Water Drops: Cohesion and Adhesion In the back of the book Science Instruction in the Middle and Secondary Schools (2002), the authors Eugene Chiapetta and Thomas Koballa suggest that "if you place a drop of water on a piece of wax paper and ask the question, you will have a puzzle that needs to be resolved Does the drop of water roll or slide across the paper? and explained." The book then goes on to describe how the exploration might proceed, explaining that some chalk dust or pepper in the drop will help students see that the drop rolls rather than slides. By way of explanation, the authors state that, "In this situation, we observe the cohesive effects of water molecules permitting a drop of liquid to form and maintain a spherical shape that will roll on a surface without breaking apart." What does this really mean? Is this explanation enough? I decided to play around with this system and see what things I could come up with. Let's start with the drop of water on the wax paper: It is easy to see that the drop seems to have a "skin" holding it into a sort of flattened sphere. It turns out that this is the result of the tendency of water molecules to attract one another (called The surface tension cohesion). lowest energy state for this drop occurs when the maximum number of water molecules are surrounded on all sides by other water molecules -- meaning that the drop should have the minimum possible surface area, which is a sphere. The effect of gravity flattens this ideal sphere into the shape we see. Surface tension can be shown with many other demonstrations, including one in which you add coins to a full glass of water and note how the surface rises well above the rim of the glass: Though teachers often talk about surface tension being like a "skin", this is not really an accurate model, as there is no other compound other than water in the glass. In addition, the surface does not react to being poked
Water Drops: Cohesion and Adhesion In the back of the book Science Instruction in the Middle and Secondary Schools (2002), the authors Eugene Chiapetta and Thomas Koballa suggest that "if you place a drop of water on a piece of wax paper and ask the question, Does the drop of water roll or slide across the paper? you will have a puzzle that needs to be resolved and explained." The book then goes on to describe how the exploration might proceed, explaining that some chalk dust or pepper in the drop will help students see that the drop rolls rather than slides. By way of explanation, the authors state that, "In this situation, we observe the cohesive effects of water molecules permitting a drop of liquid to form and maintain a spherical shape that will roll on a surface without breaking apart." What does this really mean? Is this explanation enough? I decided to play around with this system and see what things I could come up with. Let's start with the drop of water on the wax paper: It is easy to see that the drop seems to have a "skin" holding it into a sort of flattened sphere. It turns out that this surface tension is the result of the tendency of water molecules to attract one another (called cohesion). The lowest energy state for this drop occurs when the maximum number of water molecules are surrounded on all sides by other water molecules -- meaning that the drop should have the minimum possible surface area, which is a sphere. The effect of gravity flattens this ideal sphere into the shape we see. Surface tension can be shown with many other demonstrations, including one in which you add coins to a full glass of water and note how the surface rises well above the rim of the glass: Though teachers often talk about surface tension being like a "skin", this is not really an accurate model, as there is no other compound other than water in the glass. In addition, the surface does not react to being poked