Oil and Water

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ENG Study Examines Fluid Dynamics of Capillary Action

By Sara Cody

Langmuir Bird
Graduate student Peter Walls and Assistant Professor James Bird (ME)

Whether it’s a raindrop beading up on a jacket, morning dew clinging to a leaf, or a sip of wine leaving a trail along the inside of the glass, surface tension is a ubiquitous force that people encounter every day. Though it is a fundamental problem in physics, a BU study by Assistant Professor James Bird (ME, MSE), recently featured on the cover of Langmuir, revisits the relationship between gravity and surface tension and explores its underlying effects on capillary displacement of viscous liquids.

“When you place a piece of paper towel over water and the paper towel absorbs it, you are seeing capillary displacement because water is displacing the air in the paper towel and it gets wet,” says Peter Walls, a graduate student in Bird’s laboratory who was the lead author on the paper. “When you use a liquid more viscous than water, other factors like gravity and the viscosity of the other fluid come into play when it comes to measuring the rate that the liquid is absorbed. Those other factors are what we are looking at in this study.”

Walls and Bird got the idea to explore the effects of viscosity on capillary action when they were working on a different, more complex experiment. Using small test tubes filled with oil and glass beads to create an obstacle for the liquid, Walls dipped them into a tub of water with oil layered on top, in order to mimic the trapping found in oil reservoirs. When the liquid was removed from the tubes, drops of oil clung to the glass beads at the point where the curve of the spheres formed and wondered if they could predict the behavior.

When Walls looked to the literature for answers, he came up surprisingly empty. This gap in existing knowledge prompted him to take a step back, and simplify his experimental setup to get a better idea of what was happening in its simplest form. They removed the glass beads and elected to work solely with the empty glass tubes themselves, dipping them into baths of liquid varying between low, medium and high levels of viscosity.

The study, featured on the cover of Langmuir, revisits the relationship between gravity and surface tension.

To analyze their data, the research team modified the equation of motion to account for the effects of the displaced fluid. Previous works have suggested that gravity is negligible until the late stages of rise. Here, through asymptotic analysis, they showed analytically how gravity indirectly reshapes the rise dynamics in their experiments. This new insight allowed Walls to plot out what was happening in the tubes graphically, and what he found confirmed his suspicions: that gravity appears to have a role even in the early stages of capillary rise, depending on the viscosity of liquid.

“Capillary rise in a tube is a canonical fluid mechanics problem that has been examined for centuries,” says Bird. “A remarkable aspect of our work is that it untangles previous conflicting results and shows how they materialize from different limits of the same underlying model.”

To continue this research, Walls and Bird will resume working on the experiment that initially set them down this path, looking at the same phenomenon with viscous liquids and how an obstacle (the glass beads) affects the outcome.

“Ultimately, the goal of our lab is to study aspects of fluid mechanics that are driven by surface tension, so these results will be important to account for going forward,” says Walls. “By developing a deeper understanding of this simple case, we can use this knowledge and apply it to more complex cases, which can teach us a lot about when it happens in the real-world, from oil recovery in contaminated groundwater to paint coatings and many other industrial settings.”