In life, we may encounter such a situation. If there is a bowl of water, it will feel easier to stir when we keep stirring it rapidly with chopsticks in a certain direction, and the water in the bowl will show a concave vortex in the center at the same time. However, if we add flour to this bowl of water, the result will be different. At this time, if we stir again, we will find that the harder it is to stir, and the batter climbs up along the chopsticks. Why is there such a contrast phenomenon?
In fact, in the classification of fluids in soft condensed matter physics, water and batter belong to two kinds of fluids with different properties, namely Newtonian fluid and non-Newtonian fluid. Many common fluids in nature, such as water, alcohol and air, are called "Newtonian fluid". For most low molecular fluids (with small molecular weight), the viscosity coefficient remains basically fixed due to their isotropic characteristics. This kind of fluid is Newtonian fluid. The viscosity coefficient of non-Newtonian fluid changes with the shear rate. The higher the shear rate, the stronger the viscosity. In other words, if you want to stir this kind of fluid quickly, you have to work harder, and the faster the speed, the harder it feels. This kind of non Newtonian fluid includes many high molecular weight (high molecular weight) solutions and suspensions.
Flour paste is a kind of natural fluid, not a polymer paste. Therefore, when we stir the batter, the faster we stir, the faster the viscosity of the batter will increase, and the result will naturally be more difficult to stir.
At this point, we can answer the above questions more specifically. For water, when we keep stirring it with chopsticks, the water molecules diffuse to the periphery due to the action of centrifugal force, so there will be less water in the center, which looks like a concave vortex, which leads to easier stirring. However, for the batter, the flour polymer forms an anisotropic structure in the stirring, and the polymer chain will be stretched and wound on the chopsticks. The greater the shear force, the higher the degree of stretching, and the polymer chain itself will produce stronger recovery elasticity. In this way, these flour polymers are squeezed to the center, resulting in the "climbing phenomenon" (also known as Weisenberg effect), which leads to the more difficult to stir.