Electrically rotating suspended ﬁlms of polar liquids
M. R. Ejtehadi
Received: 2 March 2010 / Revised: 1 July 2010 / Accepted: 7 July 2010 / Published online: 4 August 2010
Ó Springer-Verlag 2010
Abstract Controlled rotation of a suspended soap water
ﬁlm, simply generated by applying an electric ﬁeld, has been
reported recently. The ﬁlm rotates when the applied electric
ﬁeld exceeds a certain threshold. In this study, we investigate
the phenomenon in ﬁlms made of a number of other liquids
with various physical and chemical properties. Our mea-
surements show that the intrinsic electrical dipole moments
of the liquid molecules seems to be vital for the corre-
sponding ﬁlm rotation. All the investigated rotating liquids
have a molecular electric dipole moment of above 1 Debye,
while weakly polar liquids do not rotate. However, the liquids
investigated here cover a wide range of physical parameters
(e.g. viscosity, density, conductivity, etc.). So far, no signif-
icant correlation has been observed between the electric ﬁeld
thresholds and macroscopic properties of the liquids.
Suspended liquid ﬁlms as thin as hundreds of nanometers
or less allow us to study physical phenomena in a quasi
non-directional (2D) medium (Couder et al. 1989; Chomaz
and Cathalau 1990; Rivera and Wu 2000; Huang et al.
2004). Hydrodynamical effects of applying electric ﬁeld
have been studied widely for the bulk of both in polar and
non-polar liquids (Saville 1997; Green et al. 2000), but the
electro-hydrodynamical (EHD) ﬂow on the suspended
ﬁlms of liquids has been studied just for liquid crystals (de
Gennes and Prost 1995; Chandrasekhar 1992). For exam-
ple, applying sufﬁciently large electric ﬁelds can produce
patterns of convective vortices in freely suspended ﬁlms of
some thermotropic liquid crystals in nematic (Faetti et al.
1983a, b), smectic (Morris et al. 1990; Daya et al. 1997)
and also isotropic (Faetti et al. 1983a; Sonin 1998) phases.
Recently, it has been shown that EHD effects in sus-
pended ﬁlms of soap water can induce a rotation with a full
control on the velocity and chirality (Amjadi et al. 2009).
The device, called a ﬁlm motor, consists of a liquid ﬁlm on
a two-dimensional frame connected to an electrolysis
potential difference and laid in an external in-plane electric
ﬁeld which crosses the internal electrolysis current
(Fig. 1a). When either the external ﬁeld or the electrolysis
voltage exceeds a certain threshold, while the other one
does not vanish, the liquid ﬁlm begins to rotate. The effect
may have wide applications e.g. in mixing, both in micro-
and nano-mechanical devices.
Some charge-based mechanisms may be suggested for
explaining the ﬁlm rotation, e.g. non-uniform ion distri-
bution, caused by the external electric ﬁeld (the electro-
phoresis effect) (Shiryaeva et al. 2009), ion jets in the
electrolysis double-layers close to the electrodes (Chi-
ragwandi et al. 2005), and the electric ﬁeld interactions
with surface charges induced on the ﬂuid–air interfaces
(Faetti et al. 1983a; Morris et al. 1990; Daya et al. 1997).
However, there is enough evidence to show that none of
these mechanisms are the dominant reasons behind the ﬁlm
rotation phenomenon (Amjadi et al. 2009).
In this paper, the viability of rotation in ﬁlm motors
involving liquids other than soap water is studied. Mea-
surements applied to 39 liquids capable of producing stable
suspended ﬁlm, yet covering a wide range of physical and
chemical properties, are presented. Our results indicate that
only liquids with electrically polar molecules can rotate.
R. Shirsavar Á A. Amjadi Á M. R. Ejtehadi (&)
Department of Physics, Sharif University of Technology,
P.O. Box 11365-9161, Tehran, Iran
Exp Fluids (2011) 50:419–428