Nature inspired surface hierarchical structures

Nature inspired surface hierarchical structures

by Shashini Rathnayaka

As the world thrives towards unimaginable realities with advanced technologies, it is highly essential for human beings to blend more with nature. Given all the natural disasters that have arisen in the past decade around the world, it is no doubt that we should accelerate the world towards smart and sustainable technologies. Biomimicry or the bio inspired technologies is one area of research that has greater potential in contributing towards this cause. Nature inspired surface engineering technologies are a further narrowed down path in biomimicry.

The most popular nature inspired surface functionality is superhydrophobicity mimicked from the lotus leaf or the rose petal. Lotus leaf surface has nano scale fibers on a micro pillar textured surface and a hydrophobic wax layer on top which makes the leaf superhydrophobic. Although this is a known phenomenon by many, the unpopular fact is that this special effect is created not only by the hydrophobic polar groups of the surface material, but the nano and micro level hierarchical structure present on the surface.

Surface functional properties of a solid material upon contact with a liquid depend on three factors; composition/atomic arrangement of the solid, surface roughness at the nano and micro scale, and compositional characteristics of the liquid that comes into contact with the solid surface. Surface roughness can be simplified as surface topology or surface texture at nano and micro scale. Exceptional surface properties are achieved through these surface textures which can trap air in between those hierarchical structures which manipulate the liquid to solid contact.

Differences in shape, size and density of the nano/micro features enable tremendous wetting responses of solid materials which make the surface engineering world interesting. Even though both lotus leaf and rose petals are non-wettable, there is a difference. Water droplets can easily roll off on lotus leaf, but water droplets do not roll off on rose petals and sticks to the surface instead. This is resulted by surface patterns that are unique to each species.

Let us explore the beauty of these surface science principles through a few examples. Anti-fogging surface designing is one insightful topic. Anti-fogging effect can be easily incorporated onto a solid material by introducing a hydrophilic or moisture absorbing coating on to the surface.  But a challenge poses when a hydrophobic material surface has to be converted into an anti-fogging surface because introducing a hydrophilic coating on to a hydrophobic surface is highly unlikely due to the surface energy gap. The common solution for this is the introduction of amphiphile compounds. But nature on the other hand, has a better way of achieving this. It has been long discovered that the mosquito eye is anti-fogging due to the superhydrophobic surface structure of hexagonally non-close-packed nipples at the nano scale and hexagonally closed packed hemispheres at the micro scale. This hierarchical structure prevents water condensation on the surface thereby preventing the formation of fog. This technology is ideal for incorporating the anti-fogging effect on to a hydrophobic material.

The science gets even more interesting when dew water harvesting systems adapt the same technology with a different setting. Just like the surface texture can be designed to prevent water condensation, it can also be designed for nano features to act as water nucleation sites that accelerate the condensation of water from the atmosphere. Likewise, variations in the surface patterns enable designing of smart surfaces with a wide range of unique surface characteristics including but not limited to superhydrophobic, superhydrophilic, ice-phobic, anti-corrosive, anti-fogging and antimicrobial properties.

(Mosquito eye:

However, the major barrier that is slowing down the “nature technology” transfer to “manmade technology” is the precise recreation of nano patterns synthetically. Chemical methods (layer by layer assembly, chemical etching, nanomaterial deposition etc.), lithography and laser micromachining are a few technologies that are in practice. Scalable and commercially viable technologies are yet to be developed. Scientists expect that the advancement of these nano and micro machining techniques will bring greener solutions to the world. Therefore, it is possible to envision that in future there might be robots that can imprint nano/micro level surface topologies on any surface more efficiently.

The latest technology advancement in bio inspired surfaces is the superomniphobic surfaces which cannot literally be wetted by any liquid. There have also been studies done by using these surface science principles in designing antimicrobial surfaces which prevent microorganism adhesion. With raising concerns of resistance development of microorganisms due to over usage of antiseptics, this seems a better approach. It is again inspired by species in nature including insects and plants which show protection against pathogens. Nature inspired surface engineering is not a matured area of science which makes it crucial for exploration due to the foreseen economical value of potential applications.