GOALI: Microscale fundamentals of sweat evaporation

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Sweating is important to human thermoregulation, thermal comfort, illness diagnosis, and hygiene product development. However, little is known about how droplets emerge and evaporate from our sweat glands. The scale of these phenomena is in-between molecular processes that drive sweat generation revealed by biochemists and macroscopic sweat rate measurements performed by physiologists. Engineers have studied evaporation across all these length scales but only in non-biological settings. This work will use three advanced imaging techniques to provide a unique view of droplets during various sweating stages. To identify the main physical mechanisms underlying sweat evaporation, high-resolution photographs and videos will be interpreted with theoretical modeling and additional experimentation with artificial sweating surfaces. These research outcomes will potentially impact diverse fields ranging from medical diagnostic development to energy-efficient building design. In addition, the work will inform the industrial partner how to more realistically mimic sweating using a thermal manikin. These devices are often used in apparel development and to optimize airflow for human thermal comfort in buildings and vehicles. The project will foster the academia-industry partnership with various joint activities and engage with the broader public through multimedia platforms and campus events. To systematically understand the role of various thermofluidic factors in different sweat evaporation modes, the project team proposes a two-pronged transdisciplinary approach that merges physiological and engineering perspectives. First, a new method will be developed to integrate ventilated capsule sweat rate measurements used routinely in physiology with multimodal imaging. Specifically, the team will use fast macro videography, mid-wave infrared thermography, and optical coherence tomography. A suite of non-invasive methods will be used to simultaneously measure sweat evaporation rate and visualize the corresponding microscale sweat dynamics at three skin sites. The team will study these processes under neutral, moderate, and strong thermal stimuli that will be asserted to induce out-of-pore, dropwise, and filmwise sweating modes. Second, the contributions of various factors to sweat evaporation through experimentation and modeling will be explored employing the same method but with artificial sweating surfaces with gradually increasing complexity. This knowledge will enable the formulation of reduced-order models of the out-of-pore, dropwise, and filmwise modes. In addition, imaging sweating onset and drying phases will deepen understanding of biological processes, including how sweat traverses through the duct to the skin and what happens to sweat on the surface once its secretion stops.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.