What we often call the “smell of rain” is a complex olfactory mix whipped up by some pretty fancy chemistry and physics. The ozone top note of an approaching thunder storm gradually gives way to an earthy scent known as “petrichor.” While many of the molecules in this organic base note have been known for some time, an enduring mystery has been how they get activated and become airborne. Using high-speed photography, engineers from MIT have now discovered the secret — the effervescent action of microbubbles produced when raindrops of the right size and speed strike dry, porous earth.

As you can see in the video below, a raindrop flattening against the earth puts on a show of resonating ripples. Depending on the permeability and wettability of the soil, the air trapped beneath the drop can be excited into a hydrodynamic fizz where particles gain enough energy to be ejected into the air. To see these tiny droplets, the researchers added fluorescent dye to the impact surface.

Seeing the dye light up in the ejected microbubbles indicates that other particles embedded in the surface can be ejected in the same way. The experiments reveal that this phenomenon doesn’t happen every time, but rather only when the right kind of drops hit the right kind of soil.

The researchers decided to do what any good mechanical engineer would do in these circumstances: precisely define the conditions for aromatic aerosol generation in terms of dimensionless parameters.

The beauty of dimensionless parameters (numbers with no units) is that a single number can often tell you whether or not some particular kind of behavior can be observed no matter what the scale. For instance, in aerodynamics there is a dimensionless parameter known as the “Reynolds number” which represents the ratio of the inertial to viscous forces at play on an object moving in a fluid. When that number falls within a certain range the same kind of turbulence and vortices will be generated around an object no matter what its size, the speed, or the kind of fluid.

The paper’s authors derived two main parameters for their raindrops: the Weber number, which compares inertia to surface tension in a fluid, and the Peclet number, which is the ratio of convection to diffusion in a fluid. In practice there are actually any number of variations on these parameters. The particular form of Peclet number that the authors realized was important for smell generation took into account the impact velocity and the wettability of the soil.

This is an excellent opportunity to do for your biochemistry vocabulary what we just did for your mechanical engineering vocabulary. The main component in petrichor that is responsible for the familiar pungent smell is known as geosmin. An easy way to double the total number of organic molecules that you know is to realize that if you just add the magic word “synthase” to a molecule, you now know the name of the protein enzyme that synthesizes it. For example, the bacteria that make geosmin use an enzyme called geosmin synthase. Of course there are some molecules that don’t need an enzyme to be made, and also some enzymes that will nonetheless use a different kind of name, but one can always hedge their guess with a countergambit about a different molecule and its enzyme.

This research has direct implications not just for how scented molecules are spread throughout the environment, but also for the spread of more contentious bacteria and viruses. Those elements that manage to hitch a ride on a cavitating bubble will have the advantage in getting dispersed. The fine surf generated by crashing waves is another commonplace physical mechanism which delivers a complex set of molecules from sea to land. These new experiments suggest that aerosols have been an under appreciated mechanism at work in the biosphere. It now appears that understanding the mechanics of molecular transfer is just as important as understanding the nature of the molecules themselves.

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