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No. 112: Jul-Aug 1997

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When Like Charges Attract

The above title implies that a basic law of physics has been overturned. Indeed, a commentary in Nature by C.A. Murray begins as follows:

"Larsen and Grier, on page 230 of this issue [Ref. 2], show that two similarly charged polymer spheres suspended in water can attract each other when they are several diameters apart. This surprising result casts some light on a tricky theoretical many-body problem that has been swept under the rug for a century, and it has implications for colloids in nature and in industrial processes." (Ref. 1)

Exactly what happens is not yet clear. This counter-intuitive phenomenon occurs in a many-body situation, where screening charges are established between the like-charged spheres. Although Coulomb's Law states that like charges repel one another, the presence of screening particles complicates the picture, as do the van der Waals dipole interactions.

The microscopic situation may be murky, but there is no doubt on the macroscopic level that unexpected attractive forces are operating. For example, when sub-microscopic, electrically charged latex spheres are suspended in water, one would expect a homogeneous colloidal soup. Instead, the tiny, charged spheres pull themselves together in patchy, but ordered arrays. These metastable groupings of spheres are called "crystallites." Theorists are not certain what is going on.

References

Ref. 1. Murray, Cherry A.; "When Like Charges Attract," Nature, 385:203, 1997.

Ref. 2. Larsen, Amy E., and Grier, David G.; "Like-Charge Attractions in Metastable Colloidal Crystallites," Nature, 385:230. 1997.

From Science Frontiers #112, JUL-AUG 1997. 1997-2000 William R. Corliss