Fed up with your businesses’ spiraling cleaning costs, the growing cost of labour, and the short intervals between cleaning cycles? A series of hydrophobic cleaning products are emerging on the market that delivers self-cleaning surfaces, reducing the need for regular washing, scrubbing, and polishing with chemical agents.
The Science Of Self-Cleaning
Two seemingly contradictory principles lie behind the development of hydrophobic cleaners: an extreme non-attraction to water (superhydrophobicity) and an extreme attraction to water (superhydrophilicity).
The behaviour of water landing on a surface is described by Young’s equation, which relates surface tensions between the solid surface, liquid droplets, and surrounding air to the contact angle between the water/air interface and the solid.
Water droplets consequently assume a cap-like shape when sitting on a solid substrate. The smaller the contact angle, the flatter the droplets and the wetter the surface. In contrast, large contact angles mean reduced area at the liquid/solid interface, resulting in the substrate not being as easily wetted.
Self-cleaning materials exploit what happens at the extreme ends of this situation, where the contact angle either approaches 180° (superhydrophobic) or zero (superhydrophilic) situations. Water coming into contact with a superhydrophilic surface spreads out and forms a thin film, while on the superhydrophobic surface, water forms spherical bead-like drops, minimizing the solid/liquid contact area.
In the first scenario, cleaning can occur when a thin sheet of water running off a surface washes away loose dust. The second scenario relies on the spherical droplets rolling off the surface and picking up particulate dirt in their path as the roll.
Two contrasting approaches create the same result: a surface that exploits the dynamics of drop formation and wetting to maximize dirt removal when doused with water. If the water results from rain, then no external cleaning is required to keep surfaces free of dust and dirt, hence the term self-cleaning.
Putting The Science To Work
The concepts explaining the principals of water-based, self-cleaning sound simple. However, creating a synthetic material capable of ridding itself of residual dust during a rainstorm requires a detailed understanding of the characteristics of the surface itself.
Rainwater may be repelled from a superhydrophobic surface, but what if the drops slide or stick? If light rain falling on a superhydrophilic surface evaporates rather than providing an effective clean, what happens to that dirt? Moreover, is water alone sufficient to shift the organic debris and grime that accumulates over time?
Proponents of the superdry, beading-and-rolling approach owe their breakthrough to two botanists who first outlined the basic principles of superhydrophobic, self-cleaning surfaces.
Wilhelm Barthlott and Christoph Neinhuis, both from the University of Bonn, noticed how certain leaves appeared to protect themselves from external contamination. In particular, they studied the characteristics of the sacred lotus plant, a wetland species native to Asia and revered over the generations as a symbol of purity.
Barthlott and Neinhuis analyzed the lotus leaf structure and concluded a link existed between its surface appearance and self-cleaning functionality. They coined the term the ‘Lotus Effect’ and trademarked it. Their expression is now synonymous with superhydrophobic self-cleaning.
Discovering the Self-Cleaning Phenomena
The botanists’ key observation was that the apparently smooth lotus leaves are actually rough. Each leaf surface is covered with an array of tiny lumps, 5 to 10 μm high and approximately 10 to15 μm apart. This uneven surface is itself festooned with waxy, hydrophobic crystals, each around 1 nm in diameter. This discovery prompted scientists and engineers in the coatings industry to move away from the idea that smooth surfaces equal clean surfaces.
The two-tier roughness on the leaf’s surface proved especially important to its self-cleaning properties. Water droplets meeting a superhydrophobic material effectively perch on high points on the rough surface, minimizing their contact with the solid and forming spherical drops.
The lotus leaf’s dual texture level increases the leaf’s overall surface roughness, boosting its hydrophobicity. Moreover, the two-tier setup also reduces the likelihood of surface air spaces being inundated by water. Drops sitting on top of the textured layer roll easily away.
How Superhydrophobic Self-Cleaning Works
Water landing on a ‘clogged’ surface is inclined to either smear across the surface before evaporating, or to stick where it landed. If the drop is adhesive, it does not move, so it does not take dust or grime with it. As the drop evaporates, it concentrates the dust close to the contact line, so the surface looks dirtier than before it rained.
Creating anti-adhesive, superhydrophobic surfaces involve engineering materials with two scales of roughness, just as nature does. Many materials are naturally superhydrophobic, and close examination always shows lumps and bumps of 20 μm in size, and on each bump, you see smaller lumps approximately 1 μm in size.
What Does This Mean For Your Business?
Hydrophobic cleaners containing the semiconductor TiO2 absorb ultraviolet light, which reacts with oxygen and moisture in the air to form activated oxygen. This activated oxygen helps oxidize and decompose organic grime and kill bacteria.
Water falling on the activated surface simply spreads out into a continuous film washing away any remnants of loose grime. These characteristics allow continual cleaning and prevention of contamination of the surface when exposed to the sun, artificial light or the rain.
Less grime means business can extend the time between cleaning cycles, reducing labour and materials costs. Self-cleaning surfaces also reduce the average time needed to perform cleaning tasks, further reducing costs whilst boosting productivity.
Moreover, business will be able to reduce their consumption of expensive conventional cleaners, some of which contain toxic and even caustic chemicals which need to be treated in dedicated waste handling facilities, further reducing the life-cycle costs of the current generation of industrial cleaning agents.
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Manufacturing self-cleaning, rapid-drying surfaces are increasingly becoming relatively straightforward. Today, researchers are applying modified hydrophobic cleaning methods to apply the coating, rather than chemical vapour based methodologies. These techniques are proving to be cost-effective and easily adapted to a range of applications. Engineers can apply clear, hard coatings to glass, plastics, painted surfaces, metals, and ceramics using alternative methods of coating a surface to suit a specific application.