[Paper Review] Vapour Cloud Dynamics Induced by Evaporation

Sam Dehaeck, Pierre Colinet|arXiv (Cornell University)|Oct 15, 2010

Gas Dynamics and Kinetic Theory3 references3 citations

TL;DR

This study uses Mach-Zehnder interferometry to visualize and analyze vapour cloud dynamics induced by evaporation from HFE-7100 liquid surfaces under ambient conditions. It demonstrates that the high density of the vapour cloud drives convective flows, causing significant deviations from purely diffusive behaviour, and reveals complex dynamics such as downward convection, spreading along substrates, and Bénard-like convection in the liquid phase.

ABSTRACT

In this fluid dynamics video, the vapour cloud generated near evaporating free liquid surfaces is visualised by Mach-Zehnder interferometry (MZI). More precisely, for evaporating HFE-7100 (from 3M) and ambient conditions, the vapour concentration field and its dynamical behaviour are observed in three simple experiments. Through these experiments, it is evidenced that the high density of the vapour cloud (compared to air) induces convective motions in the gas mixture, resulting in deviations of the concentration field from a purely diffusional behaviour.

Motivation & Objective

• To investigate the dynamic behavior of vapour clouds formed during evaporation of high-volatility liquids like HFE-7100 under ambient conditions.
• To overcome limitations of existing techniques such as infrared absorption and planar laser-induced fluorescence by employing Mach-Zehnder interferometry for real-time, non-invasive concentration field measurements.
• To examine how vapour cloud density influences gas-phase convection and alters the spatial distribution of vapour beyond purely diffusional predictions.
• To observe and characterize the interplay between vapour cloud dynamics and liquid-phase phenomena such as cooling-induced convection during evaporation.
• To demonstrate the feasibility of MZI as a complementary tool for studying vapour cloud evolution in evaporative systems with high vapour pressure liquids.

Proposed method

• Mach-Zehnder interferometry (MZI) is employed to measure dynamic refractive index fields, which are directly linked to vapour concentration and temperature gradients in transparent media.
• The technique is applied to visualize vapour concentration fields in air above evaporating HFE-7100 droplets and liquid pools under ambient conditions, leveraging the strong refractive index dependence on vapour concentration.
• Three experimental configurations are used: a hanging drop, a drop near a solid substrate forming a liquid bridge, and a rectangular cuvette with a removable cover to initiate sudden evaporation.
• Interferometric fringes are recorded in real time to map spatial and temporal variations in the vapour concentration field and detect convective structures.
• The method captures both gas-phase dynamics (e.g., downward vapour plumes, spreading along surfaces) and liquid-phase convection (Bénard-like cells) due to evaporative cooling.
• The integration length of 2 mm is sufficient to detect measurable optical path differences due to vapour concentration changes, even at ambient conditions.

Experimental results

Research questions

• RQ1 How does the density of a vapour cloud generated by evaporating HFE-7100 affect its dynamical behavior in still air?
• RQ2 To what extent do convective flows in the gas phase deviate from purely diffusional vapour transport models?
• RQ3 How does the presence of a solid substrate influence the morphology and dynamics of the vapour cloud near an evaporating liquid?
• RQ4 What are the characteristics of vapour cloud evolution when evaporation is suddenly initiated in a confined geometry, such as a cuvette?
• RQ5 Can Mach-Zehnder interferometry effectively resolve transient vapour concentration fields in systems with high vapour pressure liquids under ambient conditions?

Key findings

• The vapour cloud generated by evaporating HFE-7100 exhibits strong downward convection due to its higher density compared to air, deviating significantly from purely diffusive behaviour.
• In the presence of a solid substrate, the vapour cloud spreads along the surface, and the isoconcentration lines reconfigure in response to the evolving liquid bridge geometry.
• When the cover is removed from the cuvette, the initially homogeneous vapour layer rapidly evolves into a stagnant diffusive zone with intermittent spillover of excess vapour over the edges.
• Bénard-like convection cells are observed in the liquid phase, driven by cooling at the free surface due to evaporation, indicating thermocapillary instabilities.
• Mach-Zehnder interferometry successfully captures dynamic vapour concentration fields with sufficient sensitivity for HFE-7100 at ambient conditions, even with a 2 mm optical path length.
• The technique demonstrates potential as a complementary method to infrared absorption and PLIF for studying vapour cloud dynamics in high-volatility liquid systems.