  Hypothesis: Surfactants are often added to aqueous solutions to induce
spreading on otherwise unwettable hydrophobic surfaces. Alternatively, they can
be introduced directly into solid hydrophobic materials-such as the soft
elastomer, polydimethylsiloxane-to induce autonomous spreading without
requiring additional surface or liquid modifications. Given the similarity
between mechanisms of these two approaches, models that describe wetting by
aqueous surfactant solutions should also characterize wetting on
surfactant-solid systems.
  Experiments: Multiple surfactants of varying size and chemical composition
were added to pre-polymerized PDMS samples. After cross-linking, water droplets
were placed on the surfaces at set time points, and their contact angles were
recorded to track the temporal evolution of the interfacial tension. Multiple
nonlinear models were fitted to this data, their parameters analyzed, and each
goodness of fit compared.
  Findings: An empirical model of dynamic surface tension was found to describe
the wetting process better than the single established model found in the
literature. The proposed model adapted better to the longer timescales induced
by slow molecular diffusivity in PDMS. Siloxane ethoxylate surfactants induced
faster and more complete wetting of PDMS by water than oxyoctylphenol
ethoxylates did. The generalizability of this model for characterizing
surfactants of a wide range of physiochemical properties was demonstrated.
