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See Section 2.7↑ for a review of multiphasic materials, and [13] for additional details on contact interfaces involving solutes. The contact interface is defined between surfaces $\gamma^{\left(1\right)}$ and $\gamma^{\left(2\right)}$ . Due to continuity requirements on the traction and fluxes, the external virtual work resulting from contact tractions $\mathbf{t}^{\left(i\right)}$ , solvent fluxes $w_{n}^{\left(i\right)}$ and solute fluxes $j_{n}^{\alpha\left(i\right)}$ ($i=1,2)$ may be combined into the contact integral $$\begin{equation} \begin{aligned}\delta G_{c} & =\int_{\gamma^{\left(1\right)}}\left(\delta\mathbf{v}^{\left(1\right)}-\delta\mathbf{v}^{\left(2\right)}\right)\cdot\mathbf{t}^{\left(1\right)}\,da^{\left(1\right)}\\ & +\int_{\gamma^{\left(1\right)}}\left(\delta\tilde{p}^{\left(1\right)}-\delta\tilde{p}^{\left(2\right)}\right)w_{n}^{\left(1\right)}\,da^{\left(1\right)}\\ & +\sum_{\alpha}\int_{\gamma^{\left(1\right)}}\left(\delta\tilde{c}^{\alpha\left(1\right)}-\delta\tilde{c}^{\alpha\left(2\right)}\right)j_{n}^{\alpha\left(1\right)}\,da^{\left(1\right)}\,. \end{aligned} \label{eq696-1} \end{equation}$$ Note that the summation in (7.7-1) is performed only over solutes that are present on both sides of the contact interface. No special treatment is needed for solutes that only belong to one side, since the natural boundary condition for these solutes enforces zero normal flux across the contact interface.