Manipulation of Multiphase Fluid in Microgravity using Photo-responsive Surfactants*

Bubble manipulation is crucial for enhancing electrochemical reactions and heat transfer processes. Previous studies have relied on passive surface modifications or active external stimuli, which require complex fabrication or substantial energy inputs. To date, these methods have not achieved direct bubble control without involving solid surfaces. We report an approach using photo-responsive surfactants to control bubble movement. By programming low-power LEDs, we locally induce reversible changes in the surfactant molecules, altering surface tension and triggering Marangoni flow. To decouple Marangoni and buoyancy effects, we perform microgravity experiments, and successfully demonstrate real-time bubble manipulation.

The tested surfactant exhibits rapid and reversible photo-switching, allowing for long-term control. We achieve bubble propulsion with velocities up to 6 mm/s and accelerations reaching 5 mm/s² under LED illumination of only 38 mW/cm². Additionally, we develop numerical simulations to understand the dependence of bubble velocity on bubble size, switching kinetics, and light duration. These synergistic experimental and numerical findings provide key guidance for managing bubble dynamics in microgravity and designing new photo-surfactants for fluid dynamics.