One-step production of doubly re-entrant microstructures through reaction-diffusion photolithography for omniphobic surfaces

Abstract

The omniphobic surface, which is composed of biomimetic doubly re-entrant structures, has a non-wet property regardless of the wetting characteristics of any liquid. The doubly re-entrant structure is crucial because it is biocompatible and chemically stable and structurally retains omniphobic without chemical treatment. Various methods have been tried to make such a structure, but complex methods such as multistep etching and circularly polarized light have been used. In this paper, doubly re-entrant structures are fabricated by photolithography using reaction-diffusion photolithography. Reaction-diffusion photolithography is a photolithography method using diffusion of oxygen. A photoinitiator receives light to generate a radical, and the monomer is photopolymerized into a polymer through the radical. At this time, in the presence of oxygen, oxygen has a better reactivity with radicals, thereby inhibiting the reaction. Therefore, by using a photocurable material in which oxygen is dissolved, various structure can be formed by diffusion of oxygen. A constant related to the diffusion and depletion of oxygen is the Damkohler number. Depending on the size of the Dakohler number, the aspect of the structure is different. By controlling the diffusion distance of oxygen, the Damkohler number can be adjusted. In particular, in this paper, an attempt was made to fabricate an ideal doubly re-entrant microstructure by inducing the diffusion of radicals by increasing the photoinitiator concentration as well as the diffusion distance of oxygen. Different parameters show different types of photopolymerization, and it is possible to fabricate doubly re-entrant structures using different parameters. Finally, patterning the doubly re-entrant structures to the array, the omniphobic test is completed by measuring the contact angle.