Phys. Rev. Lett. 108, 016101 (2012)


It's always worth reading the classics, and Filip Beunis and colleagues would no doubt agree. Their extension to Robert Millikan's celebrated oil-drop experiment has afforded them unprecedented resolution of individual charging events on a solid/liquid interface.

In the century-old experiment, the force balance on an oil drop suspended between metal electrodes enabled Millikan to make an accurate estimate of the charge on an electron. (Millikan is pictured here, right, in 1919 with Arthur Noyes and George Hale, his fellow founding scientists of what would become the California Institute of Technology.)

The new variant developed by Beunis et al. involves a colloidal microsphere immersed in a nonpolar liquid, and optically trapped between two electrodes. By monitoring the position of the particle with a coaxial beam, the team was able to infer the particle's charge from its response to a sinusoidally varying voltage. Discrete levels in this response belied individual events, and corresponded to values within 10% of the accepted value for elementary charge.

The speed and accuracy of the method are integral to its success. But the most promising aspect of the experiment arguably lies with the stability of the trap, which allowed Beunis et al. to measure over long times, prompting a statistical analysis that resulted in a realistic physical model for the surface-charging mechanism.