Fluid Dynamics

Why physics is an important part of the search for coronavirus answers

By Christina Nunez

June 14, 2021

The 21st century is so rife with new scientific frontiers that established disciplines can sometimes seem fixed in amber. Even as the novel coronavirus drives the cutting edge of vaccine development, it is revealing what we still don’t know about the basic nature of how fluids behave. Suddenly in early 2020, the question of how droplets travel in air carried direct implications for our daily lives.

Jonathan McCoy sees the pandemic as a poignant example of how physics has an immediacy and daily relevance people often don’t recognize. The associate professor of physics and astronomy makes this point with his Physics of Fluids class, which he has been teaching since 2010.

“We’re constantly interacting with fluids. A lot of people don’t even realize that air is a fluid, for example, because it flows,” McCoy said. “To a physicist, anything that flows has fluid qualities.”

The novel coronavirus showed just how many unknowns exist about infectious disease transmission via fluids. Even as climbing vaccination rates have made it possible to resume some semblance of normal life in many parts of the world, these questions remain relevant—not only for the continuing effort to reduce infections of coronavirus, but to understand any disease that spreads through airborne droplets.

“Many students are surprised to learn how complicated the behavior of droplets, bubbles, and other interfaces where one fluid meets another can be,” McCoy said. “These interfaces are barely mentioned in many classic textbooks and are still an area of active research today.”

McCoy himself is actively learning about these interfaces. Just before the pandemic hit, he was at a conference called Dynamics Days, where he heard MIT researcher Lydia Bourouiba give a talk discussing her research on the role of fluid dynamics in disease transmission.

Two months later, Bourouiba became a fixture in news reports about the pandemic and how to stop its spread. She challenged the idea that physical distancing of six feet, by itself, would be enough to protect people. Her research had shown exhaled droplets could travel much farther—up to 27 feet. As recently as May 2021, the U.S. Centers for Disease Control and Prevention was revising its scientific brief on what is known about transmission of the virus.

“There’s always some growth that changes the way you interact, your ability to communicate, the way you formulate questions.”
Jonathan McCoy

A fluid dynamics course is unusual to find at a small liberal arts school, or even within a physics curriculum, McCoy noted. Because the science of how liquids and gases flow is integral to designing machines such as engines and wind turbines, fluid dynamics is often housed within engineering departments. That’s logical, but McCoy doesn’t want the connection to physics to get lost.

“Fluid dynamics, as a field, grew up within physics [in the 19th century],” he said. “So, studying fluid dynamics is, in a way, rediscovering key parts of physics”

As part of the course, students learn the mathematical language and principles associated with classic fluid motion problems, such as how vortices behave and how droplets form. This past spring, students also connected these ideas to questions surrounding the spread of airborne infectious diseases.

The Bourouiba Group and other members of the fluid dynamics community have shown that exhalation while breathing, speaking, or sneezing produces a turbulent cloud, the presence of which can extend the lifetimes of smaller suspended droplets by factors of 100 or more.

“Droplets inside a turbulent cloud are swirling and circulating in complicated ways, unfolding across a wide range of length and time scales,” McCoy explained. He brought up a range of “hidden small riddles” about the formation and fate of exhaled droplets based on a variety of factors, including droplet size, air temperature, relative humidity, word pronunciation, and even the properties of a person’s saliva.

For McCoy and his students, riddles like this bring home the point that, while many people associate physics with the realm of big theoretical questions about the evolution of the universe, it is also a dynamic field investigating phenomena relevant to the here and now. So, does a premed student need to study physics? He says yes.

“There’s nothing in medicine that isn’t made out of stuff, and all of that stuff is playing by the rules,” McCoy said. “But it’s hard sometimes to see the consequences of those rules, and that those are still areas of study.”

Coronavirus is a striking illustration of this point, and McCoy said the Physics of Fluids class had “an extra edge to it” for students this year. But it’s also just one instance in a larger truth that he wants to convey, which is the power of a broad liberal arts education to awaken students to everyday mysteries and the way they connect across different disciplines.

Everything that you do at Colby can feed your goals, McCoy said, no matter how seemingly unrelated a class is to your main interests.

“There’s always some growth that changes the way you interact, your ability to communicate, the way you formulate questions,” he said.

Some of those questions are new; others span centuries, enticing generation after generation. McCoy said it’s a recurring experience for him to attend a physics conference and hear a talk about a research topic first explored by, say, Leonardo da Vinci. Then the speaker will mention that the problem is unresolved.

“You’re like, wow, that’s unresolved. Huh. And why didn’t I think of that?” he said. “There are tons of examples of this in interdisciplinary science today. You just have to be creative and keep asking questions.”