Directed Assembly of Janus
Particles under High Frequency
ac-Electric Fields: Effects of Medium Conductivity and Colloidal Surface
Chemistry
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Abstract
The manipulation and assembly of polystyrene-based Janus
particles
of varied surface chemistry on one hemispherical particle surface
under high frequency nonuniform ac-electric fields is examined experimentally
by in situ microscopic observation. Strong effects of ac-field frequency,
medium conductivity, and particle surface chemistry on the structure
of Janus colloidal assembly are observed. At low medium conductivity,
σ<sub>m</sub> from 0.0007 S/m to 0.0153 S/m, pearl chains of
Janus particles are observed over the ac-frequency range from 25 kHz
to 20 MHz, indicating the dielectrophoresis (DEP)-directed assembly.
In contrast, the chaining of Janus particles is disrupted in a certain
frequency range at high σ<sub>m</sub> from 0.0153 S/m to 0.116
S/m, suggesting the combining effects of both induced-charge electrophoresis
(ICEP) and DEP. The critical transition frequency for the onset of
the fractal aggregation at high σ<sub>m</sub> from 0.0153 S/m
to 0.116 S/m is experimentally determined, showing a good agreement
with the theoretically predicted upper ICEP frequency limit. Additionally,
it is demonstrated that by using zwitterionic Janus particles, the
assembled structure of Janus particles under ac-fields can be modified
by the chemical coatings on each hemispherical surface of Janus particles