College Student Solves 100-Year-Old Physics Problem

Caroline Delbert
Photo credit: Ecole Polytechnique Federale de Lausanne

From Popular Mechanics

  • After the question has stumped scientists for more than a century, a college student has finally explained why bubbles “stick” inside very narrow tubes of liquid.
  • Observations about bubbles in glasses of beer or tap water relate only broadly to these few-millimeters-thick tubes.
  • The scientist bounced light off of the bubble and tube to measure thickness of a film holding the bubble in place.

A French college student says he’s solved a 100-year-old physics problem. The question is about an air bubble trapped in a very narrow tube of liquid, where the bubble appears to be “stuck,” but physicists have never been able to explain why. Wassim Dhaouadi’s undergraduate research showed that the bubble isn’t truly stuck—it’s just moving upward very, very slowly, like a climber inching through a narrow space between rock faces.

For a long time, scientists have observed that an air bubble trapped in a tube just a few millimeters thick appears to be stuck and can’t move freely with gravity. They theorized that a film of liquid was “holding” the bubble in place, but couldn’t prove it. The secret to Dhaouadi’s success is the way he developed to observe and measure a microscopic film of liquid that prevents free movement of the air bubble.

The tiny tube problem is a specific case that shows an exception to how bubbles behave otherwise. There are two major “what’s this bubble doing here?” questions asked by civilians having beverages, and these are different because of both what causes the bubbles and the size and shape of the vessel.

First, if you leave a glass of water at room temperature, it will sprout tiny bubbles along the sides and bottom of the glass. This is because water is filled with dissolved gases and other substances that separate back out of the water when it’s left alone and undisturbed. Especially when water is treated with added salts or minerals or run out of an aerating household tap, there’s a lot in there that will settle out.

And second, you might have an especially fizzy-looking glass of beer or sometimes soda, where bubbles gather and stream along the sides of the glass. This is caused by imperfections on the surface of the glass, which gives carbonated liquids an excuse to release their carbonation in bubble form. In other words, your glass is a little dirty, and the dirt is triggering bubbles.

Tiny bubbles in a glass of water stick to the side, but agitated bubbles in a carbonated liquid more easily float upward, because their energy exceeds the amount of surface tension holding the bubbles to the side of a water glass. In a tiny tube just millimeters across, even a very small bubble is literally surrounded by surfaces. This is why scientists long theorized that a microscopic, but intact “film” held the bubble in place, the same way a soap bubble will adhere to a surface or another bubble and form a bond. They just couldn’t prove it.

Dhaouadi cracked the problem by using an observation method similar, in a way, to mass spectrometry. He and a lab supervisor directed light at the side of the tube and then at the surface of the bubble and used the reflected rays of light to deduce the thickness of the film. This is also how they observed that the bubble isn’t truly stuck, either. It’s just moving very, very slowly, hindered by the film surrounding the bubble, which wants to cling and stick to the tube.

Solving a problem like this is a major coup and a fun thing on its own, but much of the study of these nano-scale fluid dynamics relates back to health, like how blood flows through vessels in the human body. Understanding resistance and flow can help scientists who study ways to explore and treat problems with these vessels, and recreating body-like conditions using synthetic materials is one of the biggest frontiers in science today.

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