Down with the Clip!
Students see how surface tension can enable light objects (paper clips, peppercorns) to float on an island of oil in water, and subsequently sink when the surface tension of the oil/water interface is reduced by the addition of a surfactant; such as ordinary dish soap.
Oil spills are real-life examples of surface tension in action; the oil and water don't want to mix. Adding surfactants lowers the surface tension between opposing fluids, making them useful for oil spill cleanups.Copyright (left to right) NASA; George Wardlaw, NSF; and 2010 Samuel DuPont and Ryan Cates, STARS – University of South Florida http://www.netl.doe.gov/newsroom/netlog/jan2011/images/gulf_w.jpg http://www.nsf.gov/news/news_images.jsp?cntn_id=116993&org=NSF
The key property of a surfactant is its ability to reduce surface tension between two different molecule types, allowing for mixing when the molecules would otherwise want to remain separate. This property is extensively used in research and industry. For example, in the cleanup of the 2010 Gulf of Mexico oil spill, surfactants were used as dispersants. By spraying surfactants over the oil, the oil more thoroughly mixes with the surrounding water, spreading the oil over a greater area. This allows for more interaction between the oil and native microorganism that can digest the oil into less hazardous byproducts.
After this activity, students should be able to:
- Explain how surface tension allows objects that would normally sink to remain suspended on top of a liquid.
- Describe how a surfactant, such as soap, reduces the surface tension causing the floating object to sink.
- Define the molecular property that is responsible for surface tension.