  Acid-induced release of stored ions from polyacrylic acid hydrogels (with a
free surface fully permeable to the ion and acid flux) was observed to increase
the gel osmotic pressure that leads to rapid, temporary swelling faster than
the characteristic solvent absorption rate of the gel. Here we develop a
continuum poroelastic theory that quantitatively explains the experiments by
introducing a "gel diffusio-phoresis" mechanism: Steric repulsion between the
gel polymers and released ions can induce a diffusio-osmotic solvent intake
counteracted by the diffusio-phoretic expansion of the gel network. For
applications ranging from drug delivery to soft robotics, engineering the gel
diffusio-phoresis may enable stimuli-responsive hydrogels with amplified strain
rates and power output.
