Visualization of Surfactant Dynamics to and along Oil–Water Interfaces Using Solvatochromic Fluorescent Surfactants

Although many theories exist to understand the transport of surfactants to and along interfaces under flow, few techniques are available to directly monitor surfactant dynamics at interfaces. Due to the broad use of surfactants, from oil spill remediation to healthcare and household products, a better means of assessing surfactant dynamics would be widely applicable. Here, we describe and demonstrate the use of fluorescent surfactant-dyes as a means of dynamic visualization of surfactant transport and have synthesized Nile Red/polyethylene glycol (NR-PEG) derivatives for this purpose. These NR-PEG surfactant-dyes fluoresce differently at oil–water interfaces than in bulk aqueous solution, allowing for the selective monitoring of bound and free surfactant using confocal microscopy. We have visualized the transport of surfactants to and along interfaces of droplets in a pipe flow and at the interfaces of liquid-infused surfaces undergoing shear-driven drainage. Imaging surfactant adsorption to interfaces at these time scales will allow for the development of improved dispersant systems for applications such as oil spill remediation and can also be used in understanding other fundamental transport phenomena involving dispersants.