Sunday, 11 December 2011

Omniphobic, huh?

'Slippery when icy' by ksr8s
under a CC license
That was meant in a scientific sense, of course!

'Omniphobicity' is the property that some surfaces possess to repel both polar and non-polar liquids. The former (polar liquids) typically refers to aqueous solutions and those surfaces that repel polar liquids are called 'hydrophobic'. The latter (non-polar liquids) typically refers to oils and those surfaces that repel non-polar liquids are called 'lipophobic'.

One would think that hydrophobic surfaces would tend to be lipophilic while hydrophilic surfaces would tend to be lipophobic. Well, often it is that way. But not always. The degree of hydrophobicity of a surface varies, depending on a number of factors, including composition and structure of the interfacial layers.

Interestingly, the behaviour of surfaces (or 'interfaces', to be more precise) regulates many processes in nature, including some we don't normally think about. For instance, the feathers of aquatic birds tend to be covered by a hydrophobic powder that waterproofs them, while in some cases the hydrophilicity of the feathers is controlled so as to allow some species to submerge into water.

Plant leafs tend to be hydrophobic, as they are covered by waxes. A classical example is the lotus leaf, seen in the video below:

Exploiting the properties of the different surfaces has found plenty of technological applications. TEFLON-coated pans is a humble but extremely common one. Applying coatings to surfaces can modify their behaviour, mostly regardless of the composition of the underlying material. For instance, the following video shows the behaviour of two types of wool cloth when wetted:

Hydrophobic behaviour feels a bit weird, doesn't it?

But, really, the options and possibilities are many. For instance, rendering surfaces hydrophobic, helps waterproofing and grants surfaces 'self-cleaning' abilities. Commercial products for that are in the market since several years and are extremely easy to apply (e.g., nanophos products - btw, I'm not affiliated to them).

Rendering surfaces to dislike both polar and non-polar liquids, however, has proven a bit challenging. You see, apart from achieving the desired surface behaviour, the resulting layer normally has to have some acceptable mechanical properties and reasonable endurance. Recently, a Harvard-submitted paper appeared in Nature describing the creation of 'Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity' [a more layman-friendly description can be found in the Discover magazine blog entry]. As the title suggests, the surface modification was inspired by the Pitcher plant. The authors suggest that having such a surface could be advantageous in handling liquids, medical applications, applications where anti-fouling and self-cleaning properties are needed (e.g., in the food industry).

That's certainly an interesting technology with a good potential of proving 'disruptive'. Of course, for food (and medical) applications, there are still a number of rigorous checks to make in order to ensure that the particular surface coating is toxicologically acceptable, remains stable in the range of temperature, pH, solvents and mechanical stresses used in food processing, cannot be attacked by bacteria (unlikely, since it is omniphobic) and that it doesn't break down or leak in any way to the food it is in contact with. And all that, at a reasonable cost... If it goes through, I bet we'll soon see some interesting product concepts on the market!

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