Magnetic field sensitive gels, called ferrogels, are chemically cross-linked polymer networks
swollen by a ferrofluid. The monodomain magnetic particles, with a typical size of about 10 nm, couple
the shape of the polymer gel to the nonuniform external magnetic field. Shape distortion occurs
instantaneously and disappears abruptly when an external magnetic field is applied or removed,
respectively. This work provides a thermodynamic analysis of shape transition based on the free energy
of the swollen network that includes the elasticity of network chains as well as magnetic interactions of
finely dispersed solid particles with the external field. It is shown that noncontinuous shape transition
is due to a shift of equilibrium state from one local minimum to another one, similar to a first-order
phase transition. The discussions presented here may be useful for the design of magnetically active
soft polymeric actuators.
Unidirectional elongation of magnetic field sensitive polymer gels, called ferrogels, have been studied. In ferrogels, finely distributed colloidal particles having superparamagnetic behavior are incorporated into a swollen network. These particles couple the shape of the gel to the nonuniform magnetic field. Shape distortion occurs instantaneously and disappears when the external field is removed. A discontinuous elongation and contraction in response to infinite-small change in the external magnetic field has been observed and a theoretical interpretation based on coupled magnetic and rubber elastic properties is provided.
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