Dynamic phase transitions in polyelectrolyte gels

When a polyelectrolyte gel is placed in an ionic solution, slight alterations in the environmental parameters can trigger enormous changes in the gel volume. In some cases, the gel volume will change discontinuously, resulting in a volume phase transition. Many studies of the volume phase transition focus on the equilibrium states of the gel or resolving the time dependence of the gel volume. In this talk, I present a novel phase-field model of a polyelectrolyte gel that can be used to explore the evolution of the gel structure during the volume phase transition. Numerical simulations reveal that the volume phase transition can occur via two routes. In the first case, a swelling or deswelling front emerges from the free surface and propagates into the bulk of the gel. In the second case, spinodal decomposition occurs alongside front propagation, resulting in a non-trivial distribution of ions that is coupled to the gel mechanics. Our results also show that the electric double layer that forms at the gel-bath interface can play a key role in determining the internal structure of the gel. If the Kuhn and Debye lengths are commensurate, then a localised mode of phase separation can emerge from the double layer and invade the gel, leading to a global breakdown of electroneutrality. We conclude with a discussion of how these rich dynamics can be observed through experimentally measurable macroscopic quantities.