Altering the wetting properties of FEP films by nano-structuring the surface using reactive ion etching

Abstract

Renewable energy sources has become a large field of research, and in the recent years, methods to harvest energy from rain drops have been developed. This technology is based on rain drops rolling or sliding on a polymer surface. Thus, the behavior of drops on such polymer surfaces are of interest. In this thesis work, the surface of fluorinated ethylene propylene (FEP) has been modified by creating nano-structures using reactive ion etching (RIE). Different etch parameters resulted in nano-structures with shapes like granules, hairs and holes. The granules and hairs had a diameter of approximately 100 nm, and the tallest hairs were about 500 - 600 nm tall. The highest measured aspect ratio of hairlike structures were about 5 (height/width). In order to accurately measure the wetting properties of the nano-structured FEP, a suitable measurement method had to be chosen and optimized. Experiments with a tilted plate enabled reproducible results for the nano-structured FEP of advancing contact angle (ACA), receding contact angle (RCA) and the roll off angle (ROA), i.e. when the drop starts to roll or slide off the sample. By the ROA, the adhesive force between the drop and the surface of the sample could be calculated. An alternative method to estimate the adhesive force using measurements of ACA and RCA has been found to give comparable results to the method using the ROA. Generation of nano-structures altered the wetting properties of FEP significantly. The ACA increased from 118 ± 2◦ to 144 ± 5◦ at the most, and the RCA decreased from 102 ± 2◦ to about 20◦ for FEP treated by different RIE processes. Drop-surface adhesion increased from 110 ± 10 µN for untreated FEP, to 620 ± 30 µN at most for hairlike nano-structures.