# Hydrodynamic Doppler effect in flowing soap films

## TL;DR

This paper proposes that vorticity in flowing soap films propagates at the Marangoni wave speed, leading to a hydrodynamic Doppler effect, supported by theoretical modifications and experimental validation.

## Contribution

It introduces a hypothesis linking vorticity propagation to Marangoni wave speed and demonstrates the resulting Doppler effect through theoretical modeling and experiments.

## Key findings

- Vorticity propagates at Marangoni wave speed in soap films.
- Periodic vortex arrays exhibit a redshift similar to Doppler effect.
- Experimental results confirm the presence of the hydrodynamic Doppler effect.

## Abstract

It is known for many years that the vorticity and the thickness fields in soap films are coupled and that the thickness wave propagates at the Marangoni wave speed. Based on the two observations, we propose a hypothesis that the vorticity field propagates at the Marangoni wave speed in flowing soap film channels. Using the hypothesis, we modify the point vortex model and find that the periodic array of vortices will have an expanded periodicity in the longitudinal direction, similar to the redshift of the Doppler effect. The theoretical prediction is compared with the experiments and we find that the hydrodynamic Doppler effect is present in soap film flows.