Real-Time Manipulation of Multiphase Fluids Using Photo-Responsive Surfactants

The manipulation of multiphase fluids has been identified as a crucial process for a variety of applications, including thermal management of buildings and electronics, power generation, desalination and bio-medical devices. While most previous works rely on passive control of fluid through artificial micro-/nanostructured surfaces, active methods using an electric, magnetic or thermal stimuli to control multiphase fluids offer real-time tunability and multi-functionality. However, existing methods typically rely on a high voltage (10-1000 V), a strong magnetic field (0.1 T), or a high temperature gradient generated by a high power source such as lasers. Here, we demonstrate dynamic control of liquid droplets in a multi-phase fluid system using light-sensitive surfactants that can induce large changes in the interfacial tension thereby exerting a photo-Marangoni force on a droplet. The surfactants are activated by visible light whose intensity is 1-2 orders of magnitude lower than the laser intensities used in thermal-capillary actuation. We characterize the magnitude and rate of change in interfacial tensions of various fluid-fluid systems. We also demonstrate fast and programmable movement of liquid droplet on liquid-infused-surfaces, inside microchannels, and on liquid substrates. Furthermore, we present a theoretical framework underlying the key parameters that contribute to the droplet motion. The results demonstrated in this work opens exciting doors for the use of photo-sensitive surfactants for dynamic manipulation of multiphase fluid systems for energy, building, thermal management and microfluidics applications.