It can literally be a question of life or death. Cleaning is a serious matter for all animals, even us humans. Every year, we spend a day cleaning and another two weeks bathing. Cleaning is as important to our lives as breathing, eating, and mating.

Cleaning has received little attention.

In our new Review Article, published in the Journal of Experimental Biology, we discuss whether animals have principles to clean themselves. We counted the hairs on hundreds of animals using microscope images. We read over 100 articles about cleaning nature to try and quantify the process.

Extending principles to engineering is a necessary step in science. Understanding the fly will help us to create cleaner devices and understand it better.

The hair greatly increases the surface area of a person’s body.

Scanning electron microscopy images of the hairs on a bee’s forelimb. Georgia Tech, CC BY-ND

In order to understand how animals become clean, you must first know how they get dirtied. As a result of daily life, dirt accumulates on the exterior of animals. It is difficult to determine the surface area of an animal as easily as measuring a cardboard box. From mosquitos to elephants, most animals are hairy. Hairs are a surface that extends beyond the skin of animals.

Hair increases the apparent surface area of an animal by a factor of 100. A cat’s surface area is the same as a ping-pong table. This explains why it’s so difficult to keep pets clean. The surface area of a chinchilla is the same as that of an SUV. A sea otter’s surface area is the same as a hockey rink.

We humans have approximately 100,000 pairs on our heads. Other animals have a staggering number of hairs. A butterfly has more than 100 billion hairs. This is ten times the number of beavers. The bee also has three million inches.

There are many different types of hair on animals, just as there are hairstyles. Trichia are found on animals of all sizes and shapes. Coat increases the body’s surface area, which makes cleaning more difficult. What would you prefer to clean: a linoleum or shag floor?

A fruit fly brushes cornstarch from its antennae and head. This video is slowed down 33 times.

Hair as a cleaning device

Nearly all insects use hairy legs to clean their bodies. Each leg looks like a feather duster. The interaction between the legs and the body hairs is one of the most surprising characteristics of hair. A fruit fly was observed grooming its head using its arms. We used a video camera with high speed. The particles attached to the strands of the body are catapulted with an acceleration nearly 1,000 times that of gravity. This is much faster than the fly’s limbs can move. The hairs that were used to land dust particles now function as trebuchets when the hairy arms move over them.

A high-speed video of a human hair flinging the antenna of an insect at 600 times slower speed. The cornstarch particles are flung off at a rate 1,000 times greater than the Earth’s gravity.

It is not enough to design a good cleaning tool, such as the squeegee, for effective cleaning. It is also important to create a surface which will facilitate grooming. These surfaces are often beyond imagination.

Two main principles were identified for cleaning. First, we have a cleaning strategy that is non-renewable – the animal uses its energy. Grooming is an example, as it requires energy to move a brush.

Wet-dog shaking is another example: the body uses its own inertia to shake itself, instead of a cleaning tool. When a dog is wet, it spins at high speeds its body like a washing-machine on its spin cycle. The dog expends its energy to remove the particles and drops.

There are also free cleaning methods that do not require the animal to provide any energy.

Eyelashes are great eye cleaners. Kristina Alexanderson, CC BY-SA

The eyelash is a rim of hairs that surrounds the eye. The airflow created by eyelashes can reduce the accumulation of particles in the eye by two times compared to a bare eye. By growing eyelashes, we are able to blink less often and save energy.

The pincushions that cover the wings of the cicada are another example. Bacteria behave like water balloons and explode when they come into contact. Raindrops can also roll down the fur of a hairy creature, removing particles with them.

Surface + energy + behaviors = clean

The surface of the animal, its behavior, and the energy in its environment all play a role in how animals clean themselves. If an animal has the right surface, it can get clean for free. We can also design devices that are free to clean if we adopt this mentality.

Solar panels are a good example. They must let in light, just like the eye. Solar panels can lose up to 7% of power due to dust. Squeegees are the most technologically advanced solution. The computer age has brought us light-years ahead of our predecessors in terms of communications, but our cleaning techniques are still stuck in the olden days.

A scanning electron microscope image showing the hairs of a honeybee eye. Georgia Tech CC-BY

Imagine solar panels designed like insect eyes. The solar panel could be covered with thin filaments to keep dust suspended above it while still allowing light to pass through. The panel could be cleaned by sweeping it with a new brush. The panel can be cleaned without using water or chemicals, just like insects. Video cameras and robot eyes could also be covered with eyelashes in order to reduce the amount of dust. Our National Science Foundation grant Engineering Insect Eyes is dedicated to the development of synthetic hairy systems.

We imagine future robots with smooth, shiny surfaces like those on a chromed-up automobile. In nature, however, there are few smooth surfaces. Future tabletops could have nano-sized posts that stretch on contact and kill bacteria. Robotic rovers could be covered in hairs that can sense their environment, suspend particles and allow easy cleaning. The future could be a hairy one.