Flowing fluids that act like the interlocking teeth of mechanical gears offer a possible route to novel machines that suffer less wear-and-tear than traditional devices. This is the finding of researchers at New York University (NYU) in the US, who have been studying how fluids transmit motion and force between two spinning solid objects. Their work sheds new light on how one such object, or rotor, causes another object to rotate in the liquid that surrounds it – sometimes with counterintuitive results.
“The surprising part in our work is that the direction of motion may not be what you expect,” says NYU mathematician Leif Ristroph, who led the study together with mathematical physicist Jun Zhang. “Depending on the exact conditions, one rotor can cause a nearby rotor to spin in the opposite direction, like a pair of gears pressed together. For other cases, the rotors spin in the same direction, as if they are two pulleys connected by a belt that loops around them.”
Making gear teeth using fluids
Gears have been around for thousands of years, with the first records dating back to 3000 BC. While they have advanced over time, their teeth are still made from rigid materials and are prone to wearing out and breaking.
Ristroph says that he and Zhang began their project with a simple question: might it possible to avoid this problem by making gears that don’t have teeth, and in fact don’t even touch, but are instead linked together by a fluid? The idea, he points out, is not unprecedented. Flowing air and water are commonly used to rotate structures such as turbines, so developing fluid gears to facilitate that rotation is in some ways a logical next step.
To test their idea, the researchers carried out a series of measurements aimed at determining how parameters like the spin rate and the distance between spinning objects affect the motion produced. In these measurements, they immersed the rotors – solid cylinders – in an aqueous glycerol solution with a controllable viscosity and density. They began by rotating one cylinder while allowing the other one to spin in response. Then they placed the cylinders at varying distances from each other and rotated the active cylinder at different speeds.
“The active cylinder should generate fluid flows and could therefore in principle cause rotation of the passive one,” says Ristroph, “and this is exactly what we observed.”
When the cylinders were very close to each other, the NYU team found that the fluid flows functioned like gear teeth – in effect, they “gripped” the passive rotor and caused it to spin in the opposite direction as the active one. However, when the cylinders were spaced farther apart and the active cylinder spun faster, the flows looped around the outside of the passive cylinder like a belt around a pulley, producing rotation in the same direction as the active cylinder.
A model involving gear-like- and belt-like modes
Ristroph says the team’s main difficulty was figuring out how to perform such measurements with the necessary precision. “Once we got into the project, an early challenge was to make sure we could make very precise measurements of the rotations, which required a special way to hold the rotors using air bearings,” he explains. Team member Jesse Smith, a PhD student and first author of a paper in Physical Review Letters about the research, was “brilliant in figuring out every step in this process”, Ristroph adds.
Another challenge the researchers faced was figuring out how to interpret their findings. This led them to develop a model involving “gear-like” and “belt-like” modes of induced rotations. Using this model, they showed that, at least in principle, a fluid gear could replace regular gears and pulley-and-belt systems in any system – though Ristroph suggests that transmitting rotations in a machine or keep timing via a mechanical device might be especially well-suited.
In general, Ristroph says that fluid gears offer many advantages over mechanical ones. Notably, they cannot become jammed or wear out due to grinding. But that isn’t all: “There has been a lot of recent interest in designing new types of so-called active materials that are composed of many particles, and one class of these involves spinning particles in a fluid,” he explains. “Our results could help to understand how these materials behave based on the interactions between the particles and the flows they generate.”
The NYU researchers say their next step will be to study more complex fluids. “For example, a slurry of corn starch is an everyday example of a shear-thickening fluid and it would be interesting to see if this helps the rotors better ‘grip’ one another and therefore transmit the motions/forces more effectively,” Ristroph says. “We are also numerically simulating the processes, which should allow us to investigate things like non-circular shapes of the rotors or more than just two rotors,” he tells Physics World.
The post Fluid gears make their debut appeared first on Physics World.
Connexion
