El Mosquito

Forget Snakes and Sharks; mosquitos are by far the deadliest animal on the planet. Mosquitos have killed an estimated 46 billion people – half the humans who have ever lived.  They’re the leading cause of death in many parts of the world, transmitting over 100 diseases including Malaria, Dengue and Yellow Fever.

Mosquitos lay their eggs on the surface of stagnant water. To cut the mosquito death toll, some countries spray whole regions with insecticides or smother swamp edges with biodegradable oils, suffocating the eggs before they hatch. Mosquito eggs on the surface of ponds and swamps create beautifully ordered patterns.

The eggs are either bunched up side-by-side, chained together tip-to-tip or sometimes arranged in tiny triangles (http://link.springer.com/article/10.1140%2Fepje%2Fi2011-11076-9). Why don’t they just spread randomly on the surface and stay separated from each other? Why is there so much structure and order?

    Mosquito egg-shaped particles creating beautifully ordered structures on a water surface

Mosquito egg-shaped particles creating beautifully ordered structures on a water surface

Cheerios

Have you ever noticed that Cheerios tend to stick together rather than float separately? Surface-tension – which exists at the boundary between liquids and gasses, such as the surface of ponds – brings Cheerios together. The air-water boundary is like a membrane: you can deform it, and that’s exactly what Cheerios do. (This membrane-like property of surfaces is also the reason water striding insects can walk on water). Imagine placing a bowling ball on a trampoline. You’ll notice that the bowling ball deforms the trampoline and sinks. If you place another bowling ball somewhere else on the trampoline, the second bowling ball rolls down the slope created by the first bowling ball and they coalesce. Cheerios on air-water interfaces are like bowling balls on trampolines.

water_strider_2

Water striders use surface tension to walk on water

But now imagine that you place a tennis ball on a trampoline instead, and then put a second tennis ball on the trampoline; nothing happens. The weight of the tennis ball is small compared to the strength of the trampoline and the second tennis ball doesn’t even know the first tennis ball exists. Mosquito eggs are like tennis balls, not bowling balls, on trampolines: mosquito eggs are not heavy enough to put a big dent into the interface.

American Football

symmetry_axes

Symmetry axis of a mosquito egg (top) and a cheerio (bottom)

So why, then, do mosquito eggs self-assemble and self-organise into beautiful patterns? The answer is their shape. Mosquito eggs are American football-shaped particles and their symmetry axis points along the air-water interface, where as the Cheerios’ symmetry axis points out of the air-water interface. In order to fulfil an equation known as Young’s equation, the air-water interface deforms around the American football, because its symmetry axis points along the interface and not out of it (unlike the Cheerios).  A mosquito egg depresses the interface at the tips and raises it at the sides. What’s more, the mosquito eggs depress the interface more at the tips than they elevate it at the sides because their curvature at the tips is more extreme. (15698141).

Interface deformation around a mosquito-egg shaped particle

Interface deformation around a mosquito-egg shaped particle

When many mosquito eggs are on the surface of a pond, they each feel the depressions and elevations of the interface caused by the other eggs. But there’s an added complication. The elevations and depressions of an interface behave like electric charges, with a twist; elevations attract other elevations, and depressions attract other depressions, but elevations repel depressions: opposites repel and likes attract. When an egg starts feeling the interface depressions and elevations created by another egg, they align themselves in the tip-to-tip or side-by-side orientation because these configurations allow their interface deformations to match.

What about the little triangles? Remember that the mosquito eggs depress the interface more at the tips than they raise it at the sides. So two mosquito eggs would prefer to line-up their tips where possible, and the triangle configuration allows three mosquito eggs to have all their tips lined up with another tip. That’s also why when there are many eggs, they all tend to line up side-by-side: each egg has both its tips in contact with other tips.

colloidosomes

Colloidosomes: Selectively permeable capsules composed of particles

Medicine

Why do physicists care about mosquito eggs? As we have seen, the beautifully ordered structures and patterns on pond surfaces formed by mosquito eggs have nothing to do with the fact that they come from mosquitos; they have everything to do with the particle shape. In 2002, researchers (http://www.sciencemag.org/content/298/5595/1006.full) created capsules using the self-assembly of spherical particles on the surface of droplets.

One potential application for the capsules is drug delivery; the size of the holes between the particles dictates how quickly a drug inside the capsule diffuses into the fluid outside (i.e. the permeability): drugs could be steadily released into the blood stream decreasing the necessary drug dosage and therefore reducing side-effects. The question now is whether mosquito egg-shaped particles might allow even greater control of the permeability or even tailor other properties of these capsules allowing them to target specific regions of the body.

Bibliography