SELF-ORGANIZATION IN A LAYER OF MAGNETIC
FLUID IN STRONG ELECTRIC FIELDS
V. M. Kozhevnikov, I. Yu. Chuenkova, M. I.
Danilov, and S. S. Yastrebov
North-Caucasus
State Technical University, Stavropol, Russia
e-mail: kvm@stv.runnet.ru
Abstract
The effect of a polarizing voltage
on the electrical properties of a magnetic fluid confined between the plates of
a plane capacitor connected to a series resonance circuit has been studied. The
magnetic fluid layer features the formation, development, and self-organization
of aggregates with dimensions on the order of several millimeters. These
processes influence the physical properties of the magnetic fluid layer.
The phenomenon of a change in the
physical properties of magnetic fluids as a result of their structuring under
the action of external electric fields has been extensively studied in recent
years [1-3]. In particular, we have investigated the formation of structures in
magnetic fluids exposed to relatively weak electric fields with a strength not
exceeding 400 kV/m. It was established that the average size of observed
structural elements did not exceed several microns, and their formation was
attributed to an increase in the concentration of a disperse phase near
electrodes and the subsequent aggregation processes in this phase. As is known,
strong external actions on various media may lead to the appearance of
substantially new structures as a result of the phenomenon of
self-organization.
This Letter presents the results of
investigations into the physical properties of a layer of magnetic fluid
exposed to strong electric fields.
The experiments were performed
using a setup based on a series resonance circuit comprising a standard coil
with an induction of L = 0.22 H and a plane capacitor. The capacitor
represented a cell formed by two plane-parallel glass plates with transparent
conducting coatings and a layer of magnetic fluid confined between these plates
[3]. The thickness of the magnetic fluid layer was determined by dielectric
spacers. The cell allowed a polarizing electric field with a strength of up to EP
= 5000 kV/m to be generated between the capacitor plates. The magnetic
fluid was based on kerosene and represented a suspension of magnetite particles
(with a total solid phase content of about 2%) stabilized by oleic acid.
The series resonance circuit was
excited by a sinusoidal signal from a generator with an effective voltage
amplitude of U = 1.5 V and a variable frequency. The resonance was
achieved by finding the signal frequency corresponding to a maximum of the
alternating current
in the circuit, which was
determined by measuring the voltage drop on a shunting resistor Rs
= 100 W. The
polarizing voltage was applied to the cell from a source of controlled dc
voltage and measured by a voltmeter.
Observations in the transmitted
light revealed the formation of structures in the magnetic fluid, with the
characteristic dimensions of the elements varying from 0.1 to 5 mm. The
observed patterns and the size of their elements depended on the polarizing
voltage and the duration of its application. When the polarizing field strength
was increased, the structural elements increased in size and the pattern changed
from a cellular structure to labyrinth and to fractal clusters (Figs. 1a and
1b). Simultaneous observations of interference patterns at the cell surface in
reflected light revealed the presence of autowave processes in the magnetic
fluid layer.
Figure 2 shows plots of the
resonance current versus polarizing voltage for various thicknesses of the
magnetic fluid layer. The observed variations are related to changes in the
conductivity of the cell, which are caused by the formation of aggregates,
their structuring, and self-organization. The smaller the thickness of the
magnetic fluid layer, the more pronounced the field-induced changes in the
resonance current. The structures formed in 20 to 40-mm-thick layers
under the action of a polarizing voltage of 20-30 V have the form of vortices
with spiral waves outgoing from the center (Fig. 1c). The appearance of a
maximum of resonance current for magnetic fluid layers with thicknesses in the
range from 80 to 220 mm at a polarizing voltage of 20-30 V reflects the
synchronization of autowave processes. As can be seen from Fig. 2, an increase
in the layer thickness from 80 to 220 mm does not change the character of variation of the
resonance current as a function of the polarizing voltage. The results were
reproduced well and the random errors did not exceed 1.5%.
It was established that the
resonance current in the cell circuit (and, hence, the cell conductivity) in
the absence of a polarizing voltage is independent of the magnetic fluid layer
thickness. Therefore, the electrical properties of this layer under such
conditions are determined by the near-electrode regions, which are
characterized by low conductivity.
Fig.
1. Dissipative structures observed in the transmitted light in magnetic fluid layers
of different thicknesses for various polarizing field strengths EP = 100 (a), 300 (b) and
1000 kV/m (c).
Fig.
2. Experimental plots of the resonance current IR versus polarizing
voltage UP for magnetic fluid
layers with a solid phase (magnetite) content of 2% and various thicknesses (mm):
(1) 20; (2) 40; (3) 80; (4) 110; (5) 150; (6) 220.
The application of a polarizing
voltage to the cell with the magnetic fluid gives rise to correlated motions of
the charge carriers, which is manifested by the observed macroscopic
structures. The behavior of the resonance current in the circuit as a function
of the polarizing voltage changes when the magnetic fluid layer thickness is
decreased below a certain level, which is explained by a decrease in the number
of magnetite particles in the interelectrode space,
which are involved in the formation
of dissipative structures.
REFERENCES
1. V. M. Kozhevnikov and T. F.
Morozova, Magnetohydro-dynamics 37, 383 (2001).
2.
Yu. I. Dikansky and O. A. Nechaeva, Magnetohydrody-namics 38, 287
(2002).
3.
V. M. Kozhevnikov, J. A. Larionov, I. J. Chuenkova, et al., Magnetohydrodynamics
40, 269 (2004).
Translated
by P. Pozdeev