Turning the lotus effect on its head

[Yasha]

6 April 2005

Companies that make water-repellent paints, fabrics and windscreens for cars often look to the lotus leaf for inspiration. The leaf is a symbol of purity in many cultures because of its ability to remain clean: when rain falls onto a lotus leaf, the drops of water that form on the surface roll off, taking any dirt with them. However, two researchers in the US have now discovered that although lotus leaves are superhydrophobic as far as droplets of water are concerned, they are actually hydrophilic with respect to condensed water vapour (Y-T Cheng and D Rodak 2005 Appl. Phys. Lett. 86 144101).

Microscope observations reveal that the waxy surface of the lotus leaf is made of micron-sized bumps that, in turn, are covered with nanoscale hair-like tubes (figure 1). This two-fold structure traps air under any rain drops that fall on the leaf, creating a surface that efficiently repels water. However, experiments on this so-called lotus-effect have only focused on how millimetre-sized droplets behave when they encounter the leaf's surface.


Close-up of a lotus leaf, an example of a super-hydrophobic plant. The roughness of the leaf surface results from the coexistence of micron-sized bumps and nanoscale hair-like structures. (Image credit: W Barthlott)

Yang-Tse Cheng at General Motors Research and Development Center in Michigan and Daniel Rodak of Ricardo Meda Technical Services, also in Michigan, began by placing a lotus leaf above a source of water vapour. After just a few minutes the water began condensing onto the leaf in small droplets. Moreover, as condensation continued, some of these droplets merged to form bigger drops that remained on the surface of the leaf -- contrary to what was expected.


Schematic illustrations of water drops on lotus leaf following water condensation: (a) a water drop on an area with a small amount of condensed water, and (b) on an area with a large amount of condensed water (image credit: Appl. Phys. Lett. 86 114101).

Cheng and Rodak say the droplets become trapped between the nano-scale hairs on the leaf. The drops then coalesce with other drops and eventually begin to fill the cavities created by the micron-sized bumps (figure 2). Moreover, any new drops falling onto the leaf can then stick to it, making the surface hydrophilic rather than hydrophobic (figure 3).


A water droplet sticking to a lotus leaf (photo: Y-T Cheng).

"The question now is whether truly superhydrophobic surfaces can exist," says Cheng. "Condensation experiments like ours are crucial to understanding the wetting behaviour of surfaces and should be included in any standard evaluation of hydrophobic materials."

About the author
Belle Dumé is Science Writer at PhysicsWeb

Source: PhysicsWeb

[Fernanda]

i wonder if as the larger drops fall, they drag with them the smaller in-between droplets.

I ask that cause when I used to swim competitively in middle school, I used to add water to my ear to remove that annoying water that aways stayed in my ear after swimming... unknown to me, I was using the surface tension of water to help me out, when others thought I was nuts.

I wonder if they can see that.

[marrydavidson10]

Companies that make water-repellent paints, fabrics and windscreens for cars often look to the lotus leaf for inspiration. The leaf is a symbol of purity in many cultures because of its ability to remain clean: when rain falls onto a lotus leaf, the drops of water that form on the surface roll off, taking any dirt with them. However, two researchers in the US have now discovered that although lotus leaves are superhydrophobic as far as droplets of water are concerned, they are actually hydrophilic with respect to condensed water vapour