Q. What are Newtonian Fluids and Non-Newtonian Fluids, and how do I escape from quicksand?

PIPE-FLO can be used to analyze systems with Newtonian fluids.  The program does not handle Non-Newtonian fluids.

A fluid is defined as a substance that undergoes continuous deformation when subjected to a shear stress.  The resistance of the fluid to this deformation is ordinarily measured in terms of the fluid property called viscosity.

Newtonian fluids have a constant viscosity at a given temperature.  For these fluids, the viscosity is independent of the shear stress rate of change.  Examples of Newtonian fluids are water, mineral oil, and gasoline.

A shear diagram for a Newtonian fluid is shown below.  Note that the slope is constant.

Non-Newtonian fluids have a variable viscosity at a constant temperature.  The viscosity for these fluids varies with the rate of shear of the fluid.  Examples of Non-Newtonian fluids are ketchup, blood, yogurt, gravy, and quicksand.

A shear diagram for a typical Non-Newtonian fluid is shown below.  Note that the slope is not constant.

If you picture a fluid as a series of layers, the resistance to flow arises from the friction between the fluid layers.  According to Newton, when sliding one layer over another layer twice as fast, the resisting force is two times as great.  Non-Newtonian fluids do not follow this model.  For these types of fluids, doubling the speed does not necessarily double the resistance.  It may more than double the resistance (shear thickening, like corn starch in water), or it may less than double it (shear thinning, like ketchup).

Safety Tip:  If you ever fall into a pit of quicksand, its Non-Newtonian behavior is the reason why you should wiggle out slowly rather than frantically thrashing about.  If you add stress to the quicksand, it's viscosity decreases and it acts as a liquid which is why you originally sink.  However, after its initial liquefaction, quicksand's apparent viscosity increases substantially due to the formation of sand sediment (caused by the removal of water from the quicksand around your legs).  In order to loosen the sand sediment, you must re-introduce water into the mixture by wiggling your legs to create areas for water to come into contact with the sand sediment.  This reduces the viscosity of the sand sediment around your legs allowing you to escape.