The major goals of this research project are to design and use photo-responsive surfactants to control multi-phase fluid motion through the "photo-Marangoni effect," develop a modeling framework for photo-Marangoni-driven fluid dynamics and enhance boiling heat transfer. These efforts aim to enable the control of bubble departure, migration, droplet departure and sliding (Fig. 1). Microgravity conditions allow for larger length and time scales in bubble dynamics while eliminating buoyancy and convection, ensuring accurate observation and theoretical comparison of light-controlled motion. These insights will enable exciting new capabilities for diverse terrestrial and space applications, where precision control of multi-phase fluid motion plays a key role.

To achieve these goals, we first designed and synthesized novel spiropyran-based (SP) and merocyanine-based (MC) photo-responsive surfactants. These surfactants were systematically characterized for their dynamic surface and interfacial tension responses under various conditions, including different solvents, light intensities, wavelengths, pH levels, and surfactant concentrations, demonstrating their suitability for inducing the Marangoni effect in multi-phase systems.

Using the self-designed SP, experimental testing was conducted to study air bubble departure and toluene droplet departure from solid substrates in water under UV laser illumination. To complement these experiments, COMSOL simulations were conducted to visualize droplet departure and fluid flow. The simulations were validated using tracer particle trajectories, showing strong agreement between experimental and computational results. Additionally, we experimentally achieved light-controlled droplet motion on liquid-infused surfaces (LIS), lubricant oil surfaces, and inside microchannels with solid walls. Scaling arguments were employed to investigate the relationship between droplet size and sliding velocity.

In partnership with Space Tango, we conducted experiments in a microgravity environment to study sealed air bubble migration in an MCH-para-toluene solution. Video recordings of bubble displacement were analyzed to investigate motion trajectories. The analysis explored the effects of bubble size and light exposure duration on migration velocity, while tracer particles were used to study fluid flow along the bubble surface. Furthermore, COMSOL simulations were developed to identify key factors influencing bubble migration, including surfactant concentration, bubble size, diffusion coefficient, adsorption/desorption rates, and the switching kinetics of the reversible reaction.

Lastly, we developed a mini boiling chamber with a microheater for single-bubble boiling experiments for future studies in microgravity. This system was designed to observe light-assisted vapor bubble departure and its subsequent impact on boiling heat transfer. The microheater was fabricated using advanced techniques such as photolithography, metal deposition, atomic layer deposition, and etching in a nanofabrication facility. These efforts provide a foundation for exploring the interplay between light, fluid dynamics, and heat transfer, driving advancements in phase-change applications involving bubbles.

The broader impacts of this research extend across multiple dimensions, benefiting society through advancements in science, education, and public outreach. The direct outcomes of this project have provided fundamental knowledge of the photo-Marangoni effect in multi-phase transport processes, enabling innovative solutions for precision fluid manipulation.

The integration of this research into educational programs has fostered training opportunities for both graduate and undergraduate students. PI Zhu incorporated the findings into her course, Interfacial Phenomena, offered to senior undergraduate and graduate students, providing hands-on exposure to experimental techniques and theoretical concepts related to multi-phase flow and phase-change heat transfer. This integration was further extended through guest lectures at institutions such as Johns Hopkins University, where PI Zhu shared project results with a broader academic audience.

The project provided professional development opportunities for students involved, with more than six graduate and undergraduate participants receiving hands-on research training. Highlights include presentations and award at major conferences (best poster award at MRS), as well as multiple student-led publications in prominent journals like ACS Central Science. One graduate student successfully defended their PhD thesis, while two others are scheduled to defend soon.

The project also contributed to broader dissemination through professional conferences and collaborations. PI Zhu, co-PI Luzzatto-Fegiz, Read de Alaniz, and their team presented findings at international meetings, such as the International Space Station Research and Development Conference (ISSRDC), further enhancing the visibility and impact of this research. Collaboration with the implementation partner, Space Tango, was integral to the success of the space experiments, ensuring that experimental designs met the rigorous demands of microgravity conditions and demonstrated applicability to both academic and industrial contexts.

Public outreach has been a significant component of this project. PI Zhu shared research findings with over 300 high school students through the UCSB GRIT talk series, which was broadcast on television, inspiring the next generation of scientists and engineers. Additionally, students participated in summer research internships, contributing to CubeLab development in PI Zhu’s lab. These initiatives reflect the project’s dedication to promoting STEM education and community engagement.

Last Modified: 01/30/2025
Modified by: Yangying Zhu