Researchers and scientists are interested in the nanoscale, because when many materials get down to these tiny sizes, they start to behave differently and novel properties emerge. Sometimes the material becomes explosive (for example, aluminium) or its melting point changes (for example, gold) or a new property is revealed (for example, silver becomes antibacterial and an odour eater). These novel properties are mostly due to changes in size and scale and the physics ‘rules’ that govern materials at the nanoscale.
Changes in size and scale
Changes in size and scale are factors useful in helping to predict and describe a material’s behaviour. For example, the smaller something is, the larger its surface area (the amount of surface an object has) when compared to its volume, so a nanoscale object, which is very small in at least one dimension, will have a vastly increased ratio of surface area to volume. The magnitude of this change depends on the size and shape of the nanoscale object. Consider the difference between a nanofibre, which is very thin and long, and a spherical nanoparticle, which is very small and round.
The surface area to volume ratio depends on the size and shape of an object. If you would like to explore how this ratio changes for different shapes and sizes, here is an activity where students model different shapes using clay and calculate the surface area to volume ratios with the aim of trying to develop a more efficient shape.
Catalyst nanoparticle shapes
This high surface area to volume ratio is a very important characteristic of nanoparticles when they are being used as catalysts. As chemical reactions take place at the surface of a chemical or a material, the greater the surface area for the same volume, the greater the reactivity. So, for nanoparticles, maximising the available surface area available is important for maximising possible reactivity.
Changes in the physics ‘rules’
When a material reduces to the nanoscale, there’s a shift in the physics rules that govern and predict its behaviour. Nanofibres sit in between two sets of physics rules – those that govern macroscale and microscale objects (classical mechanics) and those that govern atomic scale objects (quantum mechanics). This is illustrated in the scale ladder.
One well observed phenomenon at the nanoscale is that gravity becomes a markedly less significant force, while van der Waals forces become incredibly strong. At the nanoscale, strong van der Waals forces make materials ‘sticky’, so a nanofibre can be very effective in attracting and trapping small particles because it’s ‘sticky’ and because it has a large surface area. This makes nanofibres excellent materials for use in filtration.
Electrospun collagen nanofibres for air-filtration
SetaTM air-filters have collagen nanofibres electrospun onto the surface of a potato starch based disc. Watch the video and listen to Iain Hosie explain the importance of van der Waals forces in nanofibre material being used for air filtration.
Many novel properties are emerging as materials are being reduced from macroscale to nanoscale. This change in the properties of materials is leading to the creation of new and enhanced nanomaterials. Nanoscale materials like nanoparticles and nanofibres have an exciting future in a wide range of high-tech applications.