Shear Thickening Fluid - Hammer Press

Here you can see that when the hammer is merely pressed into the fluid, it is easily penetrated. This is due to the low level of shear force that the hammer is applying to the mass. When the shear level is low, the fluid will not thicken and the hammer sinks right through.

The simple explanation:
This substance displays four different properties on four different time scales. At very long timescales, it behaves like a liquid spreading out onto a flat surface. At moderate timescales, the substance stretches like a plastic solid. At short timescales, it bounces like an elastic solid. And, at very short timescales (the impact of a high-speed projectile), the substance simply shatters.

This substance is a dilatant compound, which means it becomes solid under the influence of pressure. If you've ever played with Oobleck or Smart Mass Putty, you've experienced the same properties.

The complicated explanation:
Some of the behaviors listed above are an example of a dilatant compound, where the viscosity (i.e. the resistance to flow) increases faster than the strain rate. Although not common, some materials do exhibit dilatant behavior (concentrated aqueous corn starch suspensions -- read: Oobleck).

However on its own, this is not enough to explain the behavior of this substance. In fact there are two mechanisms (and hence two characteristic time scales) at work.

The high molecular weight of the primary ingredient PDMS (Poly Dimethyl Siloxane) has a characteristic polymeric relaxation time, defined by the time that a random walk allows the chain to relax from a stretched state through thermal vibrations.

However due to the Boric acid (the fourth ingredient by weight), there are also transient Boron mediated crosslinks arising from associating Boron linkages. These act to give the substance a behavior more like an elastic solid than a liquid.

However since these crosslinks are dynamic, the material is not permanently locked in place and can consequently flow under the correct conditions. Therefore at longer time scales, the substance behaves like a high molecular weight polymeric fluid.

Over very short time scales the putty behaves like a crosslinked elastic solid, shattering like a ceramic on impact.

View my other videos for a demonstration of all four behaviors.

Category:

Howto & Style

Tags:

• Shear
• Thickening
• Fluid
• STF
• dilatant
• compound