Achoo!

This sequence illustrates the evolution of the multiphase turbulence cloud that suspends droplets emitted during a sneeze. Shown here are times ranging from 7 to 340 milliseconds post sneeze onset. (Image courtesy of B. E. Scharfman/A. H. Techet/J. W. M. Bush/L. Bourouiba/MIT.)

High-speed footage has demonstrated that sneezes are expelled as ballooning sheets of fluid which break apart in long filaments that destabilize and finally disperse as a spray of droplets.

The recordings showed that the fluid is flung into the air similar to paint, with all 100 volunteer sneezers producing the same pattern. Variations in fluid fragmentation depended on fluid elasticity. If the fluid traveled longer before breaking apart into droplets, then elasticity was greater.

“It’s important to understand how the process of fluid breakup, or fluid fragmentation, happens,” says Lydia Bourouiba, assistant professor of civil and environmental engineering and lead researcher. “What is the physics of the breakup telling us in terms of droplet size distribution, and the resulting prediction of the downstream range of contamination?”

The research may help to identify “super spreaders”, people with high elasticity of saliva, ultimately reducing infection rates.

The Sneezing Experiment

The MIT research showed that sneezes produced clouds of gas carrying infectious droplets up to 200 times farther than if they were simply disconnecting as drops. Bourguiba focused the camera directly in front of the subjects’ mouths to study droplet distribution at high-speed.

Volunteers were positioned against black backdrops and two high-speed monochrome cameras were focused in front of their mouths. Tickling their noses, the researchers induced and recorded the volunteers' sneezing in under 200 milliseconds.

Top and side views of the rapid fragmentation process of mucosalivary fluid occurring during a healthy sneeze. They reflect the sequence of formation of sheets and then filaments, ultimately leading to the formation of respiratory droplets outside of the mouth. These were captured with a camera operating at 6,000 to 8,000 frames per second. (Image courtesy of B. E. Scharfman/A. H. Techet/J. W. M. Bush/L. Bourouiba/MIT.)

Results showed that the fluid forms a wide sheet that balloons along with air. The balloon bursts while airborne and thin filaments separate into individual droplets. Droplets of various size then fall to the ground or remain suspended in the turbulent cloud.

Interestingly, expelled fluid remained in filament form longer while forming beads along the filaments for cases where saliva was more elastic. Eventually the beads slid off as droplets.

Super-Spreading Sprayers

Ongoing studies will hopefully obtain concrete data on droplet distribution to predict how to reduce the spread of disease.

“One of the important goals I have for the lab is to tackle cold and influenza,” Bourouiba says. “Sometimes the symptoms are difficult to distinguish. In the coming year, at different cold and influenza seasons, we will be recruiting human subjects whom we can work with to see them in infection and in health.”

Since the spray from a sneeze is more complex than simple droplets, studying sneezes could have important industrial implications. For example, a nozzle spray that replicates the fluid distribution of a sneeze would have more coverage than traditional cone sprayers. These super sprayers could be applied in the mining industry for washing decks, in fire protection spray nozzles or for dust suppression in many industries.

For more information, check out the paper “Visualization of sneeze ejecta: steps of fluid fragmentation leading to respiratory droplets” in Experiments in Fluids.

Gesundheit!