Abstract:

Evaporation in a cylindrical tube with a receding interface follows a diffusion-dominated model, classically known as the Stefan diffusion tube problem. This model is widely used to study mass transport in drying porous media, where accelerating evaporation is often desirable to shorten drying times. Yet, mechanisms to enhance evaporation at a free surface on larger length scales remain poorly understood. In this dissertation, I demonstrate that surface bubbles on volatile liquids can dramatically accelerate evaporation, with mass-loss rates up to an order of magnitude higher than Stefan’s prediction. Remarkably, these bubbles can persist for thousands of seconds, following the surface recession, with their lifetime and effect governed primarily by liquid volatility and bubble radius.

Upward flows are observed along the bubble cap, both in the liquid thin film and in the adjacent gas phase. The cap film flow is a surface-tension-driven flow resulting from the temperature gradient induced by evaporation, i.e. a thermal Marangoni flow. By seeding the liquid with insoluble solid particles, I track the cap film flow rising from the bubble base and converging at the apex.
Schlieren imaging further reveals a vapor plume rising from the apex. This plume results from forced convection of the vapor generated by the cap flow, which dominates over diffusion and thereby sustains enhanced evaporation.

I characterize the vapor plume by measuring its height as a function of bubble radius, from which a characteristic source vapor flow velocity is deduced. The plume thus represents a new mass-evacuation pathway created by the bubble. I also quantify the bubble’s contribution to evaporation and show that it remains nearly constant over time. Using boundary-layer arguments, I derive a scaling law for mass loss that agrees well with experimental data.

This study uncovers a new convection-dominated evaporation mechanism at atmospheric pressure and room temperature, up to ten times faster than ordinary diffusion-limited evaporation. Beyond advancing fundamental understanding, these findings suggest a novel, low-energy strategy for enhancing evaporation efficiency, with potential applications in industrial processes.