Some liquids exist that present really strange behaviors, due to the chemical composition and molecular structure that forms it or to the type of interaction of those molecules or particles with the solvent, when a solution or suspension is.
We will see one of those cases, that it can be located within the last mentioned group.
We put in a glass some spoonfuls of maize starch (Maizena, p.ej.) and add water as forming a quite liquid pap, tossing around with a teaspoon. From the beginning we will already notice that it is not just like to prepare other mixtures common in the kitchen: it costs enough to move the teaspoon. In fact, it is possible revolver slowly, but as soon as we increased the speed of agitation, the resistance to the movement grows remarkably. With a slow movement sera difficult not to sink the teaspoon until the bottom, but we tried an abrupt movement, will be a great resistance again.
We take the glass with a hand and hagamoslo to move quickly in circles. If the liquid were water or milk, it would already have turned upside down… But that does not happen with the white liquid that we prepared.
Another test that demonstrates the strange behavior of this liquid consists of overturning it to another container in the form of fine spurt. We will see that we do not have a spurt it uniforms perfectly and vertical as it would happen with the water, but oscillates and one moves from a side to another one.
It arrives we see as students of a university walk on a mixture of maizena and water spilled in one picina.
This little common behavior locates to the liquid that we prepared between the nonNewtonian calls Fluid, and to this in particular, between the dilatantes fluids. The calls “moving sands” work in similar form: sand mixture of in which they can be catched animal or unwary person are a water and. Also in this case an abrupt movement causes that the mixture becomes more rigid, impeding or disabling the movements.
Another nonNewtonian fluid that we found in the kitchen is the ketchup: normally it happens that when turning over the container of ketchup the content does not leave, is not spilled. It is necessary to shake strongly so that the content is spilled easily. It happens that the viscosity of the fluid falls when shaking it, promoting the movement of molecules to each other. After some minutes viscosity returns to increase. That behavior is typical of the thixotropic fluid calls, and has considerable importance in some industrial products, as paintings. Of this form, when painting with brush, for example, the movement of molecules is promoted to each other, viscosity falls and the painting extends easily. But viscosity must increase quickly after painting, to avoid the “chorreaduras”…
Video Fluid nonNewtonian