How droplets bead on ultra-waterproof surfaces
Researchers have imaged water drops rolling along a super-waterproof surface in unprecedented detail, and suggest a new way to measure superhydrophobicity. Belinda Smith reports.
If you've ever noticed a raindrop roll off a lily pad or watched self-cleaning clothes, you've seen superhydrophobicity in action.
Now high-resolution images of a water droplet rolling along an ultra-waterproof surface has prompted scientists to redefine how "superhydrophobicity" should be measured.
A team from the Max Planck Institute for Polymer Research in Germany used a laser microscope to snap the way a drop of water rolls along the top of nano "posts".
Their work was published in Physical Review Letters. In an accompanying Viewpoint article, University of Rennes physicist Laurent Courbin writes: "it is sometimes important to think about well-studied problems in a different way."
Researchers, inspired by natural water-repelling surfaces such as lily pads, have recreated them in the lab.
One surface props water droplets on tiny flat-topped pillars only a few microns wide, around the width of spider silk, and coated with hydrophobic silane molecules.
This keeps a thin cushion of air between the drop and the lower surface while minimising the amount of area the drop touches.
These waterproof coatings take advantage of water's surface tension, or the "stickiness" between water molecules. Surface tension is what keeps droplets spherical, and results in the curved surface of water in a slightly too-full glass (called the meniscus).
If water molecules are more attracted to each other than the tops of the pillars, the drop rolls away easier.
Superhydrophobic surfaces can be designed to various degrees of waterproofness. A surface's superhydrophobicity is measured by "contact angle": the angle of the advancing side of a water drop as it rolls along the surface.
A wettable surface has an angle around 90 ° – the drop spreads out like a dome. But a water-repellent surface, such as one covered with microposts, keeps a droplet spherical. Previous measurements show a drop's advancing waterfront can have a contact angle of 150 ° or more.
The contact angle is usually measured with unmagnified, or macroscopic, imaging. Is this a good enough grading technique?
Frank Schellenberger and colleagues decided to find out. Measuring such high levels of contact approaching 180 °, they write, is difficult because of the narrow gap between the liquid and the surface – remember, we're talking microns here.
Using a steady "vibration isolation" table and a hydraulic car jack, they carefully rolled water drops along the tops of silane-coated microposts and took images using a laser microscope, a super-sensitive microscope that lets researchers see down to the micro scale.
The pillars varied from five to 25 microns wide, nine to 16 microns high and 15 to 75 microns apart.
Instead of seeing the advancing waterfront "jump" to the next post, the droplets' surface gradually bent down to gently touch the top of the next pillar. So they measured a contact angle of 180 °, regardless of pillar height, separation and width.
This, they write, is the same "for many if not all superhydrophobic surfaces", rendering advancing contact angle useless.
But the researchers suggest the receding angle of contact might be a better rule. Not only did their receding contact angle measurements differ between superhydrophobic surfaces, it was consistent across microscopic and macroscopic measurements.
So how does this affect us?
Probably not terribly much, but the work could make the search for new superhydrophobic materials more efficient for researchers. And who doesn't want a self-cleaning wardrobe?