Fabrication of ultrahydrophobic surfaces by femtosecond pulsed laser micromachining
Research group: prof.dr.Ir. Meijer, ir. M.N.W. Groenendijk
Project term: September 2003 – September 2009
Keywords: femtosecond lasers, micro structuring, self-cleaning surfaces
The goal of this research is to develop plastic parts (consumer products, medical products) with a self-cleaning surface. This property can be achieved by applying a specific microstructure to a polymer’s surface. The part is fabricated by injection molding and the surface structure is directly applied to the part via the injection molding process. The mold is post machined by femtosecond pulsed laser ablation in order to machine the negative form of the surface structure into the mold.
The phenomenon of self cleaning surfaces is demonstrated in nature by the leafs of nelumbo nucifera, the lotus flower. Water droplets barely touch the leafs of this plant before forming beads and rolling off. Dirt is absorbed into a droplet and washed off this way. The basic requirement for these so called Lotus-effect surfaces is a combination of hydrophobicity and a specific microstructure. We mimicked this effect successfully. Figure 1 shows a polypropylene sample prepared by injection molding.
For the microstructure that has to be machined in a mold, research had pointed out the need for a roughness on a double scale. Besides a two dimensional pattern of cone structures with feature sizes in the 10 μm regime, a sub micrometer superimposed structure was needed.
Such structure was found by serendipity: during first experiments on the femtosecond pulsed laser, the emergence of periodic ripple structures and more chaotic rough structures in the sub micrometer regime were found on the bottom of ablated areas. The ripple spacing was usually between 600 nm – 700 nm, just beneath the wavelength (800 nm) of the femtosecond pulsed laser. Highly regular patterns can be machined by using homogenized intensities. After irradiation with further pulses, the ripple pattern is deteriorated by trenches occurring perpendicular to the ripple patterns. By applying even more pulses highly chaotic structures emerge. These self organizing microstructures are one ingredient for self cleaning structures. The second is a pillar pattern with feature sizes in the micrometer regime, which serves as carrier of the submicron structures described before. The pillar pattern can be machined into the steel mold by a direct write process. By careful selection of the parameters, the pillars and the submicron structure can be machined in one run. This method is fast and can easily be used for 3d curved surfaces.
Injection molds have been machined with different microstructures, which where used to fabricate polypropylene replicas. This cheap polymer has a relatively low surface tension, leading to hydrophobic properties. By applying a designed microstructure on such a material, ultrahydrophobicity can be achieved, comparable to the lotus leaf.
The left micrograph in Figure 2 shows a SEM image of a structure machined in the steel mold. The 13 μm spaced pillars are covered by a sub micrometer ripple patterns. The replicated structure in polymer is shown on the right. Even the ripple pattern is copied to the polymer. Figure 3 illustrates the increase of a surface’s hydrophobicity by such structures. Two drops are placed on the same polypropylene sample. On the left, the surface is micro structured, whereas on the right the surface is flat.
The right drop depicts the situation that we are used to in daily life: a drop of water wets the surface, which means that its contact area grows until equilibrium is reached. On very hydrophobic surfaces however, the water will form a droplet, making as little contact as possible. This makes drops very mobile, just a slight inclination is enough for the droplet to roll away. On conventional surfaces, a drop would stick or smear out, leaving the surface at least partly wetted. To examine the behavior of water droplets falling on a prepared ultrahydrophobic surface, high speed camera recordings have been made. The right picture in Figure 3 shows a compilation of such recording, revealing a droplet that is bouncing on our surface.
The injection molding experiments have been performed in collaboration with Philips ATC.
This research has resulted in the start up of the spin-off company Lightmotif (www.lightmotif.nl). Surface texturing using ultrashort laser pulses will be further developed in close collaboration with ongoing research of the group of Mechanical Automation / Applied Laser Technology, faculty Engineering Technologies, department Mechanical Engineering of the University of Twente.
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