Control and Manipulation of Surface Flows Using Photo Marangoni Effect

Marangoni flows are pervasive in numerous natural and engineering processes and often play a defining role in micro- and nanoscale systems. With “tears of wine” being one famous manifestation of the Marangoni effect, other notable applications included painting, printing, and wicking in heat pipes for thermal management, among others. Typically, Marangoni effect (i.e., the surface tension gradient) can be created in two ways: by a temperature and/or concentration gradient, termed thermocapillarity and/or solutocapillarity, respectively. In both cases, however, the capillary forces are inevitably coupled with other forces, such as buoyancy, which hinders our ability to study capillary forces and their role in other processes like capillary instabilities. In this study, we aim to control, manipulate, and characterize surface tension drive flows using so called photo Marangoni effect. This approach is based on the unique property of a surfactant called azobenzene trimethylammonium bromide (AzoTAB), whose tension increases subject to UV light and decreases subject to blue light. Employing this effect, an isothermal surface flow can be produced by pure surface tension effect, allowing us to investigate the behavior of capillarity with other effects effectively decoupled. Particle image velocimetry will be used to characterize the flow. Additionally, droplet manipulation in microfluidic porous media is possible with this technique.