Surfactant enhanced spreading on polymeric substrates of various surface energy

Hypothesis: In the case of complete wetting, the mechanism by which surfactant solutions can spread over solid substrates much faster than pure liquids is still not understood, despite numerous studies. Spreading should be strongly influenced by the adsorption of surfactant on the substrate depending on the specific substrate chemistry and heterogeneity, but this has been barely addressed in the literature. Moreover, it is not clear which substrate characteristics are of importance. We hypothesise that substrate energy components are of importance, whereas the contact angle of water (CAW) is not always suitable for prediction of the spreading kinetics of surfactant solutions.

Experiments: The spreading of aqueous surfactant solutions was studied on 7 different polymeric substrates characterised by surface roughness 1.6–480 nm, surface energy 38.5–61.6 mJ/m² and CAW 43–81°.

Findings: CAW does not provide a complete substrate characterisation for spreading of surfactant solutions: the same solution demonstrated different spreading kinetics on different substrates with the same CAW. Spreading kinetics was characterised by the dependence of spread area, A, with time, t, being A ∝ t^α. For complete wetting, all surfactant solutions on all substrates began the spreading process with a spreading exponent α ≤ 0.5, but two of them later went into superspreading mode with α ≥ 1. On substrates with the highest and lowest surface energy, superspreading solutions begin spreading with α ~ 0.2, similar to pure liquids. On some substrates only spreading with α ~ 0.2 was observed.