Wow! Talk about an interesting idea! A very neat experiment, called “Hydroglyphics”, has been published by Philseok Kim, Jack Alvarenga, Joanna Aizenberg and Raymond Sleeper in the Journal of Chemical Education. This experiment allows you to transform a common plastic Petri dish into a unique teaching tool that can be used to demonstrate the difference between hydrophobic and hydrophilic surfaces. Check it out in the video below.
Corona discharge from a Tesla coil is used to promote a reaction between the hydrophobic, polystyrene Petri dish with oxygen in the air. The reaction causes oxygen to be added the polystyrene, forming carbonyl and hydroxide groups, rendering it hydrophilic. Regions on the surface of the Petri dish protected by a sticker do not react with oxygen and remain hydrophobic. Water added to the corona-treated Petri dish “sticks” to the hydrophilic portions, but avoids the hydrophobic portions of the dish.
As long as you have a Tesla coil (which costs about $200), the experiment is remarkably simple to conduct. The video below outlines in detail how to make these interesting Petri dishes. See the article for more information on this experiment, including a discussion of the chemistry involved in the chemical conversion of polystyrene to oxygenated polystyrene.
I hope you get a chance to try this experiment. If you do try it, I’d love to hear about any interesting modifications you discover. I’ve tried this experiment myself, and have shared it with many of my college students (they enjoy it, too). Having conducted this experiment several times now, I have two questions I’m curious about. Maybe some of you can help me understand what is going on here; please comment if you have some possible answers. First, when you breathe on the special Petri dish, water from your breath appears to condense on the untreated, hydrophobic portion of the dish, but not on the hydrophilic, corona-treated portion of the dish. This effect seems backwards to me. Second, because oxygen atoms are inserted into the polystyrene chain during corona treatment, it would seem that the mass of the dish should increase upon corona treatment. However, (I’ve only done this experiment once), I have noticed that a corona-treated Petri dish actually showed significantly lower mass upon corona treatment. (NOTE added 5/14/15: upon several repetitions of this experiment, we have observed what one might expect. That is, the mass of the Petri dish goes up after corona treatment). Why might this be so? I look forward to hearing from you.
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Students who demonstrate understanding can develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction.
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Students who demonstrate understanding can develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction.
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Comments 2
Fog forms on hydrophobic parts
I think your breath fogs up only the hydrophobic parts because it does not get adsorbed so it beads up into visible droplets. The moisture in your breath would be adsorbed onto the hydrophilic portions so it would not be visible. Not sure about the weight difference, although heating the dish might drive off any water that was in the plate so I would heat the plate before doing the pre-weighing.
Thanks!
Hi Dr. Starkey:
I think you are right regarding the condensation pattern. Your comments make me wonder if I can view droplets with an optical microscope. I think I am going to try it out. Regarding the mysterious loss in mass, I should probably update this blog post. I have repeated this experiment several times now, and notice that the Petri dish increases in mass upon corona discharge, as expected.