Rethinking Evaporation: Thermal and Optical Evaporation from Pure Water and Hydrogels - Gang Chen

The Wouk Lecture
Ramo Auditorium
May 17, 2023

Rethinking Evaporation: Thermal and Optical Evaporation from Pure Water and Hydrogels
Gang Chen, Carl Richard Soderberg Professor of Power Engineering, MIT

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.

The Wouk Lecture is presented by the Division of Engineering & Applied Science.

Produced in association with Caltech Academic Media Technologies. ©2023 California Institute of Technology

[dagordon]

Good question at 51:37, was wondering about ammonia as well

[DerekWoolverton]

Very interesting line of inquiry. Thinking about the boundary effect, I am reminded of the development of the fiber optic laser, and how they found that while the electric dipole transitions between states of 4f electrons were not possible in free ions, in the field of host material (silica glass) the atom are perturbed and can make that transition. (leading to lasing at 1000-2000 nm wavelengths, which turn out to be very useful for a number of fields). I wonder if there is any interaction between the atmosphere and water molecules that changes its absorption behavior. It would be interesting to see if there is any variation in this with different gases.

[ДэВолшебный]

Интересно из России тут кто нибудь смотрит это?