Researchers from University College London (UK) developed a new paint which, when applied on surfaces, makes them superhydrophobic (very water repellant) and self-cleaning.
Such surfaces are resistant to abrasion and handling; moreover, they maintain their properties even if contaminated with oil. These characteristics make them suitable for new everyday applications.
Self-cleaning surfaces are coatings that clean themselves in the presence of water.
Generally, these coatings are superhydrophobic, i.e. they strongly repel water. This means that when they come in contact with water, the liquid does not spread over the surface, but it rolls on it, forming beads of water with an almost perfectly spherical shape.
These rolling water drops remove dirt/impurities which are present on the surface. Microbiological contaminants such as bacteria/viruses can also be removed, making these surfaces self-sterilizing.
Problems and Limits of New Superhydrophobic Surfaces
Surfaces with these characteristics are already commercially available and being used, for example the self-cleaning glass or anti-fogging glass used in windows and mirrors.
However, the main problem of these materials is that they are not very resistant to damage, as they can be easily scratched and abraded. In some cases, for instance, simply wiping the surface with a finger may cause damage, so that the material loses its self-cleaning properties. Moreover, contact with oils can also have the same effect.
This limits the possible applications of the self-cleaning surfaces.
New Paint Development
Researchers from University College London (UK) developed a new paint which can be easily applied to different materials and which makes the surface self-cleaning. The work was carried out in cooperation with Imperial College London (UK) and the Dalain University of Technology (China).
The paint is made of nanoparticles of titanium dioxide (TiO2) coated with a silicon-based fluorinated compound, perfluorooctyltriethoxysilane (Si(OCH3)3(CH2)2(C6F13)).
The research was published in Science on the 6th of March 2015.
Suspension in Ethanol
Professor Ivan Parkin, Head of the Chemistry Department of University College London and leading scientist in this research, explains to Decoded Science the way the paint was prepared.
“The first step was to prepare a liquid solution which contained the hydrophobic material. To do this, we dissolved perfluorooctyltriethoxysilane in ethanol; this is a species which makes the surface of the material water repellent, or hydrophobic. Then we added TiO2 nanoparticles, which are used to make self-cleaning surfaces. We used nanoparticles with two different sizes, some larger ones, with a diameter between 60 and 200 nm, and some smaller ones, with diameter of about 21 nm. Mixing everything together, the perfluorooctyltriethoxysilane formed a layer on the TiO2 nanoparticle surfaces.”
From Suspension to Coating
Professor Parkin and his coworkers then applied this suspension on the surface of different materials, to make these surfaces hydrophobic.
“To make the coating, we used different methods, depending on the material,” professor Parkin said. “For hard materials such as glass or steel, we used a spray gun; for fabrics such as cotton wool we employed the deep coating technique, while for filter paper we used a syringe. After the application of the suspension, the ethanol evaporated and a continuous coating was formed.”
In all cases, the treatment made the surface of the materials superhydrophobic and self-cleaning. For instance, when a water droplet got in contact with these surfaces, it bounced back, without spreading or being absorbed by the materials.
The picture on the side shows some treated cotton wool being dipped into dyed water (a) and then taken out (b); it can be seen how the cotton is still completely dry and white, with no dying absorbed.
Resistance to Abrasion and Contamination
According to professor Parkin, the most important and innovative aspect of this work is that these surfaces are very robust and quite resistant to abrasion.
“To improve the resistance of these self-cleaning surfaces, we decided to use them together with commercial adhesives such as tapes and sprays. We used several adhesive combination, and a very successful one was double-sided tape used with a spray; overall our system was made of (paint+adhesive+substrate). The materials made like this showed excellent resistance to abrasion; in fact they kept their structure and properties unaltered after tests such as finger-wipe, knife-scratch and sand-paper abrasion.”
Further to this, professor Parkin and his co-workers also tested the resistance of these surfaces to oil contamination. They immersed the samples in an oil solution, and then they assessed their self-cleaning behavior. Results showed that there were no significant changes, with the superhydrophobic properties maintained.
New Interesting Applications
Watch the movie here (Video by Oli Usher, from UCL Mathematical and Physical Sciences.)
to see professor Parkin and his colleagues explaining their research and to see how self-cleaning surfaces work.
Superhydrophobic Surfaces: Benefits
“These surfaces, with their resistance to abrasion and to oil contamination, can potentially be used for many more different applications in which conventional self-cleaning surfaces do not work. With our tests we wanted to simulate some of the stresses these materials can undergo for uses in everyday life; results showed that they can definitively resist in these conditions without losing their properties.” Professor Parkin said.
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