Stimuli-responsive molecules at surfaces are promising combinations for future microfluidic and integrated systems. Light as external stimulus prevails, because of its noncontact mode and precise control in application. We investigate the wettability properties of azobenzene-functionalized nanoparticles on flat glass substrates as well as on hierarchical ZnO-PDMS surfaces. Azobenzene is a photochromic molecule that alters its structure due to light excitation by an isomerization process between a trans- and cis-form. Azobenzenes anchored at a surface provide reversible changes in wettability. Here, we study the water drop hysteresis and role-off angle for varying the droplet volume for both states.
First we coated flat glass surfaces with azobenzene functionalized nanoparticles using two different deposition processes – spin coating and drop casting. Different deposition processes lead to different wettability characteristics. In the trans-state no water droplet motion is observed for any sample, hysteresis increases with increasing angle and increasing droplet volume. For the cis-state we observed a minimum volume of 6 µL for the drop-casted glass substrates for droplet movement with a roll-off angle of > 40°. At that angle the droplet starts moving and slides off the sample. In contrast, for the hierarchical ZnO-PDMS substrates the water droplets adhere in the cis-state, no motion is observed. Furthermore, we observed aging effects of nanoparticle solution on the contact-angle switch. 20° to 30° lower values for contact angles are measured, when the nanoparticle solution is older than three weeks.