# Non-Newtonian Fluids…And Superfluids

Some independent research from one of our RGS Physics pupils:

Non-Newtonian Fluids

Non-Newtonian Fluids are liquids that, unlike Newtonian fluids (for example water), have a variable viscosity when under stress. Viscosity is technically speaking how resistant a liquid is to gradual deformation by shear stress, basically speaking (my preferred explanation) viscosity is how thick a liquid is,  with the unit Pascal seconds (or in SI units (kg/SxM). The viscosity of a liquid depends on a combination of the size and shape of the particles in it and therefore the friction between them when the fluid flows, Non-Newtonian Fluids have a viscosity that depends on its’ conditions. What this means is that when there is an applied pressure the substance, which is a liquid, becomes solid, immediately after the impact it starts to liquify again (as seen in quicksand). It appears to change state while under stress. This happens because the particles in a Non-Newtonian fluid are packed tighter and closer than in a normal liquid, when the fluid is poured or mixed slowly the particles can slip past each other so it gives the appearance of a liquid, but if there is a sudden application of pressure the particles don’t slip past each other but instead pack tightly like a solid. However this is just one way fluids can disobey Newton’s laws for fluids, there are also Non-Newtonian fluids that get less and less viscous as a greater pressure is applied. A third type are Non-Newtonian fluids that get less viscous as the length of time pressure is applied increases.

Those Non-Newtonian fluids that lose viscosity as the length of time they are under pressure increase are known as Thixotropic and have a huge number of examples including; Yoghurt, many paints and Synovial fluid (found in joints in animals)

Non-Newtonian fluids that become less viscous with an increase in pressure are know as ‘Shear-Thinning’ or pseudo-plastic examples of this are; Ketchup, Nail varnish, syrups and blood.

Liquids that seemingly become solidified when under pressure are known as  ‘Shear-Thickening’ or ‘Dilatant’ Non-Newtonian Fluids and include as examples; sand in water (a sort of quicksand), Silly Putty and finally a combination of corn starch in water.

Of these types of Non-Newtonian fluids ‘Shear Thickening’ fluids have the most real world uses, excluding the many extremely cool experiments you can see on YouTube of which my favourites are:

Running on Liquid;

and putting a the mixture of cornstarch and water on a subwoofer, which is actually quite creepy;

Research is going into using Shear thickening fluids for many different applications, including Bulletproof vests and the filling of potholes. The fluids are perfect for bulletproof vests because of how easily they could be incorporated into a comfortable piece of clothing yet still reliably block a bullet on impact, even removing most of the force then impact of the bullet when it hits. Some scientists have suggested that ‘shear-thickening’ fluids would be perfect for filling potholes, the actual filling itself would be easy because of the liquid properties and it will then solidify when the wheels of the car run over it.

Superfluids

There are certain occasions when a liquid can in fact have zero viscosity or zero thickness, which is a state of matter known as superfluidity. This effect was discovered in 1937 by Pyotr Kapitsa, John F Allen and Don Misener. Helium 3 and 4 (isotopes) have been made into superfluids in the past, they have to be cooled to below their liquefaction point (which itself is extremely low at about 4 Kelvins) and it can only be liquefied using the Hampson-Linde cycle. It is therefore extremely difficult to make and sustain Helium as a superfluid but it does have some incredible properties, with a zero viscosity it flows with no friction and can escape containers literally by flowing up the sides of the container against gravity and produce fountains that in theory would last forever (although only as long as the supefluidity state is maintained).

Superfluid Helium was also used to trap and slow down light, in 1999 a Sodium Bose-Einstein condensate was used for this and was found to slow the speed of light from 299,792,458 metres/seconds to just 17 metres/seconds.