And non-newtonian fluid It is one whose viscosity varies with temperature and applied shear stress. At rest these fluids behave like a liquid, while if they are subjected to a stress force their viscosity increases. Examples of non-Newtonian fluids are ketchup, chocolate, syrup, honey, mayonnaise, and… quicksand.
In one of the scenes of ‘The Kingdom of the Crystal Skull’ (2008) Indiana Jones he ends up with Marion in some quicksand. There is an exchange of reproaches and the teacher asks her not to move because she “creates space and she will end up sinking.”
The truth is that quicksand has been one of the most repetitive resources in the seventh art. The scene is always the same: a man screams in despair, begs for help, and the more he tries to escape, the more he sinks into the sand, until he finally disappears. But what is true in all this?
A solid that behaves like a liquid
The quicksands (quick sand) are formed, basically, by a very thin mixture of silts or muds –very fine particles of clay- fully saturated with water, forming a colloidal hydrogel. Its surface is rough and the friction between the grains gives stability to the soil.
The space between grains in quicksand accounts for 30-70% of the total volume, while in non-quicky sand it is reduced to 25-30%. In other words, the space between the grains is so great that the friction decreases to the point that it stops acting like a solid and acts like a liquid.
Quicksand forms on the banks of deep rivers, such as the Amazon or the Mississippi, as well as on the banks of rivers or lakes, in swamps or on beaches with high tides.
One of the most famous places in the world, as far as quicksand is concerned, is the morecambe bay (England). It is the largest expanse of tidal flats and sands in the UK, the place where the Rivers Leven, Kent, Keer, Lune and Wyre flow into a huge salt marsh.
sinking dynamics
Daniel Bonn, a professor at the University of Amsterdam, became interested in this phenomenon and studied the dynamics of subsidence. He observed that when stepping on the ground, the sand begins to liquefy and the water and land separate, leaving a strip unable to support the weight.
He reproduced the situation in his laboratory and came to the conclusion that 100,000 Newtons are required to free a foot and that salt increases instability in quicksand.
Bonn and his team concluded that a person sinking in quicksand cannot be fully swallowed, it only sinks halfway. In other words, that the dramatic scene with the horse Artax in ‘The Neverending Story’ would never have taken place.