If you couldn’t perform the experiment, here is a video:
Brief explanation: The water that was pushed outwards by stirring flows back to the center when you stop stirring, taking the tea with it. This does not happen directly, but due to inertial forces, the flow remains circular or spiral.
More precisely, it can be explained as follows: During stirring, the centrifugal force mainly acts, pushing the water and leaves outwards. Since the wall of the cup prevents the tea from moving further outwards, a directed pressure force arises as a counter-reaction. When you stop stirring, the centrifugal force slowly subsides, so the pressure force plays a greater role. In addition, there is a boundary layer at the bottom of the cup where the flow is strongly slowed down. Since the bottom of the cup does not move and frictional forces act between the water particles and the bottom, they can no longer move so quickly there. The centrifugal force is therefore much smaller in this layer, but the inward pressure force is equally large, so that overall a force acts inwards.
The water flows inwards with the tea leaves and then rises in the center of the cup. Due to gravity, the tea leaves do not rise with it but collect at the bottom.
And what does this have to do with engineering?
Flows play a central role in engineering, especially in areas such as mechanical engineering, civil engineering, process engineering, and aerospace engineering. Here are a few examples:
- To design vehicles, aircraft, pumps, or pipeline systems, engineers must understand how liquids and gases flow.
- The forces exerted by flows must be calculated or estimated to design structures that can withstand wind and water forces, such as bridges, buildings, and dams.
- In environmental engineering, flows are important, for example, for predicting the spread of water and air pollution or for calculating wastewater treatment plants.
- For cooling systems, heating, and air conditioning, heat transfer through flows must be calculated.
Since flow processes can be very complicated, there are various methods and simplifications to find solutions for practical problems. One example is the consideration of boundary layers.