Indirect Additive Manufacturing of Amphiphobic Reentrant Surface Structures using Fused Deposition Modelling

Amphiphobic property, defined as repellency to liquids in general, can be achieved using reentrant structures with high edge angles. However, the complexity of forming an overhanging feature to create the reentrant feature with a high edge angle makes it challenging to manufacture using additive manufacturing technology. This study introduces a simple and cost-effective method for producing these structures, utilizing Fused Deposition Modelling (FDM) printed mold and silicone casting. The mold features a top layer with straight lines separated by gaps. A sharp acute angle is formed between a rounded filament and a relatively flat surface. This acute angle can be exploited to have the high edge angle of the desired reentrant structure upon replication with polydimethylsiloxane (PDMS). The best-performing reentrant surface structure obtained has an average edge angle of ~120° with additional surface topography on top of the structure due to material stacking during printing. It exhibited anisotropic wetting with an optimal water static contact angle of 160° and a minimal rolling angle of 29° for a 10 µL droplet. Furthermore, the structures also demonstrated superior repellency to ethylene glycol, a liquid with surface tension 35% lower than water. The achieved optimal static contact angle and minimal rolling angle for ethylene glycol are recorded as 163° and 14°, respectively. This study establishes the viability of a low-cost approach for attaining amphiphobicity in reentrant structures through an indirect FDM printer, presenting superior liquid-repelling properties to not just water but also to a lower surface tension liquid.