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Electrically Guided Assembly of Planar Superlattices in Binary
Colloidal Suspensions
Optical images of 2 micron polystyrene
(dark) and 1.8 micron silica (light) particle arrays assembled
in a 60 V/mm, 100 kHz electric field (oriented out of the
page).Triangular arrays form with a 2:1 ratio of polystyrene
to silica; square lattices form with 1:1 number densities.
Here the attraction stems from unequal induced particle
dipoles arising from the response of the diffuse double
layers around the differently charged particles.
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IRG 2: W. D. Ristenpart, I. A. Aksay & D. A. Saville
Binary colloidal suspensions are assembled into planar superlattices
using ac electric fields. Either triangular or square-packed arrays
form, depending on the frequency and relative particle concentrations.
The frequency dependence is striking since superlattices develop
at low and high frequencies but not at intermediate frequencies.
We explain the low frequency behavior (< 3 kHz) in terms of induced-dipole
repulsion balanced by attraction resulting from electrohydrodynamic
(EHD) flow. The flow arises from the action of the electric field
on perturbations in the free charge layers adjacent to the electrodes.
This produces a body force distribution that is balanced by fluid
motion; flow carries particles towards one another. At high frequencies
(20–200 kHz), EHD flow is negligible but aggregation occurs
since dipole-dipole interactions between unlike particles become
attractive.
Related publication [DMR-0213706]:
W. D. Ristenpart, I. A. Aksay, and D. A. Saville. “Electrically
Guided Assembly of Planar Superlattices in Binary
Colloidal Suspensions” Phys. Rev. Lett. 90 128303 (2003).
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