Question

When corn starch is added with water at 2:1 ratio and you apply strong force, the mixture will behave like solid. However, when you apply slow and steady force on the mixture, it will behave like liquid. Describe how this mixture can behave as a solid and liquid when different force is applied? Mention the type of non-Newtonian fluid for this mixture.

Answer 1

The corn starch and water mixture in 2:1 ratio sometimes acts as a solid while liquid at other times. It is so because the cornstarch forms a suspension with water where one which is finely divided is dispersed into another. In this case cornstarch disperses in water. When a strong force is applied to cornstarch solution the force allows the long starch molecules to come together. This force results in trapping of water molecules between the long starch chains to form a semi rigid structure. That is why cornstarch behaves as a solid when a strong force is applied. When the force/pressure is removed from the cornstarch mixture, it flows again. It flows because of a property known as viscosity. As a slow and steady force is applied on the cornstarch water mixture, its viscosity changes which helps in the movement of the mixture.

Corn starch with water is an example of shear thickening fluid which is also known as dilatant fluid.

As the force applied on the mixture increase, viscosity increases in this case, at lower stress there is not much increase in viscosity hence it is flowing at lower stress/force.

There is a diagram attached below which shows the relation between shear rate and shear stress of dilatant fluid. Shear stress the force per unit area applied on the material and shear rate is the rate of change of velocity. As you can see in the diagram that at very high shear stress the increase in shear rate is not much whereas at very low shear stress it follows linear relationship with shear rate.