Experimental snapshot of a Quincke roller suspension during a vortex reversal induced by a temporal modulation. (Credit: Bo Zhang, Alexey Snezhko/Argonne National Laboratory)

A self-organized vortex of rotating microparticles in a fluid reverses direction when an electric stimulus is interrupted and then reapplied with the same orientation, providing fundamental insights into mechanisms behind the sudden switch in rotation. The research offers potential inspiration for microfluidic pumps that allow the controlled flow of liquids. Such pumps allow precise fluid delivery for biomedical, chemical, and electronics applications.

These vortices of microparticles, which are engineered particles less than a millimeter in diameter, show evidence of what scientists call emergent behavior — a kind of self-organization and collective motion. In their native state, the particles are distributed randomly, but when scientists apply an electric field, they begin to synchronize their motion and rotate together either clockwise or counterclockwise.

The reversal of the particles’ rotation is caused by a combination of two kinds of forces. The first force is called an electrostatic force, which basically causes neighboring rolling particles to repel one another. This force works only over short distances — particles need to be more or less touching each other in order to experience it.

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