Upon observing colleagues struggle to keep their coffee from spilling at a math conference, UCSB professor of mechanical engineering Rouslan Krechetnikov began a research project to study this phenomenon. Krechetnikov’s work is featured in Scientific American and is set to appear in Physical Reviews E.

Although the question is simple, the answer involves a robust roast of fluid dynamics, surface physics, fluids and object interaction and the mechanics of walking.

According to Krechetnikov, the idea came to him, appropriately, during a conversation with a student over a cup of coffee.

“My student and I started discussing [the project] during a coffee break after observing how the conference participants were trying not to spill their coffee,” Krechetnikov said in an email. “While it was obvious to us and, I guess any other fluid mechanician that liquid sloshing in a cup should be excited by walking, we could not anticipate the complexity of and how interesting the phenomena are, in particular the details of the resonance and how looking at the coffee while walking affects the number of steps to spill.”

By examining high speed films of people walking with cups of coffee in a straight line, Krechetnikov and his grad student characterized the motion of coffee in a cup at varying angles and velocities. The team used LEDs and placed sensors in cups in order to record when a spill occurred. The researchers then used modeling techniques to quantify how gentle sloshing can result in spills.

According to the group’s findings, the pace and motion of walking is the prime culprit.

“The maximum acceleration occurring early on in the walking sets an initial sloshing amplitude,” Krechetnikov and fellow researchers stated in their research paper. “This interface detection is then amplified by the back-and-forth and pitching excitations.”

In essence, when the frequency of liquid reaches resonant frequency, the oscillations reinforce one another and the amplitude of movement ultimately overcomes the height of the coffee mug.

According to Krechetnikov, previous studies pertaining to fluid dynamics may provide insight on this topic.

“Of course, there are ways to control coffee spilling. For example, using ideas from liquid sloshing engineering studies, which historically were done to stabilize missiles, fuel sloshing in a tank of a missile leads to destabilization [of the missile],” Krechetnikov said.

Furthermore, Krechetnikov theorizes that annular rings or cunning mug designs could dampen the oscillations.

Krechetnikov began to study fluid dynamics in middle school while pondering how planes fly. In this same vein, his decision to investigate the science behind coffee spills and subsequent study was motivated by curiosity.

“All our studies are driven by curiosity and often we initiate a project because it is fun,” Krechetnikov said.

He also prefers a regular French Roast.