Clarified implementation of breaction

[fafroo]

Let us assume a charge density $Q$ confined to a surface (surface charge density) between domains $\Omega_-$ and $\Omega_+$. The surface charge density causes a discontinuity of the normal components of the displacement field,

$$(-\varepsilon \nabla\phi_{+})\cdot\nu_{+} + (-\varepsilon \nabla\phi_{-})\cdot\nu_{-} = -Q$$

across the surface. Here $\nu_{+,-}$ denote the outer normals to $\Omega_{+,-}$. This is consistent with e.g. Dreyer et al, Eqn 27b (beware, $\nu$ therein denotes the outer normal of $\Omega_-$).

For $\varepsilon=1$ in 1D, the equation simplifies to

$$\phi_{+}' - \phi_{-}' + Q = 0$$

See the solutions of the Example121 for homogeneous Dirichlet BCs for $Q>0$

and $Q<0$.

The plots are consistent with the 1D equation above. However in breaction, the surface charge enters with - sign as -Q.