In this study, we aim to investigate the behavior of bubbles when two bubbles collide, under what conditions will they rebound or coalesce. The properties of a bubble depend greatly on its chemical composition, environmental conditions, and its temporal evolution. Therefore, it is essential to have precise control over these factors. To achieve this, we developed our own bubble solution, the chemical components of which we know, and which is durable over time to mitigate bubble decay.
II - What is a Bubble?
A bubble solution is primarily composed of two ingredients, water, and a surfactant. The surfactant is a long molecule with a hydrophilic head and a hydrophobic tail. Mixing water and the surfactant reduces surface tension, allowing the formation of bubbles, where the surfactant accumulates on the surface, creating a thin water layer in the air. Due to gravity, the water molecules get pulled down, leading to rapid bubble decay and bursting. To counteract this effect, we added polymers to the solution, increasing its viscosity, and slowing down the decay. The chemical composition of the bubble solution, including its surfactant concentration and polymer viscosity, influences bubble behavior.
III - Behavior of Bubbles
When two bubbles meet, different scenarios may occur. They can form a lens where they stick together while maintaining a film separating the two bubbles, bounce off each other, or coalesce, forming a larger bubble. In the latter scenario, three types of coalescence may occur: complete coalescence, where the bubbles fuse completely into one bubble; partial coalescence, where the bubbles fuse but leave a residual little bubble, resulting in two bubbles, one big and one small; and coalescence leading to bursting, where both bubbles eventually explode.
a. Bouncing Conditions
Bouncing behavior occurs when two bubbles approach each other with a slow speed, but more importantly, maintain a thin film. Optical control enables us to determine film thickness, where interference causes a metallic color, black spots, and lower brightness, indicating a thin soap film. Electric forces, hydrophobic tails, and trapped air between bubbles repel the bubbles from coalescing, overcoming inertial forces.
b. Lens
If the forces mentioned above are overcome, bubbles interact through their films, sticking together to form a lens or fuse into coalescence. Destroying the thin film separating the two bubbles leads to partial coalescence. (Image: Harris 2010)
c. Coalescence
Varying the pH of the bubble solution, which influences surface tension, can help us understand coalescence behavior. For stable surface tension, we observe more interaction and higher chances of coalescence, whereas for lower surface tensions, we observe more bouncing. Balanced surface tension leads to simple coalescence, while unbalanced factors lead to partial coalescence or bursting.
IV - Surface Wave Propagation: An Explanation for Different Types of Coalescence
To explain the different types of coalescence, we examine what happens when two bubbles fuse. The different curvatures of the bubbles create a pressure profile inside the bubble, resulting in a surface wave that propagates all around the two bubbles. The speed and wavelength of the wave depend on surface tension and film thickness. Combining pressure and bubble solution properties leads to different wave propagation, creating different phenomena. Balanced factors lead to smooth propagation, causing simple coalescence, while unbalanced factors lead to wave rippling the film, leading to bursting or pinching off a separate bubble, resulting in partial coalescence.
V - Conclusion
In conclusion, by understanding the properties of bubbles, such as surface tension, age, or speed of collision, we can predict bubble behavior.
