(CONVERSATION) Watch a fly land on the kitchen table, and the first thing it does is clean itself, very, very carefully. Although we can’t see it, the animal’s surface is covered in dust, pollen, and even insidious mites that could burrow into its body if not removed.
Staying clean can be a matter of life and death. All animals, including us humans, take cleaning equally seriously. Every year, we spend a whole day bathing and another two weeks cleaning our homes. Cleaning can be as fundamental to life as eating, breathing and mating.
Yet somehow the cleanup received little attention.
In our new review the article in the Journal of Experimental Biology, we discuss how cleaning occurs in nature and whether animals actually have principles for cleaning themselves. We looked at images under a microscope to count the number and size of hairs on hundreds of animals. We have read nearly a hundred articles on cleaning in nature, trying to encrypt the cleaning process.
Extrapolation of principles is an important step for science, and even more necessary for engineering. Learning better ways to clean will not only allow us to understand the humble fly, but also build new types of devices that stay cleaner longer.
Hair greatly amplifies the surface of the body
To understand how animals clean themselves, one must first understand how they get dirty. Dirt accumulates on the exterior of an animal just as a result of living life. The surface area of an animal is not as easy to determine as it is to measure the dimensions of a cardboard box. Most animals – from mosquitoes to elephants – are hairy. Beyond the outside of a creature’s skin, the hairs provide an additional surface for dirt to collect.
We have found that on average, hair increases the apparent surface area of an animal by a hundred. Thus, a cat has the surface of a ping-pong table. (This explains why cleaning pets is so difficult.) A chinchilla is the square footage of an SUV. And a sea otter is the size of a hockey rink.
We the people have around 100,000 hairs on our head. The number of hairs on other animals is comparatively astounding. A butterfly has 100 billion hairs, more than 10 times that of a beaver. The bee has three million hairs, the same number as a squirrel.
In addition, on animals there are as many types of hair as we have types of hairstyles. Animals have trichias, thorns, macrotriches, bristles, scales, hairs in all shapes and sizes. One thing is clear: hairs increase the surface area of the body and thus worsen the problem of cleaning. Which do you prefer to clean, a linoleum floor or a high pile carpet?
Hair as a cleaning device
To cleanse its body, almost all insects have hairy legs, each leg resembling a feather duster. Observing how the legs interact with body hair reveals one of the surprising characteristics of body hair. We used a high speed video camera watching a fruit fly groom its head with its arms. Particles attached to body hair are catapulted at nearly 1,000 times the acceleration of gravity, much faster than the fly can move its limbs. The hairs that originally served as landing platforms for dust now act as trebuchets, triggered by the hairy arms moving above them.
Effective cleaning is not just about designing a good cleaning tool, like the squeegee. It also means designing a surface ready to facilitate grooming. And many times these surfaces in nature defy the imagination.
We have identified two main cleaning principles. The first is a non-renewable cleaning strategy – it uses the animal’s own energy sources. Examples include grooming, which is similar to vacuuming a carpet and requires energy to move the brush.
Other examples are the shaking of the wet dog: no cleaning tool is used, but rather the body’s own inertia. Wet dog spins his body at high speed like a washing machine in its spin cycle. Particles and drops are removed, thus spending the dog’s energy.
Alternatively, there are renewable cleaning strategies – ones that don’t require energy from the animal, but are free.
An example is the eyelash, the edge of the hairs surrounding the eye. When we move forward, the air flow generated by the eyelashes reduces particle buildup by a factor of two, compared to a naked eye. So, just by growing eyelashes, we can blink half as often, saving that energy for other uses.
Another example is the nanoscale pads on the wings of cicadas. Bacteria act like water balloons, exploding when in contact. Finally, raindrops can roll onto a furry animal’s fur, dragging particles with it and leaving it as clean as a lotus leaf.
Surface + energy + behaviors = clean
The way animals clean themselves is an interaction between the animal’s surface, its behaviors, and the energy of its environment. An animal cleans itself for free if it has the right type of surface. If we have that mindset, maybe we can design new devices that are also clean for free.
Think about solar panels. Like the eye, they must let in light. But solar panels lose 7% of their power annually due to the accumulation of dust. The most high-tech solution is the raclette. Come to think of it, the computer age gave us light years ahead of our ancestors when it comes to communication, but our cleaning methods remain stuck in the past.
Imagine solar panels designed like insect eyes. Fine filaments could be placed periodically to suspend dust above the panel, while still allowing light to enter. Cleaning the panel would just be like going through it with another brush. Just like insects, the panel could clean itself without using water or chemicals. Likewise, video cameras, the eyes of robots, could be lined with eyelashes to reduce deposits. The construction of synthetic hairy systems is the subject of our National Science Foundation grant, Engineering insect eyes.
We usually envision future robots with smooth, shiny surfaces, like a chrome polished automobile. But in nature, smooth surfaces are not the norm. Future table tops could have nanoscale rods that stretch and kill bacteria on contact. Robotic rovers can be covered with hairs that sense their surroundings, suspend particles and allow easy cleaning. Indeed, the future looks perhaps rather hairy.