What is a Non-Newtonian Fluid?
Non-Newtonian fluids are fluids that do not satisfy the Newtonian viscosity experimental law, that is, fluids whose shear stress and shear strain rate are not linear. Non-Newtonian fluids are widely found in life, production, and nature. Most biological fluids belong to the non-Newtonian fluids currently defined. Various body fluids such as blood, lymph fluid, cystic fluid, and "semi-fluids" like cytoplasm are all non-Newtonian fluids.
Non-Newtonian fluids are widely found in life, production, and nature.
Most biological fluids belong to the non-Newtonian fluids currently defined. Various body fluids such as blood, lymph fluid, cystic fluid, and "semi-fluids" like cytoplasm are all non-Newtonian fluids.
Concentrated solutions and suspensions of high molecular polymers are generally non-Newtonian fluids. Polyethylene, polyacrylamide, polyvinyl chloride, nylon 6, PVS, celluloid, polyester, rubber solution, various engineering plastics, chemical fiber melts, solutions, etc., are all non-Newtonian fluids. Petroleum, mud, coal water slurry, ceramic slurry, pulp, paint, ink, toothpaste, silkworm regeneration solution, well washing and completion fluids for drilling, magnetic slurry, coating liquid for certain photosensitive materials, foam, liquid crystal, High sandy currents, mudslides, mantle, etc. are also non-Newtonian fluids.
In the food industry, tomato juice, starch liquid, egg white, apple syrup, concentrated sugar water, soy sauce, jam, condensed milk, agar, potato milk, melted chocolate, dough, rice noodles, and minced meat such as minced fish and meat emulsion Both are non-Newtonian fluids.
Jet swell (also known as Barus effect, or Merrington effect) (Figure 1)
Figure 1 Swelling of cheese jet outlet
If a non-Newtonian fluid is forced from a large container into a capillary tube and then flows out of the capillary tube, it can be found that the diameter of the jet is larger than the diameter of the capillary tube. The ratio of the diameter of the jet to the diameter of the capillary is called the die swelling ratio (or the extrudate swelling ratio). For Newtonian fluids, it depends on the Reynolds number, and its value is between 0.88 and 1.12. For polymer melts or concentrated solutions, the value is much larger and can even exceed 10. In general, die swelling is a function of flow rate and capillary length. Die swelling is very important in die design. When the polymer melt flows out of a rectangular cross-section nozzle, the swelling at the long side of the tube cross section is more significant than that at the short side. Especially in the center of the long side of the tube section, the swelling is greatest. Therefore, if the cross-section of the product to be produced is required to be rectangular, the shape of the die cannot be rectangular, but must be a shape in which all four sides are recessed.
Climbing rod effect (also known as Weissenberg effect) (Figure 2)
Figure 2 Rod climbing effect of non-Newtonian fluid (right)
In 1944, Weissenberg publicly performed an interesting experiment at Imperial College, London, England: in a beaker with only viscoelastic fluid (non-Newtonian fluid), rotating the experimental rod. For Newtonian fluids, the liquid surface will be concave due to centrifugal force; for viscoelastic fluids, it will flow toward the center of the cup and climb up the rod, and the liquid surface will become convex, even when the experimental rod rotation speed is very low This phenomenon can also be observed. When designing a mixer, the effects of the climbing effect must be considered. Similarly, this effect should be considered and utilized when designing non-Newtonian fluid transport pumps.
No tube suction or open siphon (Figure 3)
Figure 3 Opening siphon
For Newtonian fluids, during the siphon experiment, if the siphon is lifted off the liquid surface, the siphon will stop immediately. But for polymer liquids, such as the gasoline solution of polyisobutylene and a 100% POX aqueous solution, or the slightly condensed limb system of glycan in water, it is easy to perform the siphonless experiment. When you slowly lift the tube from the container, you can see that although the tube is no longer inserted in the liquid, the liquid is continuously drawn out of the cup and continues to flow into the tube. Even simpler, do not even siphon, slightly tilt the beaker filled with the liquid to make the liquid flow down. Once the process starts, it will not be stopped until the liquid in the cup has flowed out. This ductless siphon characteristic is the basis for the spinnability of synthetic fibers.
Turbulent drag reduction (also known as the Toms effect) (Figure 4)
Figure 4 Turbulence drag reduction: water spray from fire faucet under the same power
Another wonderful property that non-Newtonian fluids show is turbulent drag reduction. It has been observed that if a small amount of polymer is added to a 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, the addition of a small amount of polymer additives to Newtonian fluids has shown drag reduction effects. Some people report that after adding polymer additives, the burst period is measured to increase, which is considered to be the role of the polymer chain. Although the reason for the turbulent drag reduction effect has not been clearly understood, it has a good application. Adding a small amount of polyethylene oxide to the fire water can more than double the head of the water sprayed from the fire truck faucet. The application of polymer additives can also improve the cavitation process and its damage.
Other properties
In addition to the above-mentioned interesting properties of non-Newtonian fluids, there are other wonderful properties that have been valued by people, such as wire drawing (can be stretched into very thin filaments, see the article "Spring Silkworm to Dead Silk") Shear thinning (see the article "Healing of Tendon Sheath Cysts"), continuous drip effect (the droplets formed by the free jet are connected by fluid rods), fluid rebound, etc. [1]
