The breaking of electroneutrality in a phase separating polyelectrolyte gel*

A common assumption in models of polyelectrolyte gels that are surrounded by an ionic solution is that the electric double layer that forms at the gel-solution interface has a passive role in the dynamics and hence can be neglected. In this talk, I will revisit this assumption through the use of a novel phase-field model of a polyelectrolyte gel. I will first discuss non-homogeneous equilibrium solutions of the model and show that when the Debye and Kuhn lengths are comparable, a novel mode of phase separation emerges. While phase separation is initially localised in the double layer, eventually it propagates to the bulk of the gel leading to global breakdown of electroneutrality. I will then showcase a range of numerical simulations that illustrate how, by changing the concentration of ions in the surrounding solution, it is possible to transform the internal structure of the gel by producing a finite number of positively and negatively charged phases near the gel-solution interface.