Light-gated electro(de)wetting with photoswitchable amphiphiles

We present an approach for controlling the surface wettability with two triggers through a combination of electrodewetting (EDeW) and photoswitchable arylazopyrazole-triethylammonium bromide surfactants (AAP-TB). Experiments were performed on hydrophilic Si/SiO₂ substrates using surfactant-laden aqueous droplets with a solution pH of 2.2. Two different cationic photoswitches were studied which differed in their terminal alkyl group. Here, either hydrogenated C₀AAP-TB or butyl-terminated C₄AAP-TB were applied. Pendant drop tensiometry revealed pronounced, light-dependent differences in surface tension and in the surfactants' critical micelle concentrations (CMC) through E/Z photoisomerization. The more surface-active C₄AAP-TB, exhibited the largest changes in surface tension upon E/Z isomerization of up to ~20 mN m^-1. Applying potentials of +3 V for C₀AAP-TB and +4 V for C₄AAP-TB induces dewetting of surfactant-laden aqueous drops on the substrate surfaces. The change in contact angle $\Delta \theta$ after application of a potential was strongly dependent on surfactant concentration, where a maximum in $\Delta \theta$ of 13° and 7° for C₀AAP-TB and for C₄AAP-TB surfactants, respectively, was observed at about 1/10 of the respective CMCs of the surfactants' E isomer. In addition, in situ light irradiation superimposed to the driving potential for EDeW triggers E/Z photoisomerization and modulates the contact angle reversibly by several degrees and with small hysteresis. Furthermore, at specific concentrations, the interplay of electric fields and spatially non-uniform photoisomerization caused by irradiation with intense UV light generates surface tension gradients that resulted in droplet deformation, droplet splitting, and ultimately in a directed motion of drops. Thus, combining potential-induced EDeW and surface energy changes by E/Z isomerization of AAP-TB surfactants provides reversible control over wettability and offers a versatile platform for programmable manipulation of liquid interfaces.