Researchers in the Netherlands are developing a novel surface treatment technique that could result in, for example, a plastic drinking cup that can be reused without the need for washing, simply because contamination has no chance to stick to the surface.
A self-cleaning surface - like that of the leaf of a Lotus plant - is useful for many applications and consumer products. These natural properties can now be imitated remarkably well.
Structuring a plastic or other surface is now possible by using an ultra-fast femtosecond laser, as has been demonstrated by PhD student Max Groenendijk of the Applied Laser Technology Group of the University of Twente.
The secret of the Lotus leaf lies in numerous tiny pillars with a wax layer on top. Water droplets are lifted by these pillars, form a spherical shape and simply cannot cover the surface. Dirt gets no chance to stick to the surface via water, as the spherical drops of water roll off and take the dirt particles with them.
Again, nature proves to be a source of inspiration for the laser scientists of the University of Twente: they aimed at creating similar surfaces, but without having to use wax. They succeeded by using an ultra-fast femtosecond laser. The light pulses are so short that they can be seen as light 'bullets' with which the surface is bombarded.
Max Groenendijk applies the laser in two separate steps. During the first step, the surface gets a fine ripple structure. This is caused by a special self-organising effect that works for almost all kinds of surfaces. Whenever the laser removes some material, a pattern of ripples is formed at the bottom. It is possible to influence this pattern with parameters such as speed, intensity and polarisation.
The second step is to write a pattern of perpendicular lines. What remains is an array of pillars. These pillars then already have the fine pattern caused by the first step. This double structure replaces the need to have wax on the pillars, and makes the surface highly hydrophobic.
Treating surfaces directly would be too expensive but, by using a mould, series products can be moulded in an economic and easy way.
Even for materials that are quite hydrophobic by themselves, the structure can improve the properties drastically. Unlike in the unstructured, smooth situation, where droplets can still smear a little, the structured surface causes water to form spherical drops, as on the leaves of the Lotus plant. The appearance of the material also changes, as well as the 'feel'; a surface that has been treated is said to feel like silk. This could be an added selling point, especially for fashionable consumer products.
To mimic the Lotus effect, several alternatives are also available. It is possible to use a coating to make materials self cleaning, but the researchers believe an unstructured coating will never be as good as the new laser technique. Structured coatings could be an alternative, but applying a coating to a structured surface is another. Groenendijk is currently investigating, together with scientists of the Membrane Technology group. Both groups are part of the Institute for Mechanics, Processes and Control Twente (Impact) of the University of Twente.
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