Oil repellents: Taking a leaf from the lotus

The lotus leaf symbolises purity, at least in some Asian cultures, on account of its remarkable ability to stay clean and dry even in the moist and damp conditions of the environment in which it grows. When a drop of water falls on a lotus leaf, it does not wet the leaf; instead, it beads up and rolls away, leaving the leaf dry and, as a bonus, clean too. The rolling water drop washes dust and other particles off the surface.

Scientists have long marvelled at this ability and tried to mimic it – and with growing success, recently. Thanks to their efforts synthetic water repellents are now used in a range of applications today. But, creating an oil repellent was quite another matter.

Oil-repellents: A sticky challenge
Oil is a more slippery (and rather sticky) customer due to its low 'surface tension', which makes oil cling to surfaces. An oil-repellent surface is, therefore, much more difficult to create than a water repellent surface.

Scientists at the Massachusetts Institute of Technology (MIT) are working on ways to make a super-oil repellent surface - or, as scientists call it, a super-oleophobic surface. They started with a polymer, containing many oil-repelling fluorine groups, developed by the US Air Force.

Fluorine chemical compounds are used in Teflon, the material used in coatings applied to cookware, to impart resistance to oil. Scientists at MIT developed a molecule with a structure that incorporates much more fluorine.

But, that is not the whole story. Chemistry alone does not produce a super-oleophobic substance. The scientists had to tweak the microscopic structure of the material in a way that allowed it to trap air near the surface. This kept the oil droplets falling on the material suspended on air and prevented them from sticking to the surface. The material was so oil repellent that oil just bounced off it.

Geography as important as chemistry
Scientists researching the lotus effect had earlier discovered that it is not the chemistry of the surface alone but also its 'geography', i.e., the roughness of a surface, that helps create a water repelling surface.

When a lotus leaf is examined under a high power microscope, the smooth waxy surface reveals itself as a mesh of microscopic bumps. When a water droplet falls on the leaf, it rests on these microscopic bumps and instead of wetting the leaf, it rolls off under its own weight with the slightest movement of the leaf.

Taking a cue from the lotus leaf, scientists looked at ways of creating a surface that would trap pockets of air in its microstructure. They came up with two ways of creating such a surface - electro-spinning and etching.

The electro-spinning process involves cutting microscopic threads on the fluorinated surface material, forming a mesh-like structure that traps air. In the etching process, scientists used silicon wafers on which they etched arrays of mushroom-shaped pillars that trapped air under the cap of the mushroom shape.

According to scientists, any material can be made oil repellent by these processes. The addition of fluorine groups imparts strength to the material.

Different applications
The scientists went on further to detail ways of creating super oil-repellent microstructures. They also created different microstructures to suit different applications (depending on which liquid was meant to be repelled).

In one instance, they were able to make materials that could repel water but not oil. Such materials could be useful in applications that involve filtering water out of fuel.

One interesting possibility scientists are working on is the development of transparent oil-repellent materials. Such materials could be used in displays that resist staining and smudging from finger prints. Cell-phone manufacturers have been trying to make just such a material for years.

Fibre mats made of repellent material are already being used to clean up hazardous waste. The US Air Force has shown interest in using the new oil repellent material to protect components of airplanes and rockets from jet fuel.

Materials made from such repellents also hold promise for development of self-washing car paints and stain-resistant clothing.