Changes in boiling controlled by molar concentration-dependent diffusion of surfactants

TL;DR

This study demonstrates that surfactant effects on boiling behavior are governed by molar concentration-dependent diffusion and dynamic adsorption, rather than the critical micelle concentration, influencing bubble formation and heat transfer.

Contribution

It reveals that boiling modifications by surfactants depend on diffusion timescales and dynamic adsorption, not CMC, providing new insights into boiling control mechanisms.

Findings

Boiling behavior changes are independent of CMC across various surfactants.

Bubble formation and nucleation are influenced by molar concentration-dependent diffusion.

Heat transfer coefficient and critical heat flux relate to the logarithm of molar concentration.

Abstract

Boiling is a prevalent phase-change process that plays a vital role in facilitating efficient heat transfer from a heating surface. While this heat transfer mechanism is generally effective, a rapid increase in surface temperature can lead to hydrodynamic instabilities, resulting in a boiling crisis. Previous studies have shown that surfactants often improve boiling performance and change the boiling crisis behavior. Conventional wisdom in this field attributes that these changes in boiling behavior are tied to the critical micelle concentration (CMC) of the particular surfactant. However, our work reveals that these changes in boiling behavior are independent of the CMC for three nonionic surfactants across a wide range of molar concentrations. In addition, visual snapshots of the bubbling behavior indicate changes in bubble formation, such as bubble size and nucleation site density, influenced by the molar concentration-dependent diffusion timescale of surfactants. Hence, these findings offer compelling evidence that boiling behavior, encompassing both boiling performance and boiling crisis, is governed by the dynamic adsorption of surfactants rather than dictated by the CMC. This becomes evident when quantifying the heat transfer coefficient (HTC) and critical heat flux (CHF) using the logarithm of molar concentration, as predicted by theory. Building upon these findings, we propose insights for controlling when CHF modification occurs in specific scenarios involving any surfactants. These insights hold significant potential for optimizing heat transfer processes and leveraging surfactants in energy-related applications to maximize boiling efficiency.