Bottom-up view of sweat droplet and film evaporation dynamics*

Sweat evaporation is vital for human thermoregulation; however, it has yet to be studied on the length scale of single pores. Here, we discuss a wind tunnel-inspired ventilated capsule with a sapphire window that simultaneously measures sweat evaporation rate and enables mid-wave infrared imaging microscale sweating dynamics [1]. Besides the experimental and numerical characterization of the parabolic flow and evaporation rate on artificial surfaces, we also discuss the use of the device in human subject experiments. The experimentation highlights multiple microscale fluid processes occurring near or on the skin including out-of-duct evaporation, pulsating droplets, crevice filling, and hydration-induced droplet-to-film transitions. In addition, the results challenge the common assumption that sweat forms an isothermal film, revealing 3x higher mass transfer coefficients during cyclic dropwise than filmwise sweating. The research highlights significant differences between various stages and rates of evaporation, suggesting the need for further exploration of these mechanisms.

[1] Jaiswal et al. iScience, 27,7,110304, 2024.