Forget droplets. Here’s how sweat really forms

On a summer day, a telltale drop of sweat tickles down your back. Next thing you know, sweat is running off you in rivers and your shirt is soaked. A new study — which wrapped six people up like overheated burritos — at last explains why.

It may seem that sweat pops out of our skin as droplets. In fact, the new data show, it rises like a tide through our pores, soaking the top layer of skin. The sweat then forms a shallow pool in the pores before joining with others to make a liquid film.

That’s how we go from a few prickles of sweat to suddenly soaking. Scientists shared these findings July 23 in Journal of the Royal Society Interface.

“Sweat has a bad reputation for being ‘icky.’ But it is incredibly important,” says Konrad Rykaczewski. Working at Arizona State University in Tempe, he studies how materials transfer heat. The dampness of sweat doesn’t cool us, he explains. It’s the evaporation of the sweat that chills our skin.

Studying how sweat works could lead to better cooling tech.

Bringing the heat

Past studies have looked at sweat on very small scales. They used light and imaging to see sweat forming in pores. Scientists also have studied sweat on larger scales. Damp skin conducts more electricity. So scientists can measure how much people sweat in different situations by assessing how conductive their skin is.

But what happens between the pores and the sweaty surface? “I was very surprised to find out that we did not understand the processes on a scale starting from [a] single sweat pore opening [to extending up] to a few centimeters,” Rykaczewski says. 

To fill in those gaps, scientists watched six people sweat. Each volunteer rested in a recliner, wearing a full-body, tube-filled suit. A pump could move hot or cold water through the suit to warm or chill the wearer.

Wearing a special suit and wrapped in a heated blanket, Konrad Rykaczewski is sweating for science. His teammates measure his sweat output with a combo of imaging, electrical and microscopic techniques.Samantha Chow/Arizona State University

Participants were also wrapped in heated blankets with a layer of waterproof paper in between, so their sweat wouldn’t soak the blankets. Each recruit was heated, cooled and then heated once more. Meanwhile, the team measured every aspect of any sweat forming on their foreheads.

The heat “feels lovely for [the] first 15 minutes, especially in winter,” Rykaczewski says. “After about two hours, it becomes [a] bit, well, sweaty.”

Breaking a sweat

Sweat comes from glands in the deepest layer of our skin. When heated subjects started to sweat, moisture emerged and evaporated from pores in a repeating cycle. But that sweat wasn’t released as a little droplet. Instead, it was nearly flat. It settled in a pore on the skin until it spilled out and connected with others, forming a puddle and then a film.

The rising tide of sweat also soaked through the outermost layer of dead skin cells. Once that outer layer had filled like a sponge, the sweat began to pool on top. The final sweaty film quickly evaporated once the subjects were cooled. But sweat is both salty and wet. So when the water evaporated, a thin salt layer stayed behind.

When the heat returned, sweat rose faster than before. This time, the leftover salt allowed the sweat to soak into the layer of dead skin cells faster. This second sweat layer skipped the flat droplet stage entirely, coming out as a film.

That’s good for cooling, Rykaczewski says. “A thin-film sweat coming out of the pores covers maximum surface area for evaporation quickly.”

Sweat begins to pool on Konrad Rykaczewski’s forehead as he participates in his own study of how we perspire.Samantha Chow/Arizona State University

Sweating for science

It’s easy to assume sweat forms as droplets. It’s “the cliché thing we picture,” says Jonathan Boreyko. We tend to imagine “a very clean, [dome-shaped] droplet — kind of like just sitting on your skin that then proceeds to evaporate.” The new study shows that at least for the forehead, that’s not what happens.

Boreyko, who works at Virginia Tech in Blacksburg, was not involved in the new research. But he does study how liquids, like sweat, move. The new study focused on sweat coming out of the forehead, he notes. The skin there is covered with very tiny hairs that help sweat evaporate. However, he adds, “It seems like this kind of stuff might play out wildly differently on any different part of the body.” Armpits and even arms have different concentrations of sweat glands, hair and more.

Understanding how sweat rises and spreads could help scientists develop things such as fabrics to help our bodies better handle heat.

Emiel DenHartog is a clothing biophysicist at North Carolina State University in Raleigh. Though he did not take part in the study, he’s excited to apply its results to his work. Clothes are currently run through sweat tests that use much larger liquid droplets. “That may not reflect how sweat and fabrics really interact and cause cooling,” he says. “It would be exciting to connect these two areas of research.”