The
Novel anisotropic and electrically responsive
colloids
Hartmut Wege (Post-doc) and Vesselin N. Paunov
Sponsors: EPSRC Duration: May 2003 to April 2006
In this project we are interested in the polarisation of o/w
and w/o emulsions and aqueous suspensions in alternating electric field. When a
homogeneous external electrical field is applied on a sample of charged
emulsion drops in water this results in: (i)
polarisation of the electric double layers of the drops, and (ii) polarisation
of the droplet material which dielectric properties practically always differ
from that of the water. Both (i) and (ii) lead to the
induction of a net electric dipolar moment in each droplet, which magnitude
depends on the droplet properties and the strength of the external electrical
field. Thus, the charged drop in external electric field are
subject to the following three forces: (a) the electrophoretic
force, (b) the force due to the interaction of the induced particle dipoles
with the external electric field, and (c) force due to the interaction between
the induced dipoles themselves. In the case of alternating (AC) electrical
field (dielectrophoresis), the result of the emulsion
droplet polarisation depends not only on the field strength but also on its
frequency. Particles of polarisability higher than that of the surrounding
fluid produce a dipole moment following the direction of the external field
while particles of lower polarisability produce dipolar moment directed against
the field. The latter two cases correspond to positive and negative dielectrophoresis, where particles are attracted or
repelled by the electrodes, respectively. The emulsion droplets usually stop
moving at frequencies above 1 kHz since they are too big to move so quickly and
the electrophoretic force (a) becomes negligible. The
electric double layers of the particles can still follow the changes of the
external AC electric field up to frequencies of around 50 kHz. However, at
higher frequencies the dielectrophoretic forces (b
and c) still operate, which leads to the following two effects, generally
observed in colloidal suspensions in the presence of AC electric field: (1) the emulsion drops are forming long chains due
to the effective alignment of their induced dipoles; (2) the chains of
particles are oriented along the lines of the electric field.
(a)
(b)
These images show the emulsion
containing mixed surfactant monolayers on drop surfaces before (a) and after (b)
application of an external AC field.
Currently we conduct similar experiments with suspensions of
rod-like particles which also show anisotropic behaviour in electric field. We
were able to align the rod-like microparticles in the suspension by using AC
field or hydrodynamic flow. The results of this project have direct application
in development of novel water-based electrorheological
fluids, smart gels, etc.
A suspension
of rod-like particles aligned in one direction by application of hydrodynamic
flow.
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