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Clean as Lotus
December 1, 2009 19:16

Lotus leaf and petals

Recently researchers from all over the world started paying attention to the surfaces, which like lotus, show the ability to self-purification. Buddhists know about this lotus property for hundreds of years, since this flower symbolizes divine purity and innocence for followers of this religion.

Well, superhuman powers have nothing to do with this property of lotus the thing is that lotus leaves have special structure. They are all covered with fanciful coarse papulas of various sizes from micro- to nano-papulas. These papulas are in their turn covered with tiny filaments. If we place a drop of water on lotus leaf, it wont be able to go between the papulas, because their surface is hydrophobic (water-repellent), and would lie on leafs surface like a fakir lies on nails. However, more often a drop easily slips off leafs surface, without wetting it, and takes all dirt with it.

Researchers from various countries of the world developed different approaches, which allow them to imitate coarse surface of the unusual flowers leaves. There are several methods to create required surface pattern, and one of them is modern technique of nanolithography. But there is a problem: many materials, which can be structured by means of nanolithography, have hydrophilic (water-loving) surfaces, thus they cannot be directly used for lotus-effect. Moreover, in his case the combination of micro- and nano-relief will only increase wetting ability and drop adhesion.

How can we change properties of such a heterogeneous surface in order to make it fully hydrophobic? Many traditional techniques appear to be non-applicable due to complex interaction of bearing surface relief and wetting processes.

Carbon dioxide pressure-temperature phase diagram
Russian scientists from the physics faculty of Moscow State University and Nesmeyanov Institute of Organoelemental Compounds applied a combination of two techniques and suggested an elegant solution to the burning problem of modern science. The scientists used the hydrofobization technique by means of supercritical carbon dioxide a compound, which is stable under high temperatures and pressures. Under extreme conditions the carbon dioxide looks like liquid, but it is not. For supercritical carbon dioxide theres not such a problem and wetting. This substance gets between papulas of a surface, treated with nanolithography, as deep as it is required, dissolving many other substances and delivering them uniformly to all surface irregularities. In normal conditions carbon dioxide is a gas, and it escapes, when hydrofobization process is over, leaving the surface modified.

As a hydrofobization agent researchers used ultrafine Teflon (polytetrafluoroethylene), which is perfectly dissolved by supercritical carbon dioxide. Researchers showed than the combination of two techniques provides a long-term stability of super-hydrophobic properties of new surfaces, w which strongly resemble lotus leaf.

Can we use these materials in our everyday life? Why not? May be we cannot get self-purifying clothes, but we can make materials for household appliances, which need clean surfaces.

Source: Science News

Kizilova Anna


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