Functionalized porous microparticles of nanofibrillated cellulose for biomimetic hierarchically structured superhydrophobic surfaces

Nanofibrillated cellulose (NFC, also called microfibrillated cellulose and native cellulose nanofibers) is an attractive sustainable nanofibrillar material to template functionalities. It allows the modification of wetting properties, but so far surfaces with NFC have suffered from high adhesion of water droplets, even when the contact angles have been large. Here we show that spray-dried NFC leads to hierarchical surface roughness, closely resembling that of lotus leaves, due to the microparticles and their porosity at a considerably smaller length scale. We present the first report on superhydrophobic surfaces from NFC with contact angle hysteresis of only a few degrees upon surface modification. The shown process to achieve the hierarchies is particularly straightforward involving airbrushing of solvent-based NFC onto the surface, followed by quick drying, and a chemical modification performed either before or after the airbrushing, with essentially similar results. The NFC microparticles also enable the formation of liquid marbles. The presented method is technically feasible, as cellulose is an economic, abundant material from nature and the spraying processes are scalable. The shown approach could allow further functionalities, such as self-cleaning and water droplet manipulation.