Clean surfaces, dirty water: Topography and chemistry in the wetting of superhydrophobic surfaces by pure liquids and surfactant solutions

The effects of surface chemistry and topography on the wetting of smooth and superhydrophobic surfaces (SHS) by pure liquids and surfactant solutions were studied. SHS of aluminum, PTFE and alkyl ketene dimmer (AKD), and smooth coated silicon wafer were probed with four pure liquids (surface tensions ∼27-73 mN/m), and aqueous solutions of sodium dodecyl sulphate, hexadecyltrimethylammonium bromide and n-decanoyl-n-methylglucamine at three concentrations below Critical Micelle Concentration (CMC). Surfaces had either fluorinated or saturated hydrocarbon coatings. Surfaces were characterized by SEM, AFM, and XPS analysis, and wettability was probed using Axisymmetric Drop Shape Analysis. On SHS, surfactant solution advancing contact angles (CA) remained hydrophobic near CMC while results for pure liquids with surface tensions similar to the solutions decreased 20-120°. Solution contact angles on coated silicon were slightly higher than pure liquid contact angles for pure liquid advancing contact angles less than 90°. For low intrinsic CA, the receding CA is zero on most SHS. Dual-scale topography (aluminum) gave higher CA for all pure liquids, with the dense spiked pattern of PTFE next highest. Results were reversed for solutions. It is hypothesized that surfactant films cover the surface pores, promoting 'metastable' Cassie wetting. Saturated hydrocarbon chemistry is shown to give lower advancing and much lower receding CA. The main finding is that dependent on topography, surfaces can remain superhydrophobic for surfactant solutions of high concentration (low surface tension).