Wetting and Self-Cleaning Properties of Artificial Superhydrophobic Surfaces

Abstract

The wetting and the self-cleaning properties (the latter is often called the "Lotus-Effect") of three types of superhydrophobic surfaces have been investigated: silicon wafer specimens with different regular arrays of spikes hydrophobized by chemical treatment, replicates of water-repellent leaves of plants, and commercially available metal foils which were additionally hydrophobized by means of a fluorinated agent. Water droplets rolled off easily from those silicon samples which had a microstructure consisting of rather slender spikes with narrow pitches. Such samples could be cleaned almost completely from artificial particulate contaminations by a fog consisting of water droplets (diameter range, 8-20 microm). Some metal foils and some replicates had two levels of roughening. Because of this, a complete removal of all particles was not possible using artificial fog. However, water drops with some amount of kinetic impact energy were able to clean these surfaces perfectly. A substrate where pronounced structures in the range below 5 microm were lacking could not be cleaned by means of fog because this treatment resulted in a continuous water film on the samples.

Self-cleaning glazing products: A state-of-the-art review and future research pathways

“…The unsatisfactory performance results by hydrophobic glass presented in Figure 5, might be explained by the results found in another study, by Fürstner et al (2005) [11]. This work investigates the self-cleaning properties of artificial superhydrophobic surfaces.…”

“…by x-ray photoelectron spectrometer (XPS) and scanning electron microscopy (SEM). These apparatuses may then be used to measure chemical elements on the surface of the self-cleaning films [5,10,11].…”

“…Furthermore, considering the results by Fürstner et al (2005) [11], the superhydrophobic property of an artificial Lotus-leaf (Nelumbo nucifera) was found to have excellent self-cleaning abilities, with a contact angle of about 158. The study also tested artificial metal surfaces of superhydrophobic ability with a contact angle of almost 165, which was found to remove over 98 % of the contaminants on its surface after it was subjected to artificial contamination and rinsing.…”

“…Illustration of how self-cleaning glass works, in three steps, from left to right: 1) activation of the coating by UV radiation and natural dirtying, 2) decomposition of the organic dirt, and 3) rain water washes away the loosened and degraded dirt. Source: Pilkington Active [6] Several investigations have been carried out on hydrophilic and hydrophobic surfaces with their respective superhydrophilic and superhydrophobic counterparts or enhancements, and on self-cleaning properties and various possible application areas in general [1,2,5,10,11,12,22,24,25,27,33,34,35,36,37,38,39,40,41,42,43,44,45,46]. It is not within the scope of this work to go into details of all these, as the focus of this work is on self-cleaning characterization methods, state-of-the-art self-cleaning glazing products of today and future research pathways for self-cleaning glazing products of tomorrow.…”

“…However, the smooth hydrophobic surface was nearly unaffected Figure 5: Haze value versus overall visual ranking for clear float glass (CFG), selfcleaning glass (SCG, hydrophilic photocatalytic glass) and hydrophobic coated glass, where the haze value is defined as the ratio diffused/direct transmitted light through the specimen, and the overall visual ranking is defined as perceived cleanliness of the glass surface, ranked from 1 (very clean) to 5 (very dirty). Source: [1] by the impact pressure, and thereof demonstrates the limited ability of smooth hydrophobic surfaces to be decontaminated by drop impact only [11]. Seeing that the glass specimens in the study by Fürstner et al are installed vertically, as most glass windows and doors are, the hydrophobic glass, especially, are dependent upon lashing rain to effectuate their selfcleaning property.…”