Teaching science and physics is not just a talking about topics from the curriculum, but also making sense of the acquired knowledge and connecting it with examples from an individual's everyday life. Surface tension is a phenomenon that is only to a lesser extent present in primary school curriculum, yet it has great importance in nature and everyday tasks. Without it, some animals (such as the water strider) and plants would not live the way they do now. Everyday household chores would be much more difficult without experience about the surface tension of water and the use of surfactants. Even very common phenomena, such as an overturned glass of water or the formation of drops, are directly related to surface tension. In the work, I discuss new experiments that have not yet been fully or at all explored. I focus on the curvature of the liquid surface and the resulting indirect forces between objects floating on the surface, investigate the floating of an ice cube in oil, and finally connect floating and buoyancy with surface tension on the example of ice in a liquid tower of water and oil.
One of the effects of surface tension that students often do not encounter is that a floating object bends the surface of a liquid. As a result, indirect forces appear due to surface tension, which tend to reduce the free surface of the liquid and manifest themselves through the movement of floating objects. Indirect forces arise between them, causing either repulsion or attraction. Thus, two object which the surfaces are wetted by the water attract each other and the same applies to two objects which the surfaces are not wetted. If a surface is wetted in one object and not in another, object then they repel each other. Students know this from experience, that corn flakes floating on milk are attracted to each other and come together. In a partially full container, they move towards the walls of the container, and in full one, with a convex surface, towards the center. The opposite effect is observed with floating metal objects; in a partially full container, they move towards the center of the container, and in an overflowing container, they move towards the walls of the container. I designed activities with interesting tasks and experiments aimed at understanding the occurrence of indirect forces between objects, between which forces do not generally exist.
In the second series of activities, I researched and designed activities about the floating of an ice cube on the layer between water and oil. I explored the surface tension on this layer in more detail, again through activity. It is less common in the literature and therefore more interesting from the point of view of research. I described qualitatively and compared the surface tension that occurs between an ice cube made of distilled, tap and sweetened water and a layer of oil. I systematically monitored the melting of blocks from frozen droplets of different densities. I compared the position of the frozen cubes depending on the different surface tension in the water layer. As the cube melts, a layer of liquid water forms around it, and the cube remains stuck to the layer of water under the oil. When adding the surfactant-detergent, I will monitor the position of the cube in the liquid tower according to the density of the liquid from which the cube is frozen. Interestingly, all three blocks float in the layer between water and oil, but when detergent is added, the blocks from distilled and tap water detach from the water surface and float to the oil surface, while the blocks from sweetened water remain in the layer between water and oil. I found the minimum concentration of sugar in a frozen cube at which the cube still floats to the surface of the oil.
In the third series of activities, I tried to find the concentration of detergent at which the cube will detach from the surface of the water. It is interesting that when the detergent is gradually added, the block first turns and only then floats to the surface of the oil. From this comparison, I estimated the value of the surface tension at the boundary between water and oil. I designed tasks and experiments with which the students could investigate the phenomenon themselves. With the help of all the activities, students will have a better insight into the operation of surface tension and its effects and will connect the acquired knowledge to everyday life.
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