Ninth-grade science fair experiment shows the glugging behavior of water bottles changes with bottle softness

It’s not every day that a ninth-grade science fair project turns into a published paper. But Rohit Velankar, the son of University of Pittsburgh Engineering professor Sachin Velankar, did just that. Watching the glug-glug of juice out of a carton, Rohit, then a ninth grader, asked, “Does flexibility of a bottle affect drainage time?” His first experiments, involving deli containers covered by patches cut out of rubber gloves, showed that containers with rubber lids emptied faster than those with the original plastic lid.

This was sufficiently interesting for Velankar père to get involved.

“The glugging of inverted rigid bottles has been studied, but not the effect of container flexibility,” Velankar said. “Yet, common containers span a range of flexibilities — rigid glass bottles, flexible cardboard juice cartons, or intravenous bags. Surely their flexibility should affect emptying.”

For the paper, a part of which also became Rohit’s tenth grade project, the duo created idealized bottles: tall acrylic boxes with rubber-membrane lids, a hole at the bottom, and a sensor to measure the pressure oscillations with each glug. To make the bottles, Rohit took advantage of the machine shop tools and electronics equipment at his high school.

“My school has a neat makerspace which made the fabrication easy,” Rohit said.

By increasing the diameter of the rubber membrane, the authors simulated containers with higher flexibility. They not only confirmed that the flexible containers drain faster than rigid ones, but also found that they drain by bigger, but less frequent, glugs.

The next step: to test whether their results extend to high-viscosity liquids like corn syrup.

Source: “Soft bottles drain faster but glug slower,” by Rohit S. Velankar and Sachin S. Velankar, Physics of Fluids (2024). The article can be accessed at https://doi.org/10.1063/5.0217553 .