Cicadas Could Help Us Develop Future Self-Cleaning Everyday Surfaces

In a multi-institutional study led by the University of Warwick, we investigated a model surface that mimics cicada wings to understand precisely how the insects get rid of contaminants from them. If you are thinking “don’t cicadas just shake the dust off their wings?”, the answer is surprisingly no. Cicadas don’t even move a bit to clean their wings because nature does it for them using condensing water droplets. Our study was published in Nano Letters.

Is self-cleaning specific to water droplets?

Self-cleaning surfaces like cicada wings, gecko skin and lotus leaves have a unique property called “super-hydrophobicity” that makes them extremely repellent to water. Due to this, when water droplets do fall on them, or condense under humid conditions, they will assume a spherical shape. These bead-like droplets will either roll down the surfaces due to gravity or “jump” off them upon merging with neighbors. In the process, they “pick up” contaminants, removing them from the surface. This has been observed in recent experiments.

However, so far there have been no theories that completely describe how self-cleaning surfaces work in nature. Once formulated and systematically validated, a complete theory would help researchers predict which liquids can remove specific contaminants from a given surface without any external intervention.

A unified theory of self-cleaning surfaces

In our study, we formulated such a theory, which explains self-cleaning of various surfaces observed in previous experiments conducted at micro-to-millimeter scale. This new theory also remarkably explains the results of our molecular simulations conducted at nanoscale. In our simulations, we placed a nanoparticle underneath one droplet sitting on a super-hydrophobic surface and pushed a second droplet towards the first. They came together, coalesced and jumped away from the surface, carrying away the particle.

Upon carefully investigating various forces involved in the system, we identified the important parameters that influenced the process. It turns out that, prior to removing contaminants from a surface, the droplet with a contaminant underneath will resemble a hot-air balloon that is about to take-off from the ground.

The key findings of the paper were:

• The new theory can be depicted in a phase diagram that gives a comprehensive idea about when and how self-cleaning occurs.
• The pulling force experienced by the nanoparticle/contaminant is at its highest when the droplet is in the hot-air balloon shape.
• The size of the droplet relative to that of the contaminant underneath must be within a particular range for self-cleaning to occur.
• The system parameters can be fine-tuned to inhibit self-cleaning, if desired, and to encourage transport of nanoparticles on super-hydrophobic surfaces.

High-performance self-cleaning surfaces in our day-to-day lives

Our results show that water droplets can only remove a certain class of contaminants from various surfaces. Furthermore, those contaminants cannot be too small compared to the size of the droplet, which is counterintuitive.

The application ambit of our improved understanding of the process includes manufacturing of windowpanes and automobile surfaces that do not require our attention to clean, and in the design of biosensors that trap macromolecules between accurately placed functionalized nanoparticles. Self-cleaning surfaces typically have anti-icing properties too.

While the new theory reasonably explains results of experiments and simulations, it is not devoid of assumptions. We have only validated the theory in scenarios where water droplets try to remove contaminants from various surfaces. Water is the most used fluid in our daily lives as well as in some industries. However, self-cleaning characteristics of other liquids might be of interest elsewhere.

More experiments required for a complete theory

The assumptions made in deriving the equations that govern the self-cleaning process must be investigated further to develop a more general theory. Moreover, there are some predictions made by the current theory that can only be validated by performing experiments at the millimeter scale, which is inaccessible to molecular simulations.

Reference: Perumanath S, Pillai R, Borg MK. Contaminant removal from nature’s self-cleaning surfaces. Nano Lett. 2023;23(10):4234-4241. doi:10.1021/acs.nanolett.3c00257

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