Propeller can't propel at Intermediate Reynolds Numbers- Experiment*

Microrobots hold immense potential for a wide range of applications, including targeted drug delivery, vascular plaque removal surgery, and various industrial fields. Enhancing microrobot performance often focuses on designing propellers across millimeter, micrometer, and nanometer scales. At the micrometer and nanometer scales, these propellers are frequently designed as helical structures. Extensive research has also been conducted on propellers operating at high Reynolds numbers (Re) in aviation. However, there is a notable lack of research on propeller performance at intermediate Re.

In this study, a propeller-driven toy submarine was used to examine thrust performance in silicone oil with varying viscosities to adjust Re. Our results show that for Re ranging from 5 to 130, the toy submarine consistently moves in reverse, regardless of the propeller's rotation direction. By modeling the submarine as a disk-propeller combination, we confirmed that at intermediate Re conditions, the interaction between the submarine body and the propeller causes the thrust direction to reverse even when the propeller rotates forward. Additionally, we observed that the range of Re where this reversal occurs widens as the disk diameter increases or as the disk is positioned closer to the propeller.