Seven years ago, scientists recorded an unusual occurrence: a pool of red dye that seemed to ‘know’ how to navigate a maze filled with milk. With just a few drops of soap, it expertly maneuvered its way through, steering clear of dead ends and making 90-degree turns on its route to the exit.
What started as a demonstration illustrating the intricacies of fluid dynamics transformed into an art piece showcased in the American Physical Society’s Gallery of Fluid Motion, and ultimately became a puzzle that the researchers have now unraveled. Their findings are detailed in the journal Physical Review Letters.
“We devised this experiment because we were struggling to convince people about certain phenomena related to drag reduction,” explained Paolo Luzzatto-Fegiz, an assistant professor of mechanical engineering, whose focus includes modeling fluid flow and exploring drag, which pertains to the resistive forces encountered by solid objects moving through fluids.
“We had a theory regarding how this occurred,” Luzzatto-Fegiz noted. “This paper presents a mathematical framework for that phenomenon.”
Especially intriguing to the research team, which included UCSB engineering professor Frederic Gibou and collaborators from Princeton University, the University of Manchester in the UK, and the Université de Rennes in France, was the dye’s remarkable capability to ‘select’ and move in the right direction when it might be expected for the dye to disperse more randomly.
Surface tension, the force that causes fluid molecules at the surface to cling together and behave like a thin skin, resisting outside forces, plays a critical role in this ink-on-milk experiment. The soap acts as a surfactant, a substance that lowers surface tension, thus facilitating movement. However, the researchers calculated that it’s the surfactants already present in the milk that assist the ink-soap mixture in navigating the maze.
“The extra surfactant and the one already in the milk work in tandem,” Luzzatto-Fegiz explained. The existing surfactant in the milk creates a variety of resistances that affect how the ink and soap mixture behaves as it travels through the maze. According to the researchers, dead ends and narrow areas apply more pressure against the mixture, while the path with the most surface area—where the exit is located—provides the least opposition.
“This means the added surfactant instantly understands the maze’s layout,” Luzzatto-Fegiz remarked.
This research supports previous studies examining the forces that propel the movement of the ink-soap mixture. This phenomenon, known as the Marangoni effect, occurs when there is a difference in surface tension, such as that caused by the additional surfactant, leading to the liquid being drawn from areas of lower surface tension to those of higher tension. This effect is a “novel consequence” not previously investigated and may have implications for scenarios involving “surfactant-driven movement in complex networks, such as lung airways,” as mentioned in the study, and could inspire enhanced techniques for drug delivery or fluid movement in complicated systems.
