Electrostriction in Field-Structured Composites: Basis for a Fast Artificial Muscle? Page: 4 of 26
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Introduction
When a suspension of dielectric particles is exposed to a uniaxial electric field, the
induced dipole moments will cause the particles to chain along the field lines to form
complex anisotropic structures, providing dielectric contrast exists between the particle
and liquid phase. These structures greatly modify the shear rheology of the suspension
and are the basis for the well-known electrorheological effect. [1] If the liquid phase is
polymerized while the field is applied, these anisotropic structures can be trapped to form
field-structured composites. These composites have anisotropic properties, some of
which have been reported in the literature, including the conductivity and permittivity,
[2,3] dielectric breakdown field, [4] and optical transmittance. [5, 6] In this paper we
discuss the enhanced electrostriction that field-structured materials exhibit, and the
possibility of using these materials as fast artificial muscles. We will show that the
expected electrostriction of these materials has a large contribution from the induced
dipoles on the particles interacting with each other.
In fact, two types of field structured composites have been demonstrated. In
addition to the uniaxial composites, of which a sample is shown in Fig. 1, it is possible to
create biaxial composites structured in a biaxial field, such as a rotating field. When a
rotating field is applied in the x-y plane at a sufficiently high frequency that particles do
not move much in one period, an average dipolar interaction is created between particles
that is exactly -1/2 the dipolar interaction produced by a uniaxial field applied along the z
axis. [7] The result of this is the formation of plates in the x-y plane, as shown in Fig. 1
for a sample of magnetic particles that was physically rotated between fixed magnets.
These uniaxial and biaxial structures form when a field is applied, in order to
reduce the free energy of the composite and its attached power supply, and a large
contribution to this is the net electrostatic energy. This energy is reduced when the
dipole-dipole interaction energy is reduced, a consequence of which is an increase of the
effective dielectric constant of the composite. Because field-structured composites
(FSCs) form to minimize the electrostatic energy, one might surmise that the electrostatic
energy might increase rapidly with deformation of these materials, and that the dielectric
constant might decrease rapidly. A major contribution to the electrostriction effect is2
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Anderson, R. A. & Martin, J. E. Electrostriction in Field-Structured Composites: Basis for a Fast Artificial Muscle?, article, January 27, 1999; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc680649/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.