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.

Inhibiting Cracks in Latte Droplets.

Latte is a mixture of coffee and milk and a model of complex fluids containing biomolecules, usually leaving complex deposit patterns after droplet evaporation. Despite the universality and applicability of biofluids, their evaporation and deposition dynamics are not fully understood and controllable because of the complexity of their components. Here we investigate latte droplet evaporation and deposition dynamics, primarily the crack development and inhibition in droplet deposit patterns. With regard to a mixture of milk and coffee, we find that the surfactant-like nature of milk and intermolecular interactions between coffee particles and milk bioparticles are responsible for achieving uniform crack-free deposits. This finding improves our understanding of pattern formation from evaporating droplets with complex biofluids, offering a clue to applications of bioinks with both printability and biocompatibility.