Photo-switching of surface wettability on micropatterned photopolymers for fast transport of water droplets over a long-distance
The present work addresses the directional movement of water droplets (2-µL droplet) across a photopolymer surface over a distance of more than 20 mm. The long-distance movement of the droplet is achieved by inscribing multi-gradients onto the surface, which is realized by a photo-induced increase in wettability amplified by a change in surface texture. To create highly hydrophobic surface characteristics, photo-reactive resin formulations containing an o-nitrobenzyl alcohol derivative with terminal acrylate groups, a multi-functional thiol and a fluorinated methacrylate monomer are patterned by visible light-assisted nanoimprint lithography. The needle-like surface microstructures in combination with the fluorine groups of the methacrylate monomer enable the formation of photopolymers with static water contact angles in the range of 140 °. Subsequent irradiation of the photopolymer with light in the UV-A spectral region leads to the localized formation of polar groups such as carboxylic acids (photolysis of the o-nitrobenzyl ester moieties) and sulfonic acids (photooxidation of free thiol groups). Along with the change in the chemical surface composition, prolonged UV exposure under air erodes the surface microstructures leading to fully wettable surfaces. The controlled photo-switching between highly hydrophobic (140 °) and fully wettable (7 °) photopolymer surfaces enables the introduction of wettability gradients by asymmetrical UV irradiation. Linear-shaped gradients rapidely drive a water droplet over a distance of 14 mm. By inscribing the wettability gradient in wedge-shaped patterns, a Laplace pressure gradient is additionally formed, which allows a directed movement of water droplets over a distance of 22 mm.