Large
deformation and instability in swelling polymeric gels
Zhigang Suo
Flexible, long polymeric molecules can covalently
crosslink into a three-dimensional network.
The resulting material, an elastomer, is capable of large and reversible
deformation. When the network is brought
in contact with a solvent, the network imbibes solvent molecules and swells,
resulting in an aggregate known as a gel.
The swelling is also reversible:
when the environment dries, the solvent molecules in the gel migrate out
and evaporate. Gels are used in diverse
applications, including medical devices, actuators in microfluidics, and
packers in oil wells. Mixtures of
macromolecular networks and mobile molecules also constitute many tissues of
plants and animals.
Swelling of a network can be markedly influenced by
a mechanical load. When the network, the
solvent, and the mechanical load equilibrate, the deformation in the gel is
usually anisotropic and inhomogeneous. This talk
describes a nonlinear field theory of gels, building upon the work initiated by
Gibbs, Biot, and Flory. The gel can
undergo large deformation of two modes.
The first mode results from the fast process of local rearrangement of
molecules, allowing the gel to change shape but not volume. The second mode results from the slow process
of long-range migration of the small molecules, allowing the gel to change both
shape and volume. The theory is illustrated with examples of swelling induced
large deformation, contact, and bifurcation.