Light-induced switching of surfaces at wetting transitions through photoisomerization of polymer monolayers

We report on a method to generate surfaces whose wettability can be reversibly switched between a superhydrophobic and Wenzel state or a Wenzel and superwetting state just by a short UV or VIS irradiation. To achieve this, we generate a silicon surface with a nanoscale roughness ("black silicon") and attach a polymer monolayer to it. The polymer contains a fluorinated azobenzene moiety which can be switched between the cis and trans state depending on the wavelength of the light used during illumination. The surface energy of the polymer coating is carefully adjusted to the energy value which separates distinct wetting regimes of the nanorough surface. This coupling of light induced switching to a transition of the wetting regimes can cause changes in the water contact angle as high as Δθ = 140° in the advancing CA or more than 175° in the receding CA even when the surface energy is changed only in a rather small range. Short irradiation times with UV or VIS light are enough to change the roll-off angle from <5° to no roll off at all and back. We discuss the requirements necessary so that large changes in the contact angle occur during photoswitching processes on rough surfaces.