Debunking Boyle's Self-Flowing Flask

This paper investigates the widespread myth of perpetual motion devices based on the so-called Boyle’s self-flowing flask, frequently popularized in online videos. A simplified theoretical model is developed to describe the dynamics of the system, showing that the flow inevitably decays and that the maximum operating time is only a few seconds for realistic geometries. Simple original tabletop experiments with hoses of different sizes confirm this picture, yielding even shorter operating times due to the additional dissipation caused by turbulence. It is of course trivial to remark that perpetual motion is impossible; what is perhaps less obvious, and what our analysis demonstrates, is that the fountain mechanism itself cannot last more than a few seconds in practice, thereby debunking the fake videos circulating on the internet. The theoretical analysis developed in this paper can serve as a useful introduction to the generalization of Bernoulli’s equation to viscous and unsteady flows, making it suitable for inclusion in a graduate-level fluid dynamics course. On the other hand, the experimental side is accessible to anyone, requiring only simple materials and thus offering an engaging way to illustrate fundamental fluid dynamics concepts.