Fluid mechanics explains viscoelastic phenomena we see every day, such as why water hurts when it hits your hand and why you can walk on a starchy solution and not sink.
Have you ever tried to belly dive to the surface of the water at a pool or beach, or flatten your hand to splash water while playing in the water and hit the water hard? In an experiment on SBS’s “Curiosity Heaven,” where starch was dissolved in water and a person walked on top of it, it was shown that if the walker rolled or walked around without stopping, the starch solution would not sink into the water, but if the walker stood still for a while, the water would sink like a swamp. If you don’t know this phenomenon, it is easy to think that if a person falls from a bridge over the Han River, most of them will drown and die, but in fact, given that the height from the bridge to the water surface of the Han River is about 40 to 50 meters, the force felt by the falling object at the moment it hits the water surface is not much different from the impact felt when falling on the asphalt at the same height, so death and accidents due to fractures and ruptures are a bigger problem.
When we study a substance, we often think of elasticity and viscosity as two separate properties, and when the substance flows or changes, we study its properties based on its elasticity if we perceive it as a solid, or its viscosity if we perceive it as a liquid. Or, conversely, we determine whether elasticity or viscosity is more dominant, and that determines whether the substance is a solid or a liquid. However, as you can see from the examples above, when we observe the behavior of real materials closely, they have both solid and liquid properties, so in fluid mechanics, we use a property called viscoelasticity to combine elasticity and viscosity. This viscoelasticity allows us to study a wide range of materials without being limited to a specific area. When studying both solids and liquids, fluid dynamics uses the Deborah number (De) to qualitatively describe the time scale, which is shown below.
Where t is the intrinsic characteristic time of the substance and T is the characteristic time of the external change process, the intrinsic characteristic time of the substance refers to the relative time that each substance has. For example, for liquid water, t is typically on the order of seconds, while lubricants that coat the surfaces of gears in machinery are on the order of seconds, and polymers used to mold plastics are on the order of seconds. At this level, the behavior of liquids goes beyond that of simply viscous liquids and begins to exhibit the characteristics of elastic solids.
For the same material, if De is large, it behaves like a solid, and if it is small, it behaves like a liquid. In the example above, where we hit the water very hard, the value of T is smaller than the value of T when we normally immerse our hands in stagnant water to wash our hands or wash our face, so the value of De is larger in this case, making the water feel more “solid” than we normally feel. On the other hand, the velocity of an object falling in free fall from a height of 50 meters before it finally hits the surface of the water is much higher, so the value of T is much smaller and the impact is much greater. We can also explain that a solution of starch in water, which is more viscous than pure water, will have a larger value of T than water, in which case it is viscoelastic enough that a person walking on it will not fall. However, if you stand still, T becomes larger and De becomes smaller, and the starch solution returns to its “normal” liquid state.
This is an interesting phenomenon that can be seen in everyday life, and it was qualitatively investigated through fluid dynamics. It is not difficult to find solids that turn into liquids and liquids that turn into solids around us. If you think about squeezing toothpaste, there is a bottleneck where the toothpaste in the toothpaste tube suddenly narrows, which is called a narrowing tube in chemical processes. Conversely, there are other types of tubes, such as those that suddenly expand after passing through a narrow tube, straight pipes, and bends that change the direction of fluid flow. Fluid dynamics helps us to quantitatively predict and calculate the flow of fluid inside these pipes, such as how fast it should flow to avoid jamming, how much pressure should be applied at the inlet, and how much pressure should be applied at the outlet to get the flow out. The study of fluids is essential in the chemical industry, which is a large-scale industry, starting with polymers for casting plastics, and therefore fluid mechanics is an indispensable basic discipline in chemical engineering.
