Soft Bottles / Glugging

In sophomore year, I did a continuation of my previous year's project, but this time with a refined experimental design including a pressure sensor and automatic water inflow and air venting. The swappable membrane design remained as a way to change container softness.

When emptying a container, there is a need for "glugging", because as water leaves, the pressure of the air above the water column decreases until it is low enough to keep the water from exiting. At this point, an air bubble must enter the container through the outlet hole, increasing the air pressure, allowing water to exit again, at which point the cycle starts again. This creates the "glugging" that we are all familiar with.

However, if there was some flexibility to the container walls, then as the air pressure decreased, those walls were sucked in slightly, which compensated for the lower air pressure, allowing the water to flow out for longer before an air bubble needed to enter. A more flexible wall can deform more, and therefore when flexibility increased, the frequency of air bubbles entering decreased, which decreased the rate of glugging.

This was verified by the pressure data collected during experimentation, which showed peaks and troughs that aligned with the entrance of an air bubble and exit of a stream of water, respectively.

These conclusions were eventually refined and published in Physics of Fluids, under the title "Soft Bottles Drain Faster But Glug Slower".