(323e) Frequency Dependent Mobility of Janus Spheres

Frequency dependent
mobility of Janus spheres

Alicia Boymelgreen¹, Tov Balli¹, Gilad Yossifon¹,Touvia Miloh²

^,1Faculty of Mechanical Engineering, Micro- and Nanofluidics Laboratory, Technion
? Israel Institute of Technology, Haifa 32000, Israel

²Faculty of
Mechanical Engineering, Tel Aviv University, Tel Aviv 69978, Israel

E-mail: yossifon@tx.technion.ac.il

Non-linear
electrohydrodynamic flows, including induced-charge and
alternating-currentelectroosmosis have received much attention amongst the
micro/nanofluidic community over the past decade for their potential to produce
net flow even under ac electric forcing, with suggested applications ranging
from electroosmotic pumps to externally controlled micromotors and mixers. The
advantages of ac electric forcing over its more commonly used dc counterpart
include lower applied voltages and the suppression of Faradaic reactions and
bubble generation which can block and degrade microfluidic systems.

The underlying mechanism relies on the
polarization of a solid suspended in an aqueous medium, under the application
of an externally applied electric field in which case the solid acquires a
non-zero surface charge. Accordingly, counterions
from within the suspending medium are driven toward the polarized surface to
shield it, creating a region of charge density or an induced electric double
layer. The applied field may then act on this electric double layer, to
ultimately drive electroosmotic flow. When the polarized medium is the actual electrodes
to which the field is applied, the phenomena is termed ?alternating current
electroosmosis? (ACEO) while the more general term of ?induced-charge
electroosmosis? (ICEO) includes the presence of non-active polarizable
structures or particles within the suspending medium. Nonlinearity arises from
the fact that the field both induces the double layer and drives it to produce
electroosmotic flow.

We examine the influence of frequency of a
uniform applied ac electric field on the mobility of metallodielectric
Janus particles, comprised of one metallic (gold) and one dielectric
(polystyrene) hemisphere. It is well-established that at low frequencies (on
the order of single kHz), the mobility is predominantly determined by the
asymmetric induced-charge electrokinetic flow around
the particle, which is strongest around the more polarizable hemisphere and
acts to propel the particle
perpendicular to the electric field in the direction of its dielectric end.
However, it has previously been observed that the particle velocity decays
well-before the charge relaxation time,
, where
 is the particle
radius,
 the thickness of the
Debye layer and
 the diffusivity of the
suspending medium. Such a premature decay suggests the presence of competing
forces, acting against pure ICEP, and result in a reduction of the Janus sphere
mobility. By tracking both freely suspended Janus particles of varying size in
electrolytes of differing conductivites, as well as
using uPIV to examine streamlines around stationary
Janus particles, we examine the interplay between induced-charge
electrophoresis, dielectrophoresis and wall effects
stemming from the proximity of the wall to the translating particle. This work
is of both fundamental and practical importance and may be used to optimize the
behavior of Janus micromotors or carriers in
lab-on-a-chip analysis systems.