Adopting the lotus position

A Dutch scientist has used a femtosecond laser to give plastic a self-cleaning surface like that of a lotus leaf, which is so effective cups made of it may not need to be washed after use.



PhD student Max Groenendijk of the Applied Laser Technology Group of the University of Twente used the ultra fast femtosecond laser to create the surface which has many potential domestic and industrial applications.



The lotus leaf is covered in tiny pillars with a waxy layer on top. Water drops are lifted by these pillars, form a spherical shape and roll off taking dirt particles with them.



Groenendijk set out to create similar surfaces using the laser to eliminate the need for wax. The light pulses of the femtosecond laser are so short that they can be seen as light ‘bullets’ with which the surface is bombed.



The laser is applied in two separate steps. During the first step, the surface gets a fine ripple structure. This is caused by a 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 like speed, intensity and polarisation.



The second step is to write a pattern of perpendicular lines which leaves 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 repeated surfaces this way would be too expensive, but by using a mould, a series of surfaces can be produced in a simple, economic way.



According to Groenendijk, the structure can dramatically improve the surface properties even for materials that are already quite hydrophobic. Unlike in an unstructured, smooth surface, where droplets can still smear a little, the structured surface gives the same spherical drops as the lotus plant.