Non-Newtonian fluid refers to a fluid that does not satisfy Newton's experimental viscosity law, that is, the relationship between its shear stress and shear strain rate is not linear. Non-Newtonian fluids widely exist in life, production and nature. Most biological fluids belong to non-Newtonian fluids now defined. Body fluids such as blood, lymph and cystic fluid, and "semi-fluid" similar to cytoplasm all belong to non-Newtonian fluid.
Features:
Jet expansion (also known as Burroughs effect or Mailington effect) If non-Newtonian fluid is forced to flow from a large container into a capillary and then flow out of the capillary, it can be found that the diameter of the jet is larger than that of the capillary. The ratio of jet diameter to capillary diameter is called extrusion swell ratio (or extrusion swell ratio). For Newtonian fluid, depending on Reynolds number, it is about 0.88 ~ 1. 12. For polymer melt or concentrated solution, its value is much larger, even exceeding 10. Generally speaking, the extrusion swell rate is a function of flow rate and capillary length. Extrusion swell is very important in die design. When the polymer melt flows out of the nozzle with rectangular cross section, the expansion of the long side of the tube is more obvious than that of the short side. Especially in the center of the long side of the pipe section. Therefore, if the cross section of the product is required to be rectangular, then the shape of the mold cannot be rectangular, but must be a shape with four sides recessed.
Rod climbing effect (also known as Wei Sen Berg effect)? Non-Newtonian Fluid 1944 Weissenberg made an interesting experiment in Imperial College London, London, England: rotating the experimental rod in a beaker filled with viscoelastic fluid (a kind of non-Newtonian fluid). For Newtonian fluid, due to the centrifugal force, the liquid level will be concave; For viscoelastic fluid, it flows to the center of the cup and climbs up along the rod, and the liquid level becomes convex, which can be observed even when the rotating speed of the experimental rod is very low. When designing the mixer, the influence of climbing pole effect must be considered. Similarly, this effect should also be considered and utilized when designing the pump for non-Newtonian fluids. ? Suction without tube or siphon (Figure 3)? Non-Newtonian Fluid For Newtonian fluid, in siphon experiment, if the siphon is lifted off the liquid surface, the siphon will stop immediately. However, for polymer liquids, such as gasoline solution of polyisobutylene and 1% water solution of POX, or slight aggregation of polysaccharides in water, tubeless siphon experiment is easy to be carried out. When the tube is slowly pulled out of the container, it can be seen that although the tube is no longer inserted into the liquid, the liquid is still continuously pulled out of the cup and continues to flow into the tube. Simpler, even without siphon. Tilt the beaker full of liquid slightly to make the liquid flow down. Once this process is started, it will not stop until the liquid in the cup is used up. This tubeless siphon property is the basis of the spinnability of synthetic fibers. ?
Turbulent drag reduction (also known as Toms effect) Another wonderful feature of non-Newtonian fluids is turbulent drag reduction. It has been observed that if a small amount of polymer is added to Newtonian fluid, a significant pressure drop can be seen at a given rate. Turbulence has always been an unsolved problem in theoretical physics and fluid mechanics. However, adding a small amount of polymer additives to Newtonian fluid can reduce drag. It is reported that the burst period increases after adding polymer additives, which is considered to be the function of polymer chains. Although the reason of turbulent drag reduction effect is not clear, it has been well applied. Adding a small amount of polyethylene oxide to fire water can make the head of water sprayed by fire hydrant more than double. The application of polymer additives can also improve the cavitation process and its destruction effect. ?
Other attributes? In addition to the above interesting properties, non-Newtonian fluids have other wonderful characteristics that people pay attention to, such as drawing (it can be stretched into very fine filaments, see the article "Filarias in spring will weave until they die"), shear thinning (see the article "Healing of tendon sheath cysts"), continuous droplet effect (the droplets formed by free jet are connected by liquid flow rods), liquid flow rebound and so on.
Ready:? In order to prepare white powder solution of non-Newtonian fluid, the volume ratio of starch to water is about 2 ∶1; If there is too much water, it is not easy to show the characteristics of non-Newtonian fluid, and the proportion can be adjusted by itself.