We all know that a dropped or shaken soda can comes with the risk of a fizzy eruption. People use different techniques to prevent these bubbly blasts, including tapping the top of the can. This advice has been passed around for years, but does it actually work?
The answer lies in the physics of what’s happening inside the can.
Building the bubbles
To make a carbonated drink, carbon dioxide is injected into the liquid — such as soda, beer or Champagne — at a high pressure and cold temperature.
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“[Bubbles] are formed as a result of what is called supersaturation of carbon dioxide,” Roberto Zenit, a professor of engineering at Brown University who studies fluid mechanics, told Live Science.
Supersaturation is an unstable state where a liquid or gas can hold more dissolved materials than normal conditions allow. Thanks to its instability, there are some actions, like shaking a can, that can encourage more bubble production.
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“The mechanical action (tapping or shaking) generates violent pressure waves inside the liquid; these waves rebound at the walls, creating low-pressure zones where tiny bubbles form,” Javier Rodríguez Rodríguez, an associate professor of fluid mechanics at Carlos III University of Madrid, told Live Science in an email. “If given enough time (minutes to tens of minutes, depending on the drink and container), these bubbles dissolve again and the can ‘forgets’ that it has been perturbed.”
However, if the soda can is open while the bubbles are suspended in the liquid, the sudden pressure drop from opening the can encourages more bubble production, resulting in more spillover, Rodríguez Rodríguez said.
In some rare cases, like spraying Champagne after a NASCAR victory, creating a geyser of foam is exactly the goal, which the drivers accomplish by vigorously shaking the bottles beforehand.
Does tapping actually work?
Unlike NASCAR drivers, most people are looking to save their drink, so they may try tapping the top of the can.
The idea behind this strategy has some merit. According to Zenit, carbon dioxide bubbles can expand out of tiny imperfections called nucleation sites, which can include dust particles or rough spots on the can.
“The reason why you tap is basically to remove possible bubbles or dust particles within the soda can,” Zenit said. If you remove those, you can reduce the rate of bubble formation, he added.
Tapping on a soda can can also work for carbonated drinks in glass and plastic bottles.
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In theory, a few gentle taps could knock loose some bubbles clinging to the walls of the can, allowing them to float into the pocket of air above the soft drink before it’s opened. With fewer nucleation sites left in the liquid, there would be fewer places for bubbles to rapidly grow once the can was opened and the pressure dropped.
But that explanation comes with an important caveat: While gentle tapping might dislodge a handful of bubbles, tapping too hard, or tapping repeatedly, could create more bubbles, Rodríguez Rodríguez said. The same principle applies to carbonated drinks in glass or plastic bottles, as tapping down on a beer bottle can encourage foam to fizz over.
Those competing effects make it difficult to determine whether tapping actually helps, and some experiments suggest that overall, it probably doesn’t.
In one preprint study from 2019, researchers tested 1,031 cans of beer by dividing them into four groups: shaken and tapped, shaken and untapped, unshaken and tapped, and unshaken and untapped. The tapped cans were flicked three times on the side with a finger before being opened. The researchers found no statistically significant difference in the amount of beer lost between the tapped and untapped cans, regardless of whether they had been shaken. They concluded that tapping “does not prevent beer loss” and that the practice isn’t supported by their observations.
That doesn’t necessarily settle the question for soda, but it hints that tapping is not an effective method.
“Beer and soda differ in composition and foaming behavior, but the study provides relevant experimental evidence that tapping is not a demonstrated solution,” Kit Yam, a food scientist at Rutgers University, told Live Science in an email.
How do you prevent a soda can from fizzing?
If tapping isn’t the answer, time probably is. Letting a shaken can sit upright for several minutes gives the bubbles time to dissolve or rise into the air pocket at the top of the can. Keeping the drink cold can also help, because carbon dioxide stays dissolved more easily at lower temperatures.
“Put the can you accidentally dropped or hit in the fridge for half an hour and open another one in the meantime!” Rodríguez Rodríguez said.
If you don’t have another can or you don’t want to wait, Rodríguez Rodríguez suggested another option: opening the can slowly.
“In this way, the gas can escape, but I only leave a tiny slit for foam to fizz out,” he explained. “Then, when you hear the gas is no longer escaping, you can safely open the can completely.”
You can use the same process to reduce the amount of foam that spills over when you pour a carbonated drink into a glass.
“If you pour it slowly,” Zenit said, “then you reduce this agitation.”
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