How small is “Nano” scale? For most of us, it is difficult to imagine such a small unit of measurement. To help you imagine how small Nano scale is, here’s a comparison: a single human hair is about 80,000-100,000 nanometers thick.
Nanotechnology is any kind of scientific application that deals with such small materials. Nanotechnology has become a very important branch of development in the last 30-40 years. As advanced microscopes, such as scanning electron microscopy (SEM) or atomic force microscopy (AFM), became more popular, nanometer scale visualizations become easily accessible in universities and laboratories. This has allowed laboratories to try modifications on different materials or biological samples. Nanotechnology is relevant for every branch of science; however, in this article we will talk about the application of it in materials science, which is the study and design of new materials.
Hydrophobic means water repelling, as the name implies. A super hydrophobic surface means a surface that does not hold any water. On the other hand, super hydrophilic means a surface that loves water, and therefore is completely absorbent (Figure 1).
A material that doesn’t get wet is desirable for many applications. These kinds of materials don’t hold any dirt or mud, and can be especially useful as military clothes or as water repellant glass for the front window of cars, etc. Even though today’s nanotechnology can make the surfaces of materials super hydrophobic, the materials can remain so only for a short period of time. These kinds of modifications either get torn off the surface or lose their hydrophobic properties due to friction, etc. Therefore, water repellant surfaces haven’t become commercially popular – yet.
There are many surfaces in nature that are super hydrophobic. Almost all of the hydrophobic nano-technological developments have been inspired by these natural surfaces. This is also called “bio-mimicking,” because nature is mimicked for scientific purposes. Examples of super hydrophobic surfaces in nature are butterfly wings, cicada wings, mosquito feet, duck feathers, and some plant leaves. There are many reasons for hydrophobicity to exist in nature. For instance, the super hydrophobicity of a butterfly’s wings allows it to fly while it is raining. The same quality allows ducks to stay dry while swimming or mosquitoes to walk on water. And the super hydrophobic nature of lotus leaves allows them to “self-clean.”
Every creature with super hydrophobic properties has different nano structures. Nano structures on animal surfaces not only allow for super hydrophobicity, they can also give the creature different properties. For example, the nano pattern on shark skin was discovered to be anti-biofilm forming. The mechanism of how this structure prevents bacteria from colonizing it is still a mystery. It is thought to have the exact surface tension for repelling bacteria. This pattern has been commercialized by Sharklet® and is sold to help make much hospital equipment bacterial resistant.
Lotus leaves also have many interesting micro/nano structures. Lotus leaves are known to grow at the bottom of ponds, but emerge above the water surface as if untouched by the contamination of the dirty water that they grow in . The water-repellant lotus leaf is often associated with extreme purity, as the surface restricts the growth of bacteria and pathogens . When a water droplet touches the lotus leaf, it immediately rolls off the surface, dragging the dirt and dust accumulated on the leaf’s surface (Figure 2 and Figure 3). Thanks to this mechanism, the surface of the leaf is constantly clean and dry. This “self-cleaning” mechanism was discovered by advanced microscopes that allow scientists to observe the micro/nano structure of the leaf’s surfaces.
When a piece of lotus leaf was observed under a SEM microscope it was seen that these surfaces have physical hierarchical surface features. What this means is that the surfaces have micro scale roughness patterns, and on top of these roughness patterns they have even smaller nano scale roughness patterns (Figure 4). These roughness patterns allow for air pockets to form on top of the surface; therefore, water does not stick to these surfaces. When the roughness is only at the micro scale, the surface becomes hydrophobic; however, for super hydrophobicity to occur, a combination of micro and nano scale roughness is necessary (Figure 5).
Many labs are trying to mimic this hierarchical micro/nanostructure roughness pattern. Some of the modifications have been successful, such as coating polymer surfaces with inorganic particles like Nano-diameter silica particles. Another possible application is making physical modifications on the surfaces of polymers using sophisticated methods like photolithography. Various suggestions have been made by laboratories all around the world. However, super hydrophobic surfaces have not been commercialized yet because these applications are either too sophisticated and time consuming to apply, or the modifications made are not permanent – meaning they get peeled off over time.
The micro/nano scale topography of every creature in nature is different, giving each of them different properties. Some of these properties allow for super hydrophobicity, some allow for super hydrophilicity, and some nano scale patterns prevent bacteria from sticking to the creature’s surface. When these creatures were created, their surface topographies were tailored according to their needs. It’s remarkable they could be made so perfectly when we as humans have to put in an incredible amount of effort and research to mimic the roughness of a single leaf. As science allows us to understand how nature works, our amazement at the perfection of the universe becomes stronger.
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