Dynamic Shaping of Femtoliter Dew Droplets.

Herein we show that wetting properties such as Giant Wetting Anisotropy and dynamic shaping can be observed when femtoliter (sub-micron scale) dew droplets are condensed on nanopatterned midly hydrophilic surfaces. Large scale, optically transparent, nanopatterned TiO2 surfaces were fabricated by direct nano-imprinting lithography of sol-gel derived films. Square, infinitely elongated, or circular droplets were obtained with square, line, or concentric patterns, respectively and were visualised in situ during formation and recession using optical microscopy and Environmental Scanning Electronic Microscopy (E-SEM). We first describe how extremely elongated droplets could form on mildly hydrophilic surfaces, naturally contaminated in real environmental conditions. In this configuration, the dew droplet shape can be dynamically and reversibly varied by controlling the out-of-equilibrium conditions associated to condensation/evaporation kinetics. As example of application, we propose a "morphological" sensor that exploits the shape of the dew droplets as transduction mode for detecting organic vapours in the outer atmosphere. Importantly, this study is underlining that environmental stable, purely hydrophilic surfaces can be smartly engineered to induce wetting phenomena at very small scale never observed so far for hydrophobic or heterogeneous surfaces. Our versatile approach based on nanoimprinted, transparent sol-gel films could open great perspectives for the implementation of environmentally stable mildly hydrophilic materials for " dew engineering" applications such as open-microfluidics, fuming for fingerprints revealing, vapour sensing or water harvesting on glass windows for instance.