Cactus-inspired material cleans oily water A

Chinese researchers had noticed that cone-shaped cactus spines harvest water from air pushing it to their base.

Copying Nature's design, they used conical copper needles to separate tiny oil drops from dirty water - a problem existing methods struggle with.

The scientists, who are based in Beijing, have reported their results in the journal Nature Communications. The discovery points the way to a new method for addressing environmental problems like oil spill clean up, with the artificial spines capturing microscopic droplets of oil underwater, continuously transporting them to the base of the sheet of spikes.

Last year, researchers from Beijing reported the discovery that the conical needles of the "bunny ear" cactus, Optunia microdasys, can collect water from the air.

Droplets of water coalesce on its barbed spines and are then driven to the spine base by the interaction between the shape of the spine and the surface tension of the water droplet.

They now report that synthetic spines are capable of separating and collecting oil droplets out of water in just the same way, and have made a synthetic "cactus skin" of needles that do exactly that.

While oil and water don't mix, if very small droplets of oil co-mingle with water it becomes almost impossible to separate the droplets out of the water again - an example is homogenised milk, in which tiny fat droplets stay suspended in the milk forever.

Describing their method, Lei Jiang, leader of the research project, said: "We fabricated needle arrays. Each conical needle in the array is a little oil collection device. The arrays can collect micron-sized oil droplets from water continuously and effectively."

Microscopic bed of nails The synthetic needles are half a millimetre long, and will remove tiny micron-sized droplets of oil from water, which are very difficult to separate out by any other method. In tests they found that hexagonal arrays of these needles could separate around 99% of oil mixed with water. The researchers have constructed conical needles made of copper and of a silicone polymer and find that the affinity of the material surface for oil, together with the shape of the cone are crucial in the operation of the device, but that rougher cones are more efficient at harvesting oil from water.