The non-Newtonian fluids A non-Newtonian fluid can be defined as a fluid whose viscosity varies according to the shear stress which is applied.

The Viscosity can be imagined as the force that should be applied to a layer of fluid belonging to the plane fixed to the velocity of the layer placed at a fixed distance. For example a fluid flowing in a tube at different speeds: the minimum speed is in the edge of the section (due to friction) and the maximum speed is at the center. The viscosity in this example is that pressure which is exerted on the wall allows a constant speed on the whole section. The shear stress on the other hand, is one of the primary efforts which may be applied to a body. For example in solid materials the shear stress is a state of tension in which the strength of a material tends to change. A non-Newtonian fluid does not have a defined value of viscosity, which depends on the intensity of the force that is applied at a certain speed, therefore, for a short period of time varies its state (from liquid to solid, or vice versa) acquiring the features. Each non-Newtonian fluid is a fluid, so it follows the laws of Newton on fluid dynamics. According to Newton, in fact, the viscosity of a fluid is independent of the force, at certain speed, to which it is subjected. Some common examples of non-Newtonian fluids are represented by blood asphalt, from toothpaste by mayonnaise, ketchup and starch from corn (used in the experiment). Therefore, it is a liqual i suspension which has these characteristcs because it is formed by a solid phase of fine prticles in suspension in a solvent, which costitutes the liquid phase. � These types of non-Newtonian fluids, whose viscosity depends on the time, are divided into:

- Reopessici flids: are substances which tend to increase their viscosity with the passing of time when subjected to shear forces and such variation of viscosity depends on the speed with which that force is applied. Being formed by a liquid and a solid phase, low speed values, the particles suspended in the liquid act as particles lubricated where the solid phase moves as a function of the motion of the liquid phase. With increasing speed, the solid phase is not sufficiently dispersed in the liquid phase and the solid particles in suspension undergo a phenomenon of "packing". This is due to the absence of Brownian motions, when the suspension becomes more dense in the slid element with respect to the liquid element, and then while maintaining the appearance the liquid form is to all effects physicaly solid. The increase of viscosity, for example, allows the cars can be driven on the beach, in fact the fast rotation of the wheels on the wet sand, causes "caking" of the solid phase, preventing the car to bog down. Another example can be the reaction of the students in a class at the sound of the bell: if students are rushing to the door all together they will form a sort of "cap" that prevent anyone from going out, on the other hand, if every student goes towards the exit in an orderly, line everyone will be able to get out. The reopessia is also the basis of the curious behavior of quicksand: This consists of a mixture of

saturated water and sand. it is generally found at the mouths of the rivers whose water beds are partially filled with sand beneath which is a layer of clay that prevents drainage. The unfortunate person that steps inside, will instinctively stir the saund to get out : unfortunately the shear forces associated with this action causing an increase of viscosity in the sandy masses which are stirred so as to. The corn starch in water gives origin to a suspension with reopessic behavoir; fact reacts the solicitins of a force increasing its viscosity. The rapid application of a force, for example by striking the surface with a finger, causes the fluid to behave as a solid. By applying weaker forces, such as the slow insertion of a spoon in the fluid, the substance will maintain its liquid state, without resistance. The experiment we conducted exploits this capability of the corn starch to vibrate with the sound waves emitted from a subwoofer film on which we put the non-Newtonian fluid. the wibration send a force to the liquid this changing its behavior. Some pictures taken during the construction of our experiment:: -Before:

-After :

-At Last :

Long live Italy!

Thixotropic-fluid (or thixotropic) : these are substances which, in contrast to the reopessic ones , tend to decrease their viscosity with the passing of time when subjected to shear forces. This variation of the viscosity depends on the speed and the intensity with which this force is applied, in fact, if a substance has internal forces (intermolecular) which prevent its movement below a certain value of shear stress, the substance itself will not begin its movement as long as this value is not exceeded. A typical example is that of the paint: it must slide under the shear stress determined by the type of application used (brush or roller), but then must congeal as soon as the effect of the applied force ceases. Other examples are: honey, ketchup, toothpaste and some creams.