Surfactant Use in Boiling Heat Transfer Alludes to a Universal Concentration?

Surfactants—molecules that change the surface properties of liquids—can enhance the rate of heat transfer in boiling, which could improve power generation, heating/cooling/refrigeration, and other applications. Boiling involves cycles of bubble nucleation, growth, and departure. These behaviors are modified as surfactants adsorb to the bubble surface, lowering surface tension and changing the bubble contact angle. Unique to each type of surfactant, there is a critical micelle concentration (CMC) above which surfactant molecules aggregate into larger structures called micelles. From conventional wisdom, adding surfactants beyond the CMC increases liquid viscosity, resulting in an overall lowering of heat transfer. Thus, to optimize heat transfer, the optimal concentration should be near the CMC. This is because the Reynolds and Rayleigh numbers of the liquid decrease—reducing the ability of the liquid to convect heat. However, our testing has found that different surfactants experience a marked “bump” in heat transfer performance occurring at the 6–7 mM range. Our results suggest the existence of a universal optimized concentration for surfactants. We hypothesize that this concentration is due to the rate surfactants enter the bubbly region via liquid motion (advection) matching the rate at which surfactants exit the region via bubble surfaces. We are continuing experiments with nonionic (TWEEN families) and ionic (sodium sulfate families) surfactants with vastly different CMCs to see if they all experience a similar “bump” around 6–7 mM. The implications could have great practical impact as engineers could consult a universal concentration to achieve optimal heat transfer efficiency with any surfactant.https://digitalscholarship.unlv.edu/durep_posters/1063/thumbnail.jp

How Dynamic Adsorption Controls Surfactant‑enhanced Boiling

Improving boiling is challenging due to the unpredictable nature of bubbles. One way to enhance boiling is with surfactants, which alter the solid–liquid and liquid–vapor interfaces. The conventional wisdom established by previous studies suggests that heat transfer enhancement is optimized near the critical micelle concentration (CMC), which is an equilibrium property that depends on surfactant type. However, these studies only tested a limited number of surfactants over small concentration ranges. Here, we test a larger variety of nonionic and anionic surfactants over the widest concentration range and find that a universal, optimal concentration range exists, irrespective of CMC. To explain this, we show that surfactant-enhanced boiling is controlled by two competing phenomena: (1) the dynamic adsorption of surfactants to the interfaces and (2) the increase in liquid dynamic viscosity at very high surfactant concentrations. This dynamic adsorption is time-limited by the millisecond-lifetime of bubbles on the boiling surface—much shorter than the timescales required to see equilibrium behaviors such as CMC. At very high concentrations, increased viscosity inhibits rapid bubble growth, reducing heat transfer. We combine the effects of adsorption and viscosity through a simple proportionality, providing a succinct and useful understanding of this enhancement behavior for boiling applications