A unified relationship for evaporation kinetics at low Mach numbers

Zhengmao Lu,Kyle L Wilke,Evelyn N Wang,Ikuya Kinefuchi,Geoffrey Vaartstra

doi:10.1038/s41467-019-10209-w

Zhengmao Lu, Kyle L Wilke + Show 3 more

Open Access

https://doi.org/10.1038/s41467-019-10209-w

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Abstract

We experimentally realized and elucidated kinetically limited evaporation where the molecular gas dynamics close to the liquid–vapour interface dominates the overall transport. This process fundamentally dictates the performance of various evaporative systems and has received significant theoretical interest. However, experimental studies have been limited due to the difficulty of isolating the interfacial thermal resistance. Here, we overcome this challenge using an ultrathin nanoporous membrane in a pure vapour ambient. We demonstrate a fundamental relationship between the evaporation flux and driving potential in a dimensionless form, which unifies kinetically limited evaporation under different working conditions. We model the nonequilibrium gas kinetics and show good agreement between experiments and theory. Our work provides a general figure of merit for evaporative heat transfer as well as design guidelines for achieving efficient evaporation in applications such as water purification, steam generation, and thermal management.

Highlights

We experimentally realized and elucidated kinetically limited evaporation where the molecular gas dynamics close to the liquid–vapour interface dominates the overall transport
The fundamental interfacial transport limit in these systems is dictated by the kinetics across a nonequilibrium gas region which inevitably forms near the evaporating surface[12]
Evaporation is induced from the pinned interfaces in the nanopores through electrically heating the gold layer which serves as a local resistive temperature detector (RTD) to address the typical temperature sensing issue

Summary

Introduction

We experimentally realized and elucidated kinetically limited evaporation where the molecular gas dynamics close to the liquid–vapour interface dominates the overall transport. The fundamental interfacial transport limit in these systems is dictated by the kinetics across a nonequilibrium gas region which inevitably forms near the evaporating surface[12] This region, which is a few mean free paths thick, is known as the Knudsen layer and originates from the net flux across the interface disturbing the local thermodynamic equilibrium. While significant theoretical efforts have offered some insights into the gas kinetics in the Knudsen layer, experimental studies have proved difficult due to limitations in vapour removal[10,15], interface temperature sensing[16,17], thermal conduction[18,19], and liquid supply[20,21]. We develop an ultrathin nanoporous membrane device to overcome previous challenges With this experimental platform and assisted by nonequilibrium gas kinetics modeling, we elucidate a fundamental relationship that unifies evaporation kinetics under different working conditions

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Conclusion