Tiny Propellers That Pull Instead of Push: Why Small-Scale Rotors Can Move Backward

What is it about?

This study explores how propellers behave in a previously unexplored “in-between” flow regime known as intermediate Reynolds numbers (Re ≈ 1–150), which lies between the worlds of large ships and tiny bacteria. Using experiments on a miniature submarine and numerical simulations, the researchers discovered a surprising thrust reversal—a propeller that normally drives forward in water instead pulls the vehicle backward when placed in a more viscous fluid. The team showed that this reversal arises from a competition between two forces: the usual backward acceleration of fluid and a newly identified mechanism called centrifugal suction, where rotation creates an inward pull along the propeller’s axis. When centrifugal suction dominates, the propeller effectively acts as a suction pump instead of a thruster. This phenomenon, validated across multiple experimental setups and simulations, reveals that propeller physics at small scales follows fundamentally different rules from those governing ships or bacteria.

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Photo by Jametlene Reskp on Unsplash

Photo by Jametlene Reskp on Unsplash

Why is it important?

The discovery fundamentally reshapes our understanding of how rotation-driven propulsion works at small scales, which is crucial for developing the next generation of micro-robots. Many emerging technologies—from medical microrobots navigating blood vessels to miniature devices inspecting pipelines or cleaning water—operate in viscous or low-inertia environments where conventional propulsion theories fail. By uncovering the role of centrifugal suction and identifying the thresholds where thrust reversal occurs, this work provides the first mechanistic framework for designing efficient propellers at intermediate Re. Beyond its technological implications, it bridges a gap in classical fluid dynamics, linking the behaviors of high-Re industrial propellers and low-Re biological flagella, and opens new directions for theory and design across physics, biology, and engineering.