Wilhelmy plats measurements showed the Nafion was hydrophobic (contact angle of ~110º) in the descending mode but was hydrophilic (contact angle of ~0º) in the ascending mode. Sessile drops were hydrophobic on Nafion, but remained pinned in position when the drop evaporated, indicating the Nafion surface under the drop must be hydrophilic. Water droplets on the membrane immediately induced local swelling; the maximum state of protrusion increased with membrane thickness, and drop size. After complete liquid evaporation, the membrane reverted to its pre-wetted state.
Wetting of Nafion was also examined with octane and ethanol. Octane wet the Nafion surface with a contact angle ~0º, but the octane did not cause any measurable deformation of the Nafion. Ethanol induced the most dramatic response; it resulted in enormous film deformation.
Nafion deformation due to sessile drops of water did not show any significant effect due to pre-straining, or adding 3% and 10% TiO2 nanoparticles to the membrane.
The results suggest that Nafion surfaces can be switched from hydrophobic to hydrophilic due to exposure to hydrogen bonding liquids: water and ethanol. Swelling properties originate from the solvent penetration into the membrane causing a differential expansion of the membrane in contact with the liquid. Even though octane wets the Nafion surface it does not appear to penetrate into the membrane suggesting that the solvent interactions are with the hydrophilic sulfonic acid domains.