Micropatterned cellulosic films to modulate paper wettability

A new class of micropatterned cellulose nanocrystal (CNC) free-standing films which modulate the surface hydrophobicity by controlling the dimensions of the channels are fabricated. Micropattern silicon molds are fabricated with photolithography and etching techniques. CNC and carboxymethyl cellulose (CMC) suspensions are vacuum dried on the molds, with a layer of nanocellulose fibers spray coated on top to create the free-standing film. Sharp 5 µm × 6 µm channels are obtained with a 2.2 wt% suspension with a 10:1 ratio of CNC:CMC. This is the smallest patterned channel size that has been achieved and is more than an order of magnitude lower than those achievable with techniques such as laser cutting. The water droplet contact angle can be tuned to resist or promote wetting, varying from 106° for 5 µm × 6 µm channels to 41° for 500 µm × 18 µm channels, without changing the surface chemistry. Dynamic advancing contact angle measurements show a slip and stick pattern, increasing linearly as the droplet volume increased and then sharply dropping, as the pinning on the channel edge is overcome by the increasing droplet volume. We demonstrate a new class of cellulose materials with finely tunable morphology to control liquid interaction and a scalable process to fabricate those at large scale.