Session: 03-03: Micro/Nanoscale Interfacial Transport Phenomena

Paper Number: 132610

132610 - Molecular Beam Experiment of Evaporating Water Molecules From a Liquid-Vapor Interface 

Abstract: 

Evaporation and condensation are ubiquitous phenomena in nature and engineering applications. The Hertz-Knudsen-Schrage equation is widely adopted for describing the net mass flux in multiphase flow analyses. However, the validity of this equation is often questioned because its derivation assumes the Maxwell-Boltzmann velocity distribution for evaporating molecules, which seems inconsistent with the nonequilibrium nature of evaporation and condensation. Molecular dynamics (MD) simulations indicated that the velocity distribution of evaporating molecules deviates from the Maxwell-Boltzmann distribution under highly nonequilibrium conditions, whereas such nonequilibrium velocity distributions have not yet been verified by experiments.
Here, we show an experimental setup for measuring the velocity distribution of evaporating water molecules from a liquid-vapor interface using the molecular beam technique. A liquid surface is kept in a vacuum chamber using a nanoporous membrane made by the MEMS fabrication technique. The membrane has 109 pores with a diameter of 450 nm. The small diameter ensures a uniform temperature on the liquid surface despite the heat of evaporation. These pores are located within a circular region with a diameter of about 10 um, roughly corresponding to one mean free path. With this geometry, the number of molecular collisions becomes far less than one, meaning evaporated molecules do not collide with other molecules and preserve their initial velocity distribution. Liquid water was degassed before experiments and was constantly supplied from a flow channel in order to prevent contamination from accumulating on the liquid surface. We used heavy water instead of regular water in order to avoid noise caused by residual water molecules in the measurement chamber. Evaporated molecules are extracted using a skimmer to generate a molecular beam. Then, the beam is modulated by a mechanical chopper and is detected by a quadrupole mass spectrometer. The velocity distribution is determined from the time-of-flight distribution.
The velocity distribution obtained from our measurement deviates from the Maxwell-Boltzmann distribution, showing fewer slow molecules than the Maxwell-Boltzmann distribution. This result is consistent with the previously reported MD simulations [T. Ishiyama et al., Phys. Fluids 16, 4713 (2004)]. We will conduct further measurements to examine the temperature dependence and the effect of more than one volatile component. In addition, several groups recently reported that evaporation rates under light irradiation exceed the thermal limit, where all the energy of incident light is converted to latent heat and sensible heat. To explain this phenomenon, a hypothesis called the photomolecular effect, where photons cleave off water clusters from liquid-vapor interfaces, has been proposed [Yaodong Tu, Gang Chen et al., PNAS 120, e2312751120 (2023)]. Our molecular beam setup is promising to clarify the microscopic perspective of enhanced evaporation at liquid-vapor interfaces under light irradiation.

Presenting Author: Ikuya Kinefuchi The University of Tokyo

Presenting Author Biography: Kinefuchi received his B.S. (2001), S.M. (2003), and Ph.D. (2006) from The University of Tokyo. He is an associate professor at Department of Mechanical Engineering, The University of Tokyo. He is interested in micro/nanoscale heat and mass transfer, rarefied gas dynamics (kinetic modeling of evaporation at a liquid-vapor interface), and mesoscale modeling (non-Markovian dissipative particle dynamics).

Authors:

Ikuya Kinefuchi The University of Tokyo