Schlieren imaging for the visualization of particles entrapped in bubble films

Bubbles are one of the primary sources of transferring substances from water to air. When bubbles burst, small droplets containing microparticles and microorganisms previously suspended in water disperse. Hence, visualizing small objects in bubble films can provide a new methodology for investigating the material transfer from water to the environment and the dynamic behavior of objects in the films. We used Schlieren imaging of bubbles to visualize small objects such as bacteria and microplastics. Remarkably, black spots (Schlieren spots) appeared when light rays passed parallel to bubbles formed on the water containing microparticles and bacteria. The simulation method of Schlieren imaging of bubbles was developed to clarify the underlying mechanism and experimentally validated with different sizes and concentrations of microparticles. We found that a specific water meniscus is formed around a particle when the bubble film thickness is smaller than the particle diameter, and the meniscus plays an important role in enlarging the Schlieren spots. The Schlieren spots are forty times larger than the bubble film thickness in this work. To understand the magnification rule, we illuminated the correlation between bubble film thickness, particle diameter, and Schlieren spot diameter.