Depending on the way it is made, molasses is between 5,000 to 10,000 times more viscous than water. The Reynolds number for an adult man in water is around one million; the Reynolds number for the same man in molasses is about 130. To make matters worse, a man immersed in molasses will not get anywhere with the kinds of symmetric swimming strokes that would propel him in water. Each repetitive stroke would only undo what was done before. Pulling his arm towards himself would move molasses away from his head, but reaching up to repeat the stroke would push the molasses back where it was before. He would stay in place, like a gnat trapped in tree sap. Even burly men struggled to tread molasses in the wake of the Boston Molasses Disaster; horses flailed and brayed, straining to keep their heads aloft and snorting to clear their airways.
From the perspective of humans and animals of comparable size, swimming through syrup is a bizarre nightmare scenario; for some of the most abundant life forms on the planet, however, a quagmire of molasses is an everyday reality.
One summer afternoon in 2011 my friend Mara and I were wandering the streets of Boston when we spotted a small plaque near the harbor memorializing the Great Molasses Flood. Around the same time I had been speaking with microbiologists about a newly discovered method of microbial locomotion: a bacterium that strategically flung and detached sticky threads a la Spiderman to slingshot itself through fluid coating a solid surface. The bacterium, Pseudomonas aeruginosa, which lives all over the place—in soil, in people’s houses and in the human body—often encounters fluids and slimes of various kinds, whether mud or mucus. By slingshotting itself around, the researchers proposed, Pseudomonas aeruginosa was taking advantage of “shear thinning”: it used the movement of its body to reduce the viscosity of surrounding fluid. Because bacteria are so tiny, the researchers explained, even a fluid we consider thin—such as plain water—is as thick as molasses to them. Microbes permanently inhabit a low Reynolds number world—a truth made famous by the American physicist Edward Mills Purcell in his 1973 lecture "Life at Low Reynolds Number.” Some bacteria must combat Reynolds numbers as low as 10^-5 (0.00001).
I became fascinated by the idea of microbes battling viscous forces many times greater than those unleashed on Boston in 1919—forces to which most of us are oblivious. So I started researching. I called up my fellow science writer Aatish Bhatia, who had written a fascinating essay called “What it feels like for a sperm,” that I highly recommend. I looked up the transcript of Purcell’s original talk and old papers by pioneers in research on microbial movement, such as Howard Berg. And I searched the research literature for the most recent studies on how microorganisms swim.
Many bacteria and other microorganisms have obvious adaptations to overcome low Reynolds numbers; they row thousands of hairlike projections called cilia or corkscrew their way through fluid with powerful spinning tails known as flagella. Other bacteria and their kin have puzzled researchers by swimming just fine without such external accoutrements. In recent years scientists have revealed how some of these more mysterious microbes get around: a few rely on complex internal motors that ripple the cell surface; one bacterium can turn mucus in the human stomach into a much thinner fluid; and another microbe has, shall we say, a rather kinky way of moving. I describe such adaptations in more detail in a feature article in the August issue of Scientific American—the culmination of my slip-‘n-slide journey into the wonderfully weird world of microbes in molasses.