project, is called shear thickening fluids, or dilatant fluids. Dilatant fluids are the opposite of pseudoplastics; their viscosity increases with increasing rate of deformation. These fluids tend to get more "syrupy" or even possess solid-like properties the faster it is deformed.

Demonstration

To create our cheap dilatant fluid, which is commonly known as oobleck, a reference to the Dr. Seuss book Bartholomew and the Oobleck, we simply mix approximately 2 parts corn starch to one part water. Red dye was added to distinguish the oobleck, which is otherwise a less noticeable pale colour. We then applied shear force in various ways to demonstrate its solid properties, while also indicating that it was indeed very much a fluid the whole time.

The oobleck experiences the pull of gravity as it trickles down, becoming more viscous in the process

As long as I keep rolling this ball of oobleck, it will remain solid

Link to demonstration: http://www.youtube.com/watch?v=STl3E5dS7U0 (Saving in full HTML destroys my images)

Discussion

Dilatant fluids, and non-Newtonian fluids in general, are part of a group of materials called "smart materials" that change their properties in reaction to external stimuli. As such, they have many useful engineering applications.

As dilatant materials will thicken under shear, they have are presently used in industrial applications to deliver a torque smoothly. For example, the viscous coupling units used in some all-wheel drive systems allowing for an even transfer of power between the front and rear drives. Although this use is limited to lower torque applications as the dilatant fluid's viscosity will only increase so much.

Research is underway to develop body armor that employs dilatant fluids to stop and sudden impact, from a quick knife stab to a bullet. This technology would allow the wearer to have maximum flexibility the majority of the time and would only activate upon sudden hard impact. A dilatant fluid armor would disperse the impact from an object much like old -fashion chain mail would, though over and even larger part of the body. The flexible nature of the fluid would also allow the wearer to be more completely protected (i.e., there would not be much unguarded areas as the fluid armor suit would easily wrap over any part of the body).

Conclusion

Dilatant fluids have viscosities that vary with the rate of shear. This property is has applications in industrial settings for transferring torque. Research is also going underway for the use of dilatant fluids in body armor. You too can make your own dilatant fluid by mixing about two parts cornstarch with one part water.

References

• R. W. Fox, P. J. Pritchard, and A. T. McDonald Introduction to Fluid Mechanics 7th Ed., John Wiley & Sons Inc., USA (2009).
• http://www.ccm.udel.edu/STF/pubs1.html
• http://www.instructables.com/id/Oobleck/
• http://en.wikipedia.org/wiki/Non-Newtonian_fluid