Sunday, 15 July 2012

Impact Induced (liquid-solid) Temporary State Change In Non-Newtonian Fluids



A Newtonian fluid obeys the general mathematical formula:

where tau represents the shear stress exerted by the fluid, mu represents the fluid viscosity (the constant of proportionality and the differential w.r.t to y represents the velocity gradient perpendicular to the shear (strain rate).

Any fluid that disobeys this equation is said to be non-Newtonian and displays varying properties when different strain rates are applied.

Examples of non-Newtonian fluids:

Oobleck: This is a simple example of a non-Newtonian fluid as it is just a suspension of starch in water.

Where the name of the substance originated from

Flubber: Consists of a substance which is based on the cross linking between a polyvinyl alcohol and a boron compound. It is also classed as a Maxwell solid (named after James Clerk Maxwell) as it flows easily under low stresses but can break under great impact.



Another non-Newtonian fluid is quick sand, which we can walk on if our steps are quick enough. This unusual property has never been thoroughly examined by physicists. Prior to Scott R. Waitukaitis and Heinrich M. Jaeger research this 'quicksand' effect was thought to be due to sudden energy distribution towards the surface allowing for objects to temporarily not penetrated the fluid.


This theory was shown false according to Jaeger's and Waitukaitis' experiments, which consisted of impacting oobleck in a container using different blunt objects such as a metal ball and a flat head rod at varying speeds. They used techniques such as x-ray monitoring to see what was going on below the surface and lasers to see how the surface changed upon impact and after. The pair also used different sized containers, the behavior of the oobleck remained unchanged hence the theory of 'energy-surface distribution' was shown false.


A new explanation has been given, which is explained thoroughly and numerically in their paper. The sudden hardening of non-Newtonian fluids upon impact is due to suspended particles clumping together in a cone like shape within the fluid. This cone like three dimensional shape is much denser than the fluid under low stresses and is dense enough to be classed as a solid, this explains how we can walk on quick sand if we are quick enough. 


More research could unveil more properties of non-Newtonian fluids and we could use new materials and use their properties in industry, vehicles, packaging and other impact based safety measure.

http://www.nature.com/nature/journal/v487/n7406/full/nature11187.html


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