I believe that everyone has played "soap bubbles" when they were young. As long as you have enough patience and skill, you can definitely blow out a big and beautiful "soap bubble". We all know that the basic principle of soap bubbles lies in surface tension. By adding surfactants to water, surface tension is significantly reduced and the radius of curvature of the stable liquid surface is larger, thus forming "soap bubbles". Speaking of this, I believe everyone will have questions, why our soapy water never blows beautiful bubbles, because the presence of surfactants is not enough, and a small amount of but important polymer needs to be added as a stabilizer. Beautiful soap bubbles are fleeting and easily destroyed. This is because it is in a metastable state and is unstable. When touched by the outside world, the steady state is destroyed, causing the bubble to burst.
New Discovery: Surface Tension Is the Decisive Factor Causing Bubbles to Burst in Viscous Liquids
The blowing and bursting of bubbles is not only a game, but also has a wide range of uses in industrial production. In the food industry, adhesives and other fields require a lot of foaming to form cavities and prepare low-density fillers; in the coating industry and other fields, the presence of defoamers is needed to improve the quality of the paint film. Therefore, the principle of bubble gas-liquid interface rupture is an important problem that needs to be solved urgently in the industry. On the other hand, it is also an important basic scientific problem in the field of colloidal interface chemistry. Recently, the research team of Professor James C. Bird of Boston University conducted in-depth research on this issue and found that surface tension is the decisive factor leading to the rupture of bubbles in viscous liquids, subverting the traditional belief that gravity is the decisive factor. The related research is titled "A new wrinkle on liquid sheets: Turning the mechanism of viscous bubble collapse upside down" and is published in the world-class journal "Science" in the form of a cover paper.
The gas inside the liquid will gather to form bubbles, and then due to the buoyancy of the bubbles, they will inevitably rise to reach the interface to form a film dome supported by the gas trapped inside, and then the film dome will burst. In low-viscosity liquids, due to surface tension and inertia, the bubble life is very short and will burst instantly within a few milliseconds. However, the process of bubble bursting in high-viscosity liquids widely existing in nature and industry is quite different. Bubbles in high-viscosity liquids will have a longer life. At the initial stage of bubble formation at the interface, due to the integrity of the bubble, the gas can be restrained, and the internal gas can support the bubble. Then the top of the bubble bursts and a hole is formed. The internal and external pressures are consistent and the bubble cannot be supported. , The formation of wrinkles around the bubbles, resulting in unstable structure, and then rupture. Previous research believed that gravity and the hole on the top were the decisive factors in this evolution.
However, in this study, the researchers studied the bursting process of bubbles in different positions (upright, side, and upside down) in the viscous liquid of the bubbles. We found that the direction of gravity of the bubbles at different positions is quite different. But the rupture process is basically the same, so the preliminary judgment is that gravity is not the decisive factor in determining the rupture behavior. Taking a bubble with a diameter of 1 mm in a liquid with a viscosity of 106 cPD, calculate the ratio of its capillary force to the influence of gravity, Fc/Fg≈80, it can also be seen that the capillary force determined by the surface tension is the process of bubble bursting The decisive factor.
The gas inside the liquid will gather to form bubbles, and then due to the buoyancy of the bubbles, they will inevitably rise to reach the interface to form a film dome supported by the gas trapped inside, and then the film dome will burst. In low-viscosity liquids, due to surface tension and inertia, the bubble life is very short and will burst instantly within a few milliseconds. However, the process of bubble bursting in high-viscosity liquids widely existing in nature and industry is quite different. Bubbles in high-viscosity liquids will have a longer life. At the initial stage of bubble formation at the interface, due to the integrity of the bubble, the gas can be restrained, and the internal gas can support the bubble. Then the top of the bubble bursts and a hole is formed. The internal and external pressures are consistent and the bubble cannot be supported. , The formation of wrinkles around the bubbles, resulting in unstable structure, and then rupture. Previous research believed that gravity and the hole on the top were the decisive factors in this evolution.
However, in this study, the researchers studied the bursting process of bubbles in different positions (upright, side, and upside down) in the viscous liquid of the bubbles. We found that the direction of gravity of the bubbles at different positions is quite different. But the rupture process is basically the same, so the preliminary judgment is that gravity is not the decisive factor in determining the rupture behavior. Taking a bubble with a diameter of 1 mm in a liquid with a viscosity of 106 cPD, calculate the ratio of its capillary force to the influence of gravity, Fc/Fg≈80, it can also be seen that the capillary force determined by the surface tension is the process of bubble bursting The decisive factor.
The Influence of Bubble Thickness and Viscosity on the Kinetics of Bubble Collapse
On this basis, the researchers studied the kinetics of bubble collapse. It is proved by theoretical reasoning that in the process of bubble collapse by surface tension, the height and collapse speed of the bubble depend on the thickness and viscosity of the bubble. Researchers use experiments to study the collapse process of silicone oils with different viscosities. First, by using a high-speed camera, the relationship between viscosity and bubble height is proved, and the collapse of bubbles can be effectively delayed by increasing the viscosity. Then, the thickness of the bubble tip was measured by monochromatic light diffraction fringes, which proved the relationship between bubble thickness and collapse speed. It is further demonstrated that surface tension is the main driving force in the process of bubble collapse.
Experimental Result
The non-rupture ablation mechanism of bubbles is considered in previous studies that the holes created on the bubble surface are also one of the decisive factors for wrinkles. The researchers here designed a device that releases the gas from the bottom after the bubble reaches the maximum, so that the bubble is ablated without creating holes, and wrinkles are still produced on the surface of the bubble in the process. The researchers believe that this is due to the fact that after the gas is released and the bubble loses its support, the compression of the hoop during the contraction of the bubble is greater than the surface tension, resulting in wrinkles. James C. Bird's research team studied the bursting morphology of bubbles at different locations and found that surface tension is the main driving force for bubble bursting. Gravity plays a negligible role. Surface tension is also the main factor that determines the behavior and wrinkling of viscous bubbles. . They also developed a predictive model so that the bubbles in the viscous liquid would not wrinkle under all conditions, and described a scenario like this. This research also has important guiding significance for the regulation of bubble behavior in industrially produced viscous liquids.
