Nucleate boiling has found immense applications in a wide range of industrial applications such as power generation, water purification, electronics cooling and space technologies. Bubble dynamics play a critical role in determining the heat transfer performace of nucleate boiling. The intense vaporization of saturated liquids, the bubble-growth-induced agitation of liquids, and the thermo-capillary convection around active vapor bubbles all contribute to the high heat transfer coefficient of nucleate boiling. On the other hand, inefficient removal of bubbles from the substrate can create a vapor film and trigger the critical heat flux (CHF). Recently, applying boiling in space applications has attracted increasing interests due to its relevance to space thermal control, space power and electronic thermal management. However, achieving a high boiling heat transfer coefficient in microgravity is challenging, since the absence of buoyancy, which constitutes the primary mechanism of bubble departure on earth, facilitates the accumulation of stagnant vapor bubbles.

In this study, we realize the spontaneous removal and active manipulation of bubbles using a photo-responsive surfactant, Spiropyran (SP), whose properties undergo reversible changes triggered by light, leading to a change in the liquid-vapor interfacial tension. Firstly, an optically sensitive solution is created by dissolving SP in water or toluene. When subject to a UV light (wavelength of 365 nm), SP undergoes a reversible isomerization and is transformed into Merocyanine (MC). In this case, the surface tension of as-formed solution can be substantially lowered. In addition, MC can be readily transformed back into SP once exposed to strong white light. The equilibrium state of such reversible reaction is tuned by meticulously adjusting the intensity of UV and white light. To demonstrate active bubble removal without the influence of thermal Marangoni and convection effects, we then fabricate a glass chamber with a pin hole which can continuously generate discrete air bubbles. The temperature of the chamber is maintained at a constant of 23 degrees Celsius so as to eliminate possible thermo-capillary effect.  By applying a spatial gradient of the activation lights, we are able to produce a heterogeneous distribution of SP/MC. The height-dependent surfactant concentration hence leads to a  surface tension gradient along the bubble interface and a net force from the subsequent Marangoni flow eventually drives the bubble to depart. In addition, the bubble departure frequency can be tuned by utilizing a pulsed UV light for optimal thermal performance under different thermal loads. Last but not least, active manipulation of air bubbles can be easily realized by applying a single laser beam, which pins air bubbles around. We also carry out numerical simulations using COMSOL to understand the coupling between photo-responsive reactions, species transport and flow field. In conclusion, spontaneous bubble removal and active bubble manipulation can be achieved by combining photo-responsive surfactants and activation lights, which promises further applications in enhanced nucleate boiling under microgravity environment.


Manipulating Bubble Dynamics via Photo-Responsive Surfactants and Light for Microgravity Boiling

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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