GOALI: Local thermoreflectance measurement of evaporative heat transfer in the thin film region of a dynamic meniscus

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Cooling by evaporation is used in a wide range of important industrial and power-generation processes. Improved understanding of evaporative heat transfer will lead to more energy-efficient equipment designs for the thermal management industry. The objective of this project is to measure local evaporative heat transfer in the thin film region of menisci with unprecedented spatial resolution. Theory suggests that heat flux will spike in the thin evaporating region of the meniscus, yet prior experiments cannot adequately resolve this key feature. Thermal management technologies exploit this region using micro/nanostructured surfaces that expand the meniscus. New experiments that measure local evaporation are critical to improve such nanostructure designs for the thermal management industry but are also of fundamental value to meniscus driven processes like boiling and desalination. The educational activities will expose students to industry-driven academic research through curriculum development, guest seminars, and internships. Through collaboration with a local non-profit, regular outreach opportunities will be formalized with underrepresented middle school students. Evaporative heat fluxes will be measured using frequency domain thermoreflectance (FDTR) and simultaneous measurements of meniscus thickness will be made using interferometry. The microscale extent of the thin film region (100-102 ?gm in-plane, 100-103 nm thick) and sub-Kelvin liquid-vapor temperature differences (superheats) challenge conventional thermometry, while parasitic conduction into the underlying substrate obfuscates actual evaporative heat fluxes. As a result, prior measurements effectively integrate evaporation rates over large swaths of liquid-vapor interface, permitting a range of satisfactory theoretical interpretations. Thermoreflectance approaches, which have revolutionized heat conduction research, will help to overcome these barriers in thin film evaporation metrology. These accurate measurements of the evaporative heat flux (?b10%) will uniquely discern the role of surface attractive forces and accommodation coefficients on thin film evaporation.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.