Modeling Marangoni flows induced by photo-responsive surfactants

Photo-responsive surfactants can reversibly change their molecular conformation when illuminated, thereby tuning interfacial tensions in a multi-phase flow. These "photosurfactants" can therefore induce targeted and complex Marangoni flows, including droplet and bubble motion in the fluid interior or along interfaces, and may enable new avenues for enhancing the performance of microfluidic, thermal, and water harvesting devices. However, to the best of our knowledge, there are currently no general models for predicting the resulting flows from a given combination of photosurfactant type, geometry, illumination and fluid properties. As a consequence, the potential capabilities and benefits that could be derived from photosurfactants remain largely unknown. We examine several photosurfactants, and leverage experimental data from nuclear magnetic resonance, time-dependent tensiometry, as well as photosurfactant-driven flows of increasing complexity, both on Earth and in microgravity. We deduce models for the photo-switch mechanism and the attendant tension change, which we use to implement numerical simulations, as well as to derive scaling relations for several key flows of fundamental and technological importance.