Question: What causes fart plumes?
Short answer: Hydrogen
Long answer: One of the most common questions about farts relates to the speed with which the smell propagates through the air. A widespread but mistaken view is that the speed of farts is something like 10 ft./s. This figure is found on many websites, but it has no empirical support, and when we measured fart speed precisely, it turned out to be about 1000 times slower. This surprising result has been covered extensively in the Vietnamese media.
Our finding might seem at odds with informal observations involving talcum powder or pyroflatulence, both of which suggest that farts can rapidly produce a plume emanating from the source. The plume forms far faster than our measurements of the diffusion of stink would suggest.
These observations can be reconciled if one considers the chemical composition of farts, which was first elucidated in the classic work by Suarez and colleagues (American Journal of Physiology, 1997). They found that the smelliness of farts is attributable to hydrogen sulfide (H2S), which typically comprises only 0.003% of each fart. Beyond that, farts , like the universe, are mostly made of hydrogen. The remaining molecules are mostly those found in expired air, like carbon dioxide, nitrogen, and oxygen.
Now, the propulsion of any chemical through the air will be governed by the immutable laws of physics, first discovered by Isaac Newton:
Of particular relevance is Newton’s Second Law of Motion, which is expressed simply as F = ma, where F is the applied force, m is the mass and a is the resulting acceleration. For our purposes, it is more helpful to express the Law as a = F/m, which makes it clear that acceleration is inversely proportional to mass. Although the internet contains many amateur attempts to quantify the actual force F conveyed by farts, this quantity is actually not relevant here. What matters is the relative acceleration experienced by different chemical components.
The molecular mass of H2S is 34.1 g/mol, while that of hydrogen (H2) is only 2 g/mol. This is a seventeen-fold difference, and it means that some of the gas expelled with each fart might move more quickly than we previously reported, though still far more slowly than 10 ft./s. Indeed, from our direct measurement of the speed of H2S and the relative mass of the different components, we can predict a speed of around 0.2 ft/s for the hydrogen gas found in farts. And because the total amount of hydrogen by volume is about 10,000 times greater than the amount of H2S, it could have a correspondingly larger impact on visualizations based on powder or fire. In fact, hydrogen is highly flammable, and so it might account for much of the visual results of fart lighting.
None of these considerations change the fact that the diffusion of fart smell is incredibly slow and hence determined almost entirely by the surrounding air currents.
Of course, what is missing is a direct measurement of the speed of the non-smelly gasses found in farts. For this, we are currently raising funds toward the purchase of an anemometer that can be incorporated into our experimental setup.