Rethinking Evaporation:
Thermal and Optical Evaporation from Pure Water and Hydrogels

Evaporation is a ubiquitous phenomenon in nature, yet our understanding on evaporation is surprisingly insufficient. While the name "thermal evaporation" suggests that evaporation is driven thermally, the true driving force for evaporation is the chemical potential difference between the liquid and the vapor phases. There are many fundamental unanswered questions on evaporation. For example, how can we describe thermodynamic efficiency of evaporation that involves both temperature and chemical potential gradients? Answering this question leads us to generalize the Carnot expression for the limit of heat engine efficiency to thermal and chemical processes. In heat transfer, large temperature discontinuities across liquid-vapor interfaces had been reported experimentally, which have defied modelling efforts so far. We established a set of interfacial conditions to determine the interfacial temperature, density, and pressure drop across a liquid-vapor interface, which lead to modeling results in reasonable agreement with experimental data. Finally, I will address the puzzle of superthermal solar interfacial evaporation. Our experiments, as well as by many others, have reported that evaporation under sunlight from hydrogel and other porous materials can exceed the thermal evaporation limit by several times. We hypothesize that photons can directly cleave off water clusters at the liquid-vapor interface in a way similar to the photoelectric effect, which we call the photomolecular effect. We use several independent experiments to support this hypothesis and rationalize that the quadrupole force on water hydrogen network is the reason for the photomolecular effect.

Gang Chen
Carl Richard Soderberg Professor of Power Engineering, MIT

Gang Chen is the Carl Richard Soderberg Professor of Power Engineering at Massachusetts Institute of Technology (MIT). He served as the Department Head of the Department of Mechanical Engineering at MIT from 2013 to 2018. He obtained his PhD degree from the Mechanical Engineering Department at UC Berkeley. He was a faculty member at Duke University and UCLA, before joining MIT in 2001. He received an NSF Young Investigator Award, an R&D 100 award, an ASME Heat Transfer Memorial Award, an ASME Frank Kreith Award in Energy, a Nukiyama Memorial Award by the Japan Heat Transfer Society, a World Technology Network Award in Energy, an Eringen medal from the Society of Engineering Science, and the Capers and Marion McDonald Award for Excellences in Mentoring and Advising from MIT. He is a fellow of American Association for the Advancement of Science, the American Physical Society, The American Society of Mechanical Engineers, and the Guggenheim Foundation. He serves on the board of the Asian American Scholar Forum (aasforum.org). He is an academician of Academy Sinica, a fellow of the American Academy of Arts and Sciences, a member of the U.S. National Academy of Engineering, and was recently inducted into the National Academy of Sciences.