Newtonian and non-Newtonian fluids have recently been of active interest not only to scientists, but also to ordinary people. This is due to the fact that non-Newtonian fluid is easily made with its own hands and is suitable for home experiments. To begin with, we will understand what kind of substances they are. Newtonian fluid obeys Newton’s viscous friction law, which is why it got this name. According to this law, the shear stress in the contact planes of the fluid layers is directly proportional to the derivative of its flow velocity in the direction of the normal to these planes.
It sounds rather complicated, but it will be more clear to the reader if we say that Newtonian fluid is water, oil, and most of the fluids that are familiar to us in daily use, that is, those that retain their state of aggregation, whatever you would do ( if it’s not about evaporation or freezing, of course). But if the dependence described in the definition above is inversely proportional, we can talk about non-Newtonian fluid.
Such a liquid is always heterogeneous; it contains large molecules that gather in crystal lattices; therefore, the viscosity directly depends on the flow rate of the compound. The higher the speed, the higher the viscosity. Partially, thixotropic liquids are attributed to this type of substance, that is, those that change viscosity over time, for example, putty or chocolate. Also, some scientists are inclined to consider blood a substance that does not act according to the laws of Newton’s viscous friction, because it is a heterogeneous liquid, it is a suspension of plasma and many blood cells. Any doctor will confirm that blood viscosity can vary in different parts of the vascular system, which is often a pathology. However, not every substance is in principle capable of such metamorphoses.
Non-Newtonian fluid can be prepared at home very easily. You need to take 1.5 parts of starch (ideally corn, but also potato is suitable) and one part of water. The ingredients should be mixed slowly so that there are no lumps. Ideally, it is worth distributing it with a sufficiently thin layer on the baking sheet, but, of course, any interactions can be experienced. Try to quickly “sweep” the liquid with your fingers, and it will look and feel like a frozen plastic mass. Relax your fingers and the liquid will drain. Newtonian fluid is not capable of such tricks! You can pick up a substance in a handful and start tossing it up. Very soon it will become viscous and plastic, and therefore as if it will dance in your palms - this is a very interesting sight! Roll the liquid into a lump, it will be elastic and pleasant, but if you relax your palm, it will spread. It is interesting to add dyes to it to play with children. Some go further and even try to run around non-Newtonian fluids, roll objects on it and so on, but for such experiments, of course, much more material is needed than for home experiments. You can find many video reports and continue to explore the fascinating world of physics.