The cactus' ability to remove oil droplets from water could have implications for cleaning oil spills in the future.
Bunny Ears cactus
As reported last week in Nature Communications, Chinese researchers had noticed that cone-shaped cactus spines of the 'bunny ear' cactus, Optunia microdasys, 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.
Lei Jiang and co-workers observed how cacti spines condense moisture from air, helping to keep the plant hydrated in desert environments. They reasoned that artificial spikes could do a similar job for oil droplets in water, and designed arrays of copper spikes that act as highly efficient oil absorbents. The advantage of the method reported in this work is that it can be used to trap microscopic sized drops that could be missed by conventional separation technologies
With so much environmental damage caused by oil spills, ways to easily separate oil from water are in increasing demand.
Their 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.
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.'
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.
The phenomenon relies on the fact that the radius of a cone surface is very small near the tip, and larger further from the tip towards the base. This causes a pressure difference for a droplet at the tip compared to near the base forcing the liquid to move to the wider part of the cone. As a droplet moves along it captures any others on the way, coalescing at the cone's base.
Different types of oil-water mixtures were tested, including vegetable oil, gasoline, and organic solvents mixed with water, and all were successfully separated in the same process.
Previous examples of biomimetic inventions include Velcro fasteners (copying plant burrs that get stuck onto animal fur), the friction-reducing sharkskin swimsuits used at the Beijing Olympics, and artificial photosynthesis employed by some of the latest solar cells.
The next step is to scale up and test the theory in field experiments.