Two touching or 鈥渒issing鈥� soap bubbles can detach, slide along each other sideways or shift to the side when they are pushed together or pulled apart.
You never see a soap bubble form in the shape of a cube or a diamond because it takes too much energy to keep the bubble鈥檚 molecules bound into these shapes. Instead, bubbles are spherical, which lets them keep their surface area, and the energy cost of maintaining it, as small as possible while also enveloping as much gas, like air, as they can. at the University of Strasbourg in France and his colleagues wanted to see what shapes form when instead of one bubble, there are two and they are touching.
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They pushed a soapy liquid through two circular plastic frames that were stacked one above the other to make two bubbles, keeping the frames vertically separated just enough for the two bubbles to push into each other as they formed. After each was fully formed, the researchers pushed them into each other or separated them by moving the top frame up or down while the bottom one stayed fixed.
When they pulled the top frame upwards, the bubbles detached, but when they lowered it to squish them together, sometimes the top bubble slipped along the side of the bottom one and sometimes one would disappear, and the remaining bubble would shift sideways. The researchers catalogued when exactly each scenario happened by repeating the experiment with bubbles of many different sizes and by pushing them into each other with more or less pressure.
To understand the laws of physics behind the way the bubbles 鈥渒issed鈥�, they also simulated many of the experiments on a computer and calculated some bubble shapes from mathematical models of fluid behaviour. Ultimately, Walzel and his colleagues found that the touching bubbles were also forming shapes that minimised the energy needed to maintain them.
Walzel says that they can now predict what any two touching bubbles will do depending on their properties, such as their size. This could help them understand how more solid objects like soft silicone capsules or very light foam shells deform when they touch, like when they are used as packaging and squished together during transport, he says. The team will experiment with making these objects 鈥渒iss鈥� next.
The team鈥檚 video of these experiments was part of the , a competition held by the American Physical Society at a meeting in Las Vegas, Nevada, earlier this month.
