In 2020, Héctor Alarcón, at O’Higgins University, Chile, and his colleagues started studying the forces that act on particles moving through the fluid in a vibrating tank. The research led them to the discovery of a new type of pattern, that they called “hedgehog” based on the similarity to the nocturnal animal’s shape. The discovery came as a surprise since researchers have conducted similar experiments, without observing the new patterns.
In their experiment, the team adapted a setup devised by Michael Faraday in the 1830s. He sprinkled particles into a shallow, water-filled container and by vibrating it, the particles would sink to the bottom of the container. In response to the vibrations of the container, they are arranged into various patterns: parallel stripes, checkerboards, or hexagonal structures, depending on the vibration frequency. Today, these patterns are called Faraday waves.
To evaluate the role of the particle nature of tracers in pattern formation, we ran the experiment using soluble dyes (ink), and yet patterns remained. Moreover, increasing the tracer concentration by a factor of 5 keeps the observed patterns unaltered. These observations demonstrate that patterns are generated by the flow itself, and tracer interaction or buoyancy are not relevant, contrary to what was observed in larger-particle experiments.
In Alarcón’s experiment, the particles remained buoyant, floating just below the surface. When the container was shaken vertically at 8.3Hz, they could see the particles collecting into four groups, two on each side of the antinodal line. Depending on the size of the oscillation, the researchers saw a variety of shapes for these groups, ranging from spinning vortices to spiky hedgehogs. They also used simulations to duplicate the patterns, discovering that the particle groupings are caused by circulating fluid currents caused by the fluid pushing and pulling along the walls on a regular basis.
Faraday-Wave Contact-Line Shear Gradient Induces Streaming and Tracer Self-Organization: From Vortical to Hedgehoglike Patterns, Héctor Alarcón, Matías Herrera-Muñoz, Nicolas Périnet, Nicolás Mujica, Pablo Gutiérrez, and Leonardo Gordillo
Published: December 2020