If you had a collision with something 50 times your weight, it would probably (pun intended) have a major impact. But mosquitos do this all the time. Raindrops can weigh 50 times more than a mosquito, yet the animals thrive in rainy and humid areas. For some perspective, imagine walking down the street and suddenly being hit by a mass of water with the weight of a Chevy Suburban.

Fortunately for our collective curiosity, a group of mechanical engineers and biologists from Georgia Tech, led by Professor David Hu, decided to figure out how the pests manage it.

During a thunderstorm, a mosquito would likely be hit about every 25 seconds. They obviously aren’t dodging raindrops, as appealing as the mental image may be.

So the researchers created a “flight arena” to study mosquito-raindrop collisions—essentially a five centimeter wide, 20 centimeter tall rectangular mesh cage. In order to simulate raindrops falling at their terminal velocity (which would otherwise require a height of 10 meters) they shot a water jet point-blank into the cage of mosquitos.

To figure out what was happening during midair collisions, the authors shot a strong, 9m/s jet into the cage, and observed the impact with six mosquitos. In all of these collisions, the insects tumbled down the cage, getting hit repeatedly, before separating from the jet and landing on the side of the cage. This jet is actually stronger than terminal velocity raindrops, so this test shows that mosquitos would survive such an impact.

That didn’t answer the question of how the mosquitos survive a typical impact, so next they used slower and more realistic water drop speeds (10–260 cm/s), which allowed more accurate tracking and higher resolution imaging.

Confirming an earlier prediction, the team found that most drops impacted on the wings and legs, rather than the bodies—the body only makes up one-quarter of the potential impact area. This results in a quick, glancing blow that essentially rotates the insect (either pitch, yaw, or roll). The contact lasts a fraction of a second, after which the mosquito quickly recovers.

But direct hits to the body did happen. When rain falls on a stiff object, like a tree branch, the drops spread out very quickly. This rapid momentum transfer results in a strong impact force, roughly 10,000 times the weight of a mosquito. In other words, splat.

In the experiment, the rarer direct impacts lasted longer and pushed the mosquitos downward several (5–20) body lengths. However, unlike impacts on stiff surfaces, the mosquito and water drop quickly separated—the drop remained intact and retained nearly the same speed. Unless the insect was flying too low to the ground—in which case it would suffer a second, more forceful impact—it was always able to recover.

The key detail is that the raindrops maintain most of their speed after the impact. Transfer of momentum, and therefore impact force, depends on the difference in the drop speed before and after the impact. Compare this to a drop hitting a solid surface: the drop has zero speed after the impact, so all of the momentum was transferred during the collision. The mosquito, on the other hand, just rides along with the raindrop before pulling away safely, much like a surfer riding a wave.

The authors hypothesized that the reason for minimal momentum transfer is the small mass of mosquitos. In collisions, the transfer of momentum depends on the masses and velocities of the objects. To test this, they created Styrofoam spheres to mimic the raindrops and mosquitos (since, in physics, everything is easier to represent with a sphere), then studied collisions for a wide range of speeds and mass ratios.

They found that the collisions are inelastic—meaning that the drop and insect (or Styrofoam mimics) combine into a larger, slightly slower lump. (Elastic collisions, on the other hand, are like those between billiard balls, where all the kinetic energy is conserved.) Insects or objects with low mass barely impede the raindrop, while a much larger insect (like a dragonfly) would decrease the drop velocity to nearly zero. Following the same logic, the impact force of an inelastic collision decreases with insect size.

The acceleration caused by direct impacts appears to be high: 100–300 g, or the equivalent of 50–150 mosquito weights. By comparison, a typical human can only withstand a downward acceleration of 2–3 g, or up to about 5g upward—although trained pilots and astronauts can increase this. However, the actual force of this acceleration is low, due to the small mass of the mosquito: around 0.6g force (600 dynes). The force of an impact on a stiff surface is nearly two orders of magnitude higher.

The team also performed compression tests using a micromanipulator to determine just how much force a mosquito could withstand and still be able to fly. They found that, thanks to their strong exoskeletons, the mosquitos could survive forces over an order of magnitude higher than the ones they experience due to raindrop impacts. (Fortunately, our fingers are capable of delivering blows of this magnitude.)

Up to this point, our knowledge of how insects and birds fly through rain was minimal. Prior studies of larger animals like bats showed that rain doubled the energy expenditure needed to maintain flight. Heavy rain can significantly hinder aircraft flight by reducing lift and increasing both drag and the potential for stall. These new results not only help improve our understanding in this area, but could also be used to help design rain-resistant insect-sized micro air vehicles (MAVs).

PNAS, 2012. DOI: 10.1073/pnas.1205446109 (About DOIs)