Nanoporous monolithic microsphere arrays have anti-adhesive properties independent of humidity

TL;DR

This study demonstrates that nanoporous monolithic microsphere arrays maintain their anti-adhesive properties across a wide humidity range, unlike fibrillar adhesive pads whose adhesion varies with humidity.

Contribution

The paper introduces nanoporous monolithic microsphere arrays that exhibit humidity-independent anti-adhesive properties, contrasting with traditional fibrillar adhesives affected by humidity.

Findings

Anti-adhesive properties retained at 90% RH

Nanoporous structure allows humidity tolerance

Contrasts with humidity-sensitive fibrillar adhesives

Abstract

Bioinspired artificial surfaces with tailored adhesive properties have attracted significant interest. While fibrillar adhesive pads mimicking gecko feet are optimized for strong reversible adhesion, monolithic microsphere arrays mimicking the slippery zone of the pitchers of carnivorous plants of the genus Nepenthes show anti-adhesive properties even against tacky counterpart surfaces. In contrast to the influence of topography, the influence of relative humidity (RH) on adhesion has been widely neglected. Some previous works deal with the influence of RH on the adhesive performance of fibrillar adhesive pads. Commonly, humidity-induced softening of the fibrils enhances adhesion. However, little is known on the influence of RH on solid anti-adhesive surfaces. We prepared polymeric nanoporous monolithic microsphere arrays (NMMAs) with microsphere diameters of a few 10 {\mu}m to test their anti-adhesive properties at RHs of 2 % and 90 %. Despite the presence of continuous nanopore systems through which the inner nanopore walls were accessible to humid air, the topography-induced anti-adhesive properties of NMMAs on tacky counterpart surfaces were retained even at RH = 90 %. This RH-independent robustness of the anti-adhesive properties of NMMAs significantly contrasts the adhesion enhancement by humidity-induced softening on nanoporous fibrillar adhesive pads made of the same material.