Flow Visualization of a Marangoni Surfer: Relating Flow to Surfer Motion

Marangoni surfers are self-propelling particles that move due to surface tension gradients. Understanding their behavior in the presence of surfactant gradients is crucial for advancing microfluidic systems and for controlling particle dynamics in both scientific and industrial applications. The Marangoni effect, responsible for these surfers' motion, plays an important role in enhanced oil recovery by releasing trapped oil from rock pores and in drug delivery by improving the distribution of therapeutic agents.

In this study, we developed an experimental setup to visualize and quantify fluid flow around objects at an air-water interface under surfactant gradients. Using Particle Image Velocimetry (PIV), we characterized the fluid flow around 5 mm Marangoni surfers. Our setup, simpler than traditional PIV systems, consisted of a tank, tracer particles, and a camera. We initially investigated how surfactant concentration and the method of application affect the swimmer's maximum velocity. By analyzing the velocity field and determining the time constant, we derived a Gaussian fit equation with adjustable parameters, A and b. Fitting this equation to experimental data revealed that the values of A and b varied depending on the surfactant concentration and application method, suggesting that convective forces, rather than purely diffusive ones, influence the transport dynamics due to high particle velocities.

Additionally, we optimized tracer particle concentration by testing various levels to balance resolution, variability, and efficiency of tracer usage. We conducted experiments to identify the ideal concentration that provided the best combination of these factors. Finally, we studied the flow around free swimmers with different surfactant application methods, integrating results from tracer concentration, surfactant effects, and application methods. This allowed us to visualize the flow field around Marangoni surfers and compare it to free particle velocity. Future work will focus on stabilizing tracer solutions and achieving a precise quantitative match between tracer PIV experiments and free particle tracking to further investigate the dynamics of these systems.