Heat Transfer by Sweat Droplet Evaporation

Sweating regulates the body temperature in extreme environments
or during exercise. Here, we investigate the evaporative heat transfer
of a sweat droplet at the microscale to unveil how the evaporation
complexity of a sweat droplet would affect the body’s ability
to cool under specific environmental conditions. Our findings reveal
that, depending on the relative humidity and temperature levels, sweat
droplets experience imperfect evaporation dynamics, whereas water
droplets evaporate perfectly at equivalent ambient conditions. At
low humidity, the sweat droplet fully evaporates and leaves a solid
deposit, while at high humidity, the droplet never reaches a solid
deposit and maintains a liquid phase residue for both low and high
temperatures. This unprecedented evaporation mechanism of a sweat
droplet is attributed to the intricate physicochemical properties
of sweat as a biofluid. We suppose that the sweat residue deposited
on the surface by evaporation is continuously absorbing the surrounding
moisture. This route leads to reduced evaporative heat transfer, increased
heat index, and potential impairment of the body’s thermoregulation
capacity. The insights into the evaporative heat transfer dynamics
at the microscale would help us to improve the knowledge of the body’s
natural cooling mechanism with practical applications in healthcare,
materials science, and sports science.