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The material type for a Donnan equilibrium swelling pressure is “Donnan equilibrium”. The swelling pressure is described by the equations for ideal Donnan equilibrium, assuming that the material is porous, with a charged solid matrix, and the external bathing environment consists of a salt solution of monovalent counter-ions [55, 37]. Since osmotic swelling must be resisted by a solid matrix, this material is not stable on its own. It must be combined with an elastic material that resists the swelling, using a “solid mixture” container as described in Section 4.1.3.22↓. The following material parameters need to be defined:

<material id="1" type="solid mixture">
  <mat_axis type="local">0,0,0</mat_axis>
  <solid type="Donnan equilibrium">
    <phiw0>0.8</phiw0>
    <cF0 lc="1">1</cF0>
    <bosm>300</bosm>
  </solid>
  <solid type="ellipsoidal fiber distribution">
    <ksi>0.01,0.01,0.01</ksi>
    <beta>3,3,3</beta>
  </solid>
</material>
<LoadData>
  <loadcurve id="1">
    <loadpoint>0,0</loadpoint>
    <loadpoint>1,150</loadpoint>
  </loadcurve>
</LoadData>
<Globals>
  <Constants>
    <R>8.314e-6</R>
    <T>310</T>
  </Constants>
</Globals>