Transition to Superwetting for a Nanostructured Surface

According to traditional Wenzel theory, superhydrophilicity emerge when introducing roughness on an intrinsically hydrophilic surface. However, recent studies have shown a deviation from this behavior [1]. Understanding the failure mechanism will aid the design of surfaces that exhibit superhydrophilic behavior.

In particular, moderately hydrophilic materials, such as polymers and other low energy materials, need a careful design, as they are particularly prone to failure. In this study, we employed block copolymer nanolithography [2] with a subsequent injection molding replication in poly(methyl methacrylate).

Compared to the flat reference, the roughness increased the water contact angle (from 67.6° to 99.4°); a contraction to the traditional Wenzel theory. For moderately hydrophilic substrates, a nanoscopicly pillar-built surface has a Laplace pressure barrier that prevents droplet spreading. Increasing intrinsic hydrophilicity could lower the barrier to allow superwetting.

Consequently, we characterized the transition by applying a low-pressure Argon plasma to increase the surface free energy in a continuous fashion. Using apparent contact angle to probe the transition, we found a threshold of 55°. Furthermore, we demonstrate how macro- and microscopic wetting phenomena are interconnected.

As an example of the barrier implications, we study the condensation of water on both sides of the threshold. While flat surfaces and untreated, structured surfaces both show indelible dropwise condensation, the plasma treated, structured surface gives rise to filmwise condensation. Using a transparent polymer and designing structures to be below the diffraction limit for visible light, the threshold defines the emergence of anti-fogging properties relevant to a plethora of optical applications such as endoscopy [3].

References: [1] D. Kim et al., Wetting theory for small droplets on textured solid surfaces, Scientific Reports (2016) 6, 37813 [2] A. Telecka et al., Nanotextured Si surfaces derived from block-copolymer self-assembly with superhydrophobic, superhydrophilic, or superamphiphobic properties, RSC Advances (2018) 8, 4204. [3] S.

Sunny et al., Transparent antifouling material for improved operative field visibility in endoscopy. Proceedings of the National Academy of Sciences of the United States of America (2016), 113, 11676.