Polarization and interactions of colloidal particles in ac electric fields

Manish Mittal, Pushkar P. Lele, Eric W. Kaler, Eric M. Furst

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Micrometer-sized polystyrene particles form two-dimensional crystals in alternating current (ac) electric fields. The induced dipole-dipole interaction is the dominant force that drives this assembly. We report measurements of forces between colloidal particles in ac electric fields using optical tweezers and find good agreement with the point dipole model. The magnitude of the pair interaction forces depends strongly on the bulk solution conductivity and decreases as the ionic strength increases. The forces also decrease with increasing field frequency. The salt and frequency dependences are consistent with double layer polarization with a characteristic relaxation frequency ωCD ∼ a2 /D, where a is the particle radius and D is the ion diffusivity. This enables us to reinterpret the order-disorder transition reported for micrometer-sized polystyrene particles [Lumsdon, Langmuir 20, 2108 (2004)], including the dependence on particle size, frequency, and ionic strength. These results provide a rational framework for identifying assembly conditions of colloidal particles in ac fields over a wide range of parameters.

Original languageEnglish (US)
Article number064513
JournalJournal of Chemical Physics
Issue number6
StatePublished - 2008

Bibliographical note

Funding Information:
We thank the following individuals for helpful discussions: O. Velev, S. Gangwal, N. Wagner, J. Brady, J. Swan, C.-H. Chang, and A. Grillet. Funding from NSF-NIRT (Grant No. CBET-0506701) and Sandia National Laboratories (Contract No. 678286) is gratefully acknowledged.


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