Photoresponsive hierarchical ZnO-PDMS surfaces with azobenzene-polydopamine coated nanoparticles for reversible wettability tuning

Azobenzene-bearing surfaces are promising for light-controlled wettability switching in microfluidic systems. We investigate the wettability and stability of flat glass, flat polydimethylsiloxane (PDMS) and hierarchical ZnO-PDMS surfaces functionalized with azobenzene bearing polydopamine coated nano particles (Azo-PDA-NP). We analyse the stability of the contact-angle switching when using new droplet locations for each measurement and for using the same droplet location with compressed-nitrogen drying between measurements. For flat glass surfaces we observed a wettability change of 15 and 60 for spin-coating and drop-casting deposition of the Azo-PDA-NP, respectively. For the PDMS samples, 60 contact-angle change are obtained, but these surfaces show degradation for repeated switching cycles. In contrast the hierarchical ZnO-PDMS surfaces exhibit a wettability switch of 50 and no degradation for five switching cycles at the same droplet location. We captured water drops during ultraviolet (UV) irradiation on video and obtained a one-sided flowing motion with a speed of 1.9 Contact-angle hysteresis measurements show that the minimum criterion for droplet movement is fulfilled for the Azo-PDA-NP functionalized hierarchical ZnO-PDMS surfaces. (C) 2017 The Authors. Published by Elsevier Ltd.