Self-supporting superhydrophobic thin polymer sheets that mimic the nature's petal effect

Abstract

The high adhesive force between the red rose petal and the water droplet on its surface is termed as the ‘petal effect’, which is caused by the hierarchical array of micro papilla on the surfaces together with the nano-folds existing on top of each papilla. Because of that special surface topography, the surface is superhydrophobic, but at the same time highly adherent to the water droplet such that the droplet cannot move even if the surface is turned upside down. In this work, we produced a thin (thickness below 1 μm) self-supporting polymer sheet that mimics the surface of a red rose petal. The product is a two-layer polymer sheet made from poly(glycidyl methacrylate) (PGMA) as the supporting layer and a hydrophobic poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) on top of it as the functional layer, both of which were deposited by initiated chemical vapor deposition (iCVD) process. The integration of conformal and solvent-free iCVD process into the classical two-step molding procedure allowed exact transfer of surface topography of the petal surface, which was verified by SEM analysis. The static contact angle of water droplet on the surface of the polymer replica was found to be 152 ± 3°, and the water droplet did not roll-off even the polymer sheet is tilted or turned upside down.

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.