Self-supporting superhydrophobic thin polymer sheets that mimic the nature's petal effect
Highlights
► Polymeric thin films were deposited by initiated CVD method on a biological template. ► Conformal nature of iCVD helps to make exact copies of natural surfaces. ► Biomimetic polymer sheet has the same wetting characteristics of the rose surface.
Introduction
Many of the biological surfaces in nature have unique properties due to their well-organized surface structures. For example, many plant leaves exhibit excellent self-cleaning characteristics [1], [2], [3]. Water droplets on a lotus leaf cannot stay on the surface and roll off quickly with very small inclinations, removing the dirt on the surface. The roughness caused by the hierarchical micro and nano structures throughout the surface together with the low surface energy wax is the reason behind the superhydrophobic behavior of the lotus leaves [4], [5]. Such useful properties caused by the natural orientation of the living surfaces inspired many scientists to develop similar artificial surfaces with superior capabilities. The petal surfaces of red rose exhibit a superhydrophobic behavior (water contact angle greater than 150°) with a difference in their contact angle hysteresis with respect to the behavior of lotus leaves. Their surfaces are so adhesive to the water droplets that, even if they are turned upside down, water droplets remain attached to their surfaces for very long times. This phenomenon is known as rose petal effect, and it is caused by the array of micro papilla on the surfaces of the petal together with the nano folds existing on each papilla top [6]. These hierarchical micro and nano structures provide sufficient roughness for superhydrophobicity but have high adhesive force with water. Biomimetics is termed as the applications of biological systems found in nature to design engineering materials having special functionalities. Artificially prepared surfaces having special wettability and functionalities that were inspired from plant leaves have been the subject of many academic studies [7], [8], [9]. The main route to achieve such hydrophobic surfaces involves the creation of surface roughness caused by the macro and nano domains out of low surface energy materials [10], [11], [12], [13]. Template assisted methods are based on the formation of organic or inorganic materials over rough templates which are later removed or peeled-off to obtain self-standing replicas [14], [15]. Replica formation techniques involve atomic layer deposition, nano casting, chemical vapor deposition and physical vapor deposition (PVD). The desirable method for replication of a natural surface functionality should reproduce the surface topography with high accuracy, while producing replicated surfaces with controllable chemistries. A special CVD technique, called initiated chemical vapor deposition (iCVD) is capable of forming thin polymeric layers over complex substrates [16]. In iCVD, resistively heated filaments above the substrate provide the energy for reaction; therefore the substrate to be coated remains free from solvents, high temperatures, plasma or light sources, which can alter the chemical and/or physical nature of the fragile substrates [17]. Also, the conformal nature of the iCVD process allows uniform coatings around substrates having complicated geometries. For these reasons, iCVD is considered to be ideal method for biomimetic studies.
In this work, we used initiated chemical vapor deposition (iCVD) method, for the first time, to form polymeric duplicates of rose petal structures. First, the patterns on the petal surfaces were copied on a poly vinyl alcohol mold. The ability of iCVD to form functional polymer films on fragile polymer surfaces like PVA mold allowed the exact replication of nano and micro structures on the mold surfaces.
Section snippets
Materials
PVA was purchased from Sigma–Aldrich (Mw = 13,000–23,000). The monomers glycidyl methacrylate (97%, Aldrich) and 1H,1H,2H,2H-perfluorodecyl acrylate (97%, Aldrich), and the initiator tert-butyl peroxide (TBPO, 98%, Aldrich) were used as received.
Negative mold preparation from rose petal
6 wt% poly vinyl alcohol (PVA)–water solution was poured on the surface of a red rose petal. After waiting 24 h at ambient conditions to evaporate all of the water, the remaining PVA thin film on the petal surface was peeled off. The obtained PVA film has
Results and discussion
Fig. 1 shows ESEM and SEM images of the fresh and dry red rose petal surfaces, respectively. The surface of the fresh petal is covered by papillae that have unique semi-spherical shapes. The top of each papilla contains nano folds heading toward the center. These structures on the rose petal surface allows for hydrophobicity, with a measured static contact angle of around 151 ± 3°. Because of a large adhesive force between the water droplet and the rose petal surface, 5 μL water droplets on the
Conclusions
In this work we demonstrated the first-time use of solvent-free initiated chemical vapor deposition process in biomimetic studies. Conformal nature of iCVD helps to make exact copies of natural surfaces through a two-step replication process. Biomimetic polymer sheet templated from the fresh petal surface of a red rose possessed the same wetting characteristics of the rose surface. By using the same duplication process, many other natural surfaces can be copied into chemically well-defined
Acknowledgement
This research was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) (Project No. 110M088).
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