Replicating and resolving wetting and adhesion characteristics of a Rose petal

摘要

A Rose petal is well known for a unique form of hydrophobicity, exhibiting simultaneously high apparent equilibrium contact angle (theta*) as well as high adhesion that hinders the rolling off of droplets. This behaviour is fundamentally different than that commonly encountered in other bio-mimetic superhydrophobic surfaces (like a lotus leaf) that have air entrapped in the solid-liquid contact zone (Cassie Baxter wetting state), thereby exhibiting low adhesion. Based on laser scanning confocal microscopy as well as underwater in-situ atomic force microscopy, we obtain high resolution image of the water-substrate contact region (similar to 0.9-20 mu m) which is significantly higher than that reported so far in the literature. We clearly demonstrate that this unique "Petal effect" can indeed be attributed to the Cassie impregnating wetting state, thereby resolving the prevailing ambiguity on this topic. We have also replicated the structure of the actual rose petals using soft lithography on cross-linked polydimethylsiloxane (PDMS, Sylgard 184) and show that both theta* as well as the adhesive properties of the replicated surface to be nearly identical to that of an actual petal. This implies that the force of adhesion depends on the wetting state and the area of contact. Incidentally, the negative replica of the petal, which is obtained as an intermediate during the replication process, exhibits slightly higher adhesion and identical theta*, as compared to the actual rose petal and its positive replica. However, the prevalent wetting state on the negative replica turns out to be Cassie like due to the presence of entrapped air. Both the negative and the positive replica of the Rose petal can potentially he used as biomimetically fabricated sticky hydrophobic surface.