Tikalon Blog by Dev Gualtieri

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Shear Thickening

March 14, 2014

Most readers of this blog should be familiar with the special properties of certain corn starch -water mixtures. This substance is sometimes called oobleck after a similar substance described in Bartholomew and the Oobleck, a 1949 book by Theodor Seuss Geisel (a.k.a., Doctor Seuss).

The usual formulation is one part water to 1.5-2 parts cornstarch, by volume . Corn starch is insoluble in water, so the mixture is a suspension of fine particles in a water solution, a colloid . This mixture is a non-Newtonian fluid .

Simply, this means that you can easily poke your hand into the mixture if you do this slowly; but, if you punch the mixture, it's as hard as a rock . You can roll the mixture into a ball with your hands, but it slumps back into a liquid when you stop working it. There are several interesting online video demonstrations of the various properties of Oobleck,[1] and an accessible tutorial article.[2]

We can best explain the properties of non-Newtonian fluids by first describing a Newtonian fluid . A Newtonian fluid has the linear mechanical property that its viscous stress ("drag") is proportional to the local strain rate . For an isotropic fluid , the local strain rate is the rate of change of the fluid's velocity vector as it flows perpendicular to the shear direction. This is expressed by the equation ,

where τ is the shear stress, μ is the shear viscosity of the fluid, and du/dy is the derivative of the velocity component in the shear direction, relative to the perpendicular direction. Water is a good example of a Newtonian fluid.

The shear viscosity of a non-Newtonian fluid is not constant, and it may even be time-dependent . The difference in mechanical properties between a Newtonian fluid and a dilitant fluid, such as Oobleck, can be seen in the figure. Oobleck can be cycled up and down the same curve; that is, the process is reversible .

Newtonian and dilitant fluids

Newtonian and dilitant fluids

(Illustration by the author using Inkscape .)

Oobleck was always thought to be a non-Newtonian fluid of the dilitant type called shear-thickening; that is, it becomes thick in response to shear stress. In 2012, Scott R. Waitukaitis and Heinrich M. Jaeger of the James Franck Institute and Department of Physics , the University of Chicago (Chicago, Illinois), performed experiments that showed the effect was more akin to the jamming transition of granular media .[3-5] While shear-thickening can explain such mixtures' response to small forces , the Chicago experiments showed why they're able to support the weight of a human running on their surface .

Waitukaitis and Jaeger calculated that the material would need ten times more shear stress than it has to support a running person.That's because running generates more compression than shear . To prove this by experiment, they shot a 370 gram aluminum rod into a cornstarch colloid at a meter per second and recorded the results with high speed photography and X-rays .[5]

The impacting rod compressed the cornstarch particles so that they jammed against each other to form a rigid core. This is a known phenomenon for granular materials, and such rigid cores exert an upward force sufficient to counteract applied pressure . A movement into the cornstarch colloid of just five centimeters forms a core to a depth of 50 centimeters.[5]

Stress distribution in a granular medium

Compressive stress distribution in a granular medium.

The compressive stress is distributed to lower particles in roughly a cone -shaped core.

(Via Wikimedia Commons .)

This idea that shear-thickening mixtures have much in common with granular media has been modeled by researchers at the City College of New York (CCNY, New York, New York). Their model, which included hydrodynamic interactions and contact between the particles, showed that contact friction is essential for having discontinuous shear thickening.[6] Colloids with a large enough volume fraction of particles show two states. There's a low viscosity state with few particle contacts, a high viscosity state like a jammed granular material, and a critical shear rate that transitions from one state to the other.[6]

According to the model, this is a second-order phase transition , since the viscosity is discontinuous at the critical shear rate. The transition is reversible, since the colloid behaves like a normal liquid when the shear is removed.[7] As shown in the figure, particles become linked together above the critical shear rate, and the lubricating liquid is not able to flow between them.[9] The particles in the colloid act as a dry, granular medium. The CCNY model accurately reproduces experimental observations.[7,9]

Discontinuous shear thickening of frictional hard-sphere suspensions

Discontinuous shear thickening of frictional hard-sphere suspensions at a 52% volume fraction. At low shear, there are few particles in contact (right). After a critical shear rate, there is an abrupt transition with formation of many contacting particles.(left and right images via City College of New York .)[8]

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