In a narrow vertical tube, alcohol flows upward, surrounded by water, and carbon dioxide bubbles are born inside it. As soon as they appear, they rush toward the axis of the flow, as if an invisible hand gathers them into a neat chain. The reason lies in the concentration difference between alcohol and water: at the edge of the jet, this difference changes sharply, creating a force that pushes the bubbles toward the middle.
Experiments showed that bubbles of various sizes, from fractions of a millimeter to several millimeters, reliably align along the center. Larger bubbles may slow down or even momentarily move downward when the flow around them is distorted. This contact-free self-assembly could be useful in microfluidic chips and phase separation.
Even in ordinary soda, bubbles do not rise strictly vertically: they drift toward regions with fewer dissolved impurities, the same mechanism as in the alcohol experiment.