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Direct measurement of the effective charge in nonpolar suspensions by optical tracking of single particles

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Original languageEnglish
Pages (from-to)194503
Number of pages12
JournalThe Journal of Chemical Physics
Issue number19
Publication statusPublished - 2007


We demonstrate a novel technique for the measurement of the charge carried by a colloidal particle. The technique uses the phenomenon of the resonance of a particle held in an optical tweezers trap and driven by a sinusoidal electric field. The trapped particle forms a strongly damped harmonic oscillator whose fluctuations are a function of \gamma, the ratio of the root-mean square average of the electric and thermal forces on the particle. At low applied fields, where \gamma \ll 1, the particle is confined to the optical axis while at high fields (\gamma \gg 1) the probability distribution of the particle is double-peaked. The periodically-modulated thermal fluctuations are measured with nanometer sensitivity using an interferometric position detector. Charges, as low as a few elementary charges, can be measured with an uncertainty of about 0.25 e. This is significantly better than previous techniques and opens up new possibilities for the study of nonpolar suspensions.

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