Hundred-year-long mystery in bubble physics solved by EPFL undergrad student

A bachelor’s student at the École Polytechnique Fédérale de Lausanne (EPLF) seems to have solved a mystery that has puzzled scientists for a century. He found a compelling theory to explain why gas bubbles in narrow vertical tubes seem to remain stuck instead of rising upwards, as bubbles do.

Wassim Dhaouadi, the engineer in question, found and documented an ultra-thin film of liquid forms around the bubble that prevents it from rising as freely. The mechanics of bubble movement in a wide tube or glass is fairly easily explained by fundamental laws of science. That said, those same laws of science don’t offer a satisfying explanation for why air bubbles in tubes of millimeter thickness don’t rise in a similar fashion.

Dhaouadi found that, in fact, the bubbles aren’t “stuck”, but moving very, very slowly.

Dhaouadi, a student at the Engineering Mechanics of Soft Interfaces laboratory (EMSI) at EPFL’s School of Engineering, viewed, measured and described  the properties of the thin liquid film around bubbles – something that hasn’t ever been done before experimentally. A method of ‘optical interference’ was used to make the measurement, and the film was found to be a few dozen nanometers (1 x 10^-9 meters) in thickness. The technique involved a light source pointed at an air bubble inside a narrow tube, such that the intensity of light reflected after passing through it could be analyzed.

From the light reflected by the tube’s inner wall and the surface of the bubble itself, an accurate estimate of the film’s thickness was made. Dhaouadi, who worked on the project with help from his professor and project mentor John Kolinski at EPFL, also found that the film changes shape if heat is applied to the bubble. It also returned to its original shape when the heat was removed.

“At first, we did not know if there would even be a solution to this problem,” Dhaouadi told  EPFL press . Dhaouadi, who is now completing a Master’s degree at ETH Zurich, participated in the project out of his own interest in the research, and wound up publishing a paper from his work that brings to rest a centuries-old puzzle, Kolinski said.

The study and its findings were recently published in   Physical Review Fluids .