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What Makes Sticky Things Stick?


Last month, we read a story in the newspaper that got us thinking about one of those really basic science questions, we'd like and try to answer every week in Science Out of the Box.

(Soundbite of music)

LYDEN: Researchers at Northwestern University have made an adhesive that mimics two very sticky creatures found in nature - the gecko, famous for creeping along walls and ceilings; and the mussel, equally famous for clinging to rocky shores through hurricanes. So it made us wonder, how do things stick to each other?

We turn again to our friend, Jerry Bell, a senior scientist with the American Chemical Society. Hello, Dr. Bell.

Dr. JERRY BELL (Senior Scientist, American Chemical Society): Hello.

LYDEN: Now, we did a little poking around here and found out that this is pretty complicated. Can you please give us an explanation that isn't it, if you will, too sticky? What are these forces - chemical and physical - that cause adhesion?

Dr. BELL: There are several. The ones probably most responsible for the ones that we're talking - you're talking about with the gecko and the mussel are probably ionic. One of the surfaces has a plus charge; the other surface has a minus charge, and they attract one another. These are at the molecular level ions that are formed from sugars, from proteins and so forth.

LYDEN: Dr. Bell, could you explain for those who really need help, like me, exactly what an ionic charge is?

Dr. BELL: Ionic charge is a charge on a molecule that is formed because it has either more electrons than it should have or fewer electrons that it should have to make it neutral.

LYDEN: So that's the chemical explanation, and then there are physical properties that cause things to stick as well.

Dr. BELL: My feeling would be that underlying the physical are chemical. If we're not talking about ions - pluses and minuses - we're probably talking about what are called induced charges where one species induces a charge on another and so the interaction becomes, once again, a plus and minus interaction.

LYDEN: How about something like this, there's more to one kind of bond, right? So here, I've got…

Dr. BELL: Yes.

LYDEN: …some packing tape.

(Soundbite of sticking scotch tape on a cardboard)

LYDEN: Let me try to unstick this tape from this cardboard package.

(Soundbite of ripping off scotch tape from a cardboard)

LYDEN: So what happened, chemically?

Dr. BELL: You, again, broke - probably broke ionic interactions. And you can actually do an experiment. Take two pieces of a sticky tape, like a scotch tape, maybe about 10 inches long. Put them down on a flat, smooth surface and then rip them off and try to bring them together and see whether or not they will actually come together or whether they're going to repel one another.

LYDEN: We actually have some…

Dr. BELL: Good. Yeah.

LYDEN: …clean tape here. Hang on a sec. Let me see if I can get two pieces.

(Soundbite of scotch tape)

LYDEN: Okay. Here's tape number one.

(Soundbite of sticking scotch tape on a table)

LYDEN: I'm sticking it on the table. Here's tape number two.

(Soundbite of sticking scotch tape on a table)

LYDEN: Sticking it on the table here in Studio 2A. All right. So now I just rip these things off at the same time.

Dr. BELL: Rip them off and try to bring them together.

(Soundbite of ripping off scotch tape from table)

LYDEN: They are forming an oval.

Dr. BELL: They're trying to…

LYDEN: They're trying to come together but they won't.

Dr. BELL: That's right because…

LYDEN: Like a love story.

Dr. BELL: They repel one another because they both have picked up the same charge from the ripping off.

LYDEN: Fate is requiring that they always gaze upon each other just from the distance. Well, I love that. So that is because we have changed the ionic charge.

Dr. BELL: Right. Right.

LYDEN: I like it. Well, thank you so very much for coming in. Dr. Jerry Bell is a senior scientist with the American Chemical Society. Thanks so much.

Dr. BELL: Thank you. Transcript provided by NPR, Copyright NPR.

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Longtime listeners recognize Jacki Lyden's voice from her frequent work as a substitute host on NPR. As a journalist who has been with NPR since 1979, Lyden regards herself first and foremost as a storyteller and looks for the distinctive human voice in a huge range of national and international stories.
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