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If you float two coffee beans in a cup of water that is being vibrated by your subwoofer (go to a sin wave generator online), the two beans will either be attracted to the sides of the cup or if positioned right will be attracted to each other in an accelerating manner. What phenomenon is this, and could it be related to gravity in space?

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marked as duplicate by Qmechanic Oct 10 '14 at 17:59

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Are they floating on the water surface? If so, there is a name for this phenomenon: the Cheerios effect. To learn more, check out…. Not exactly sure how the subwoofer vibrations would effect it, other than producing standing waves on the surface whose nodes might attract the beans. – DumpsterDoofus Mar 30 '14 at 22:56
@DumpsterDoofus he didn't have two balls in the glass though. – Enjoys Math Mar 30 '14 at 23:00
True, but the same explanation still applies when you have two objects floating in the glass, both to the bean-glass attraction and to the bean-bean attraction. It's a surface tension-related effect, as LDC3 mentioned. And the Cheerios effect is specifically about the interaction of multiple floating objects on a surface, so I think the case of two beans floating in water is covered by it. – DumpsterDoofus Mar 30 '14 at 23:05
up vote 2 down vote accepted

I think it has more to do with surface tension.

Added: When an object is floating on the surface of water, it bends the surface to support it. This bending extends outward from the object decreasing with distance (much like gravity). When 2 objects are near each other, they feel the bent surface and move down the slope (they attract each other).

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The beans just happen to float well, but what if the phenomenon persisted when completely surrounded by the fluid? Also maybe there's "surface tension" in space? – Enjoys Math Mar 30 '14 at 22:54
Yes, there is surface tension in space (with an atmosphere). Water will form a sphere in space due to surface tension. – LDC3 Mar 30 '14 at 22:58

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