Superhydrophobic Surfaces Make Cleaning A Task Of The Past

Have you ever seen a drop of water strike a lotus leaf? It immediately beads up and runs off, taking any dirt particles with it. This self-cleaning functionality is known as the Lotus Effect and is a coveted property for many man-made materials as well. Recently, scientists have reported a new manufacturing technique that allows glass surfaces to be easily endowed with this function. The basis of the effect is superhydrophobicity.

Lotus leaf: this computer generated image shows the nanostructure on a lotus leaf surface that allows it to be self-cleaning. Generated by William Thielicke.Lotus leaf: this computer generated image shows the nanostructure on a lotus leaf surface that allows it to be self-cleaning. Generated by William Thielicke.

When a droplet of water is left to its own devices, its high surface tension leads to the formation of a sphere because this is the shape with the least surface area and consequently the lowest energy. When the droplet contacts a surface, competing adhesive forces may cause it to spread out and “wet” the surface. However, certain surfaces, those termed superhydrophobic, may be so inhospitable to water that a spherical shape remains the most energetically efficient. In these cases, the droplets do not wet the surface, but rather remain beaded up and can roll right off. Superhydrophobicity can be conveyed either through chemical coatings (think of your car after a fresh waxing) or by nanostructuring of the surface (as in the case of the lotus).

The use of the lotus as model is not a novel development, it has already been used in the production of mildew-resistant paints and anti-fog coatings, among other applications. The researchers in this case therefore decided to go one step further by employing a second biological inspiration in their work. They also modelled their surfaces on a moth’s eye. Moth’s eyes are inherently antireflective thanks to naturally occurring nanostructures that alter the refractive index of incoming light as it travels to the cornea. Antireflectivity is particularly valuable in the development of solar cells where the more light that is absorbed, the more power can be generated.

Moth's eye: the surface of a moth's eye is naturally antireflective, a highly desirable property for solar cells. Image by John Alan Elson.Moth's eye: the surface of a moth's eye is naturally antireflective, a highly desirable property for solar cells. Image by John Alan Elson.

Couple these two principles and you have higher efficiency solar cells that are also self-cleaning. The scientists produced the coating by first depositing a thin layer of glass which is then heat-treated and chemically etched into the desired nano-topology. The resultant surface is not only superhydrophobic and antireflective, but also remarkably durable. This distinguishes it from many previously reported chemical-based coatings which wear off over time, particularly when exposed to inclement weather as would be expected in many proposed scenarios. Prototype solar cells equipped with the coating were found to give a 3 – 6 % increase in electricity production while also offering significantly reduced maintenance costs.

Aside from solar panels, any number of optical devices could benefit from such a surface. These include items like goggles, sensors, photodetectors and periscopes. This research, like much that has come before, truly highlights how much we stand to learn and benefit from the “natural technologies” that have evolved on our planet.

Via Science Daily and the Journal of Materials Chemistry C.